Hi folks. Brand new to this forum. Got a question regarding DHW and DSH's (yeah I know... this topic has been covered ad nauseum!).

Tried to find the thread; "Justifying two electric hot water heaters", as recc by others but have been unable. Have read many other posts regarding this issue, but so far none that I have found answered my question(s). Here goes:

New construction (just finished framing), have selected two GSHPs for this project both with DSH option (WF Envision NDV049 & NDV064). They will both be on the same closed loop with the coils placed in lake. Location is west of Knoxville, TN so our heating/cooling is somewhat evenly divided. Expecting the house to be very tight and am using open cell spray foam to seal/insulate. Expect to have only two occupants (age late 50s) with average hot water needs (i.e. no hot-tub, no hydronic systems, modest masterbath shower). Natural gas is not available. Expect to use LP for cooking, clothes drying, fireplace, and maybe ... DHW (?). I'm also planning on installing a demand recirc system for HW using a loop with branch mainfolds located at various places in the house to shorten the wait time for hot water.

The plumber is on the job now and I'll need to be deciding on the DHW system best suited for us. Since I'm selecting the DSH option for both HP's I'd like to plumb them to the same pre-heat tank, or the main HW tank. I have read some threads on the differing views of 1 vs 2 tanks. I'll also add that after much research, I've ruled out a tankless system due to the desire to use the DSH's and the need for a HW recirc system - can be done, but makes for a rather complicated DHW system. Too bad because I'd like to go tankless (that's probably a whole nother thread).

1. What are your views of using a pre-heat tank in this configuration vs running the DSH's to the main HW tank (either gas or electric)?

2. I like the concept of using a single HW tank for cost and space saving reasons, but wonder if it is the most efficient? Additionally, how do you plumb two DSH's to the same HW tank and prevent the thermostat from running the heater (LP or electric)?

2.  How does the recirc system work with either a one tank or two tank system?

3. Now to make it more complicated - if I use the two tank strategy, I'm considering installing shutoffs/bypasses to the main tank for the purpose of using DHW produced only from the pre-heat tank during Summer/Winter months. Any comments or suggestions about this configuration? Has anyone attempted this before?

That's probably about it for now.

Very interested to hear any feedback.

For the most part, twin tanks are the best way to go. Make sure they are piped like the drawing and set up for a temp diff of 8~12 deg.

Bergy

Be quite certain that LP is truly a better deal on a cost per btu basis than electricity before selecting it to fuel the finishing tank or the clothes dryer - don't forget to factor combustion efficiency / energy factor into that calculation.

I endorse the Bergy pic (for about the 37-teenth time). The compromises required to configure DSH into a single tank that is also powered / fired are such that in renders DSH virtually worthless.

If you haven't already ordered the Envisions, give careful thought to purchasing the DSH option on just one - the one that will be most convenient to a DSH buffer tank. Connecting over 9 tons of geo DSH to a water heating system serving just two folks is radical overkill. For comparison, I get 50+% of my water heat for a family of 5 from a DSH working off a single NDV038 locked in low stage, about 2.25 nominal tons...all I need to condition 3400 SF under air near Jax, FL.

I have a couple years' HVAC hourmeter, water heater hour meter, heat pump water heater watthour data, and buffer tank temperature data to back all that up.

While on the subject of tonnage, is the selection of 9.5 tons of geo justified by a careful, ACCA Manual J compliant load calculation? Knoxville has a fairly mild climate, and the ground temperature there is just about as good as it gets for efficient geo year round. Is this a 10,000 SF house?

Finally, I wouldn't worry about valving off a tank in summer. Standby losses are lower in warm weather, and if you use Marathon tanks they'll be almost negligible relative to the aggravation and cost of a Frankenvalve system. Adding hardware such as valves and bypass lines increases standby loss since it increases the number of lines and fittings exposed to ambient conditions, even if they are insulated, good pipe insulation runs around R2-R3, vs a tank at R12 or higher.

Bergy, thnx for the post. Picture is worth ... a whole bunch of words.

Regarding this; "Make sure they are piped like the drawing and set up for a temp diff of 8~12 deg.", I'm not sure how a person would ensure a temp diff of 8 to 12 deg other than to set the main tank temp 8 to 12 deg warmer than the temp limit of the DSH which as I understand is about 129 deg. Not sure I want my DHW coming out at 141 deg.

What am I missing?

Thnx again.

Posted By Scewter on 20 Nov 2010 10:07 AM
Bergy, thnx for the post. Picture is worth ... a whole bunch of words.

Regarding this; "Make sure they are piped like the drawing and set up for a temp diff of 8~12 deg.", I'm not sure how a person would ensure a temp diff of 8 to 12 deg other than to set the main tank temp 8 to 12 deg warmer than the temp limit of the DSH which as I understand is about 129 deg. Not sure I want my DHW coming out at 141 deg.

What am I missing?

Thnx again.

The temperature differential of 8~12 deg. is for the Desuperheater. The water being put back into the bottom of the tank should be 8~12 deg warmer than the water being pulled out of the top.

Bergy

Curt; excellent post and points. Many thnx for your time.

Couple of responses;

Regarding "Be quite certain that LP is truly a better deal on a cost per btu basis than electricity before selecting it to fuel the finishing tank or the clothes dryer - don't forget to factor combustion efficiency / energy factor into that calculation." Great point. I have not looked at that. I'm not quite sure how to go about that calc, but I do have access to LP costs as well as electricity rates in our area (LP gas is about $2 per gal, give or take, while electricity runs about $.09/kWh). Probably a conversion thing that I need to look at. I have always assumed that gas was cheaper/more efficient than electricity. That may not be the case anymore, I don't know.
 
Regarding "Connecting over 9 tons of geo DSH to a water heating system serving just two folks is radical overkill. For comparison, I get 50+% of my water heat for a family of 5 from a DSH working off a single NDV038 locked in low stage". This probably goes to the heart of the issue I'm struggling with right now in designing this system. Here're my thoughts - if you can get 50+% of your DHW from a DSH as you indicate (and by the way, reading your other posts I'm convinced your calculations are pretty darn accurate as you are very detailed in your work) then adding another DSH might get you closer to 100%? Obviously you can't just add another DSH without adding another unit. But if you start with two units to begin with as we are, why not take advantage of the most efficient means of creating DWH by way of the DSH. Our contractor priced out the DSH option at approx $1,500 (less 30% for Fed Tax Credit), so we're looking at some amount of time for each of those DSHers to pay themselves off. How long is that...? I don't know, but given the lifespan of the equipment (20+ years?), I would think it would happen well before the end of their useful service life. So, if can approach 100% of DHW during the times they are producing, why wouldn't it be prudent to take advantage of it?

Regarding "I wouldn't worry about valving off a tank in summer. Standby losses are lower in warm weather, and if you use Marathon tanks they'll be almost negligible relative to the aggravation and cost of a Frankenvalve system. Adding hardware such as valves and bypass lines increases standby loss since it increases the number of lines and fittings exposed to ambient conditions, even if they are insulated, good pipe insulation runs around R2-R3, vs a tank at R12 or higher." This is related to the previous point. If I can approach 100% of my DHW usage being supplied by the two DSH during heating/cooling seasons, wouldn't it make sense to isolate the main hot water heater (i.e. shut it down for the season) and supply the house from the buffer tank only. Otherwise everytime I turn on the faucet to get hot water it has to run through the main tank which then has to maintain it at the same 125 deg temp during periods of no flow (such as throughout the night). I'm assuming a 50 gal buffer tank (such as Marathon Themal Storage tank), and a 40 gal main tank (TBD-electr or LP fired?).

Thnx again for the great info. Look forward to reading the replies. Just a simple mind on this end struggling with concepts and ideas that are beyond my ability to resolve. You folks are a godsend.

Scewter

Bergy,

Okay, starting to understand. What's the purpose of the 8 to 12 deg temp difference? Is it so the DSH will heat the water and not just "bypass it" (if that's what happens when it limits out at 129 deg)? And how do you ensure there is that temp difference? (Sorry, I'm just a little slow and uninformed)

Got another question regarding your schematic: my read is that the outgoing water fm the DSH is plumbed back into the buffer tank at the bottom (in this case the drain valve). Assuming that is a correct read, I'm confused about the actual flow. The drain valve is at the bottom, so entering hot water fm the DSH would be entering at the coldest part of the tank? Further confusing me is the schematic from Water Furnace regarding their Geo Tank found here:

http://www.waterfurnace.com/literat...1006TW.pdf

Referencing page 1 and 2, the supply from the buffer tank going into the DSH comes out of the bottom, 7" below where the cold water enters (59" vs 52" dip tubes), yet returns back to the tank at the top. This seems counter to what your diagram indicates. Just a little unsure.

Maybe more to the point - how would you recc I plumb the DSH to this tank (I'm considering using a 50 gal Marathon Thermal Staorage tank for the buffer and am gonna try and insert an image here):



Again, thnx so much for your time in this and look forward to your comments.

We have done it multiple times, namely piping multiple heatpumps' DSH together into one larger buffer tank, works great. You simply capture more heat, especially when you depend on LP or electric for your supplement heat. On the other hand, how much heat do you need, certainly not 9 tons of DSH capacity for 2 people.
People are not aware of many issues there. First, regular gas heated tank have a horrible efficiency, usually between 50-60%. Secondly, due to the principle of operation, a large area is non insulated (e.g. contact area of the burner) and they are usually vented through the tank, so much of the standby loss is lost to the chimney. Overall, the standby heatloss is many times higher than a well insulated electric tank. Third, electric tanks are much cheaper to install. With LP, you are probably looking at around $800 a year in hot water costs, with electric you are probably looking at a few hundred $ less, thanks to the higher efficiency and the lesser amount of heatloss, and thanks to your relatively cheap electricity costs.

I am not big fan of throttling flow in order to achieve a higher delta. The amount of of BTU's transfered is directly proportional to the flow and the delta. You reduce the flow by increasing the delta T , but it will not impact the amount of BTUs transfered into the tank.
Do not run your DHW only of the buffer tank (or one tank), you will be very unhappy and uncomfortable. Every new water entering the tank will be at ambient ground temperature, causing the tank to fluctuate too much, and the DSH don't have have enough capacity for quick recovery, and in the shoulder months there might not be enough heat. Just adding 2 instead of 1 desuperheater does not bring you up to 100%. Especially in A/C mode with the new, more efficient R-410 units, the DSH will heat your water from 60F to 90F since there is simple not enough excess heat to get your water above 90F. So it preheats your water nicely, saving you a lot of money, but cannot do your whole domestic hot water heat load. Even going off one tank in the peak of the heating season, has slow recovery time.
Best and most efficient way in your scenario: One buffer tank running all year around with 2 DSH piped into it (again, you do not need the total load, but recovery of your buffer tank will be quicker, so once you have the DSH already ordered....), and a second way to "boost" your temperature, either by an electric tank, or an on demand heater. Since that secondary source of heat does not have to make many BTUs, probably a high efficiency electric tank with a low standby loss is the best way to go.

I'm with Engineer, what is the heat loss/gain that requires 9 tons in a house described as follows:
"New construction (just finished framing), have selected two GSHPs for this project both with DSH option (WF Envision NDV049 & NDV064). They will both be on the same closed loop with the coils placed in lake. Location is west of Knoxville, TN so our heating/cooling is somewhat evenly divided. Expecting the house to be very tight and am using open cell spray foam to seal/insulate".
Making sure we don't miss the forest for the trees here......

RE DSH piping Bergy's is one I'd endorse.
Joe

At $2 per gallon consumed at 60% energy factor LP costs $36.23 per million Btu of delivered hot water

At $0.09 per kWh consumed at 90% energy factor, electricity costs $29.30 per million Btu of delivered hot water.

The same basic calculation applies to clothes drying.

Pricier gas fired water heaters can be had that bring that EF up a good bit, but they are very expensive, involve moving parts and, unless ducted to get combustion air from outside them home, pull conditioned air out of the house, imposing additional load on the HVAC system. Natural Gas at a buck or so per therm and / or 20 cent kWhrs might justify gas water heating and clothes drying. Trucked LP is an excellent fuel for grilling steaks and powering standby generators, but it is generally a poor deal for bulk heat.

I differ sightly with Bergy on the Delta T across a DSH coil. I have never throttled one to raise Delta T to within the 8-12 range. I rely instead on the DSH option pump being quite dinky, and I admit I assess operation by an unscientific laying on of hands - verify by feel that return water is perceptibly warmer than supply water. Having confessed that I'll comment on the thermodynamics applicable to DSH operation: There are two competing outcomes governing flow through a DSH:

1) Delta-T: Higher flow through DSH coil reduces temperature differential across it and increases total heat recovery...that's good, right?

2) Nope - higher GPM requires more pump power, and circulating pump efficiency is quite poor ~30%. More importantly, higher than necessary flow into a buffer tank will stir it up. That is very bad, because successful (comfort, efficiency) operation of a domestic hot water tank depends on maintaining thermal stratification in that tank - simply put, we want the tank to be hot on top and cold down low whenever water is drawn from it. Bergy's diagram accomplishes this, though to convince yourself of that mentally add the dip tube that carries cold water inlet water deep down into the tank without disturbing the hot water on top. Incidentally, though thermodynamics would seem to favor doing so, we do NOT withdraw water from the boiler drain at the very bottom of the tank. That is to keep any sediment there out of the DSH system.

Finally, I expand and strengthen my recommendation against valving off any part of a domestic water system for any reason. First, some background: Mainstream institutions (Energy star, various conservation groups, utilities) recommend domestic hot water be set at 120 F for reasons of scald safety and energy efficiency. I agree, and run my own even lower at 110. That said, there is a small but vocal minority that suggests hot water be set at 140 to ward off Legionella. Though Legionella is an issue with cooling towers and other institutional water systems, I have yet to encounter convincing evidence of it being a problem in a domestic system. Sometimes I think the 140*F stance is a product of a plumbers union somewhere, seeking additional revenue from pricey tempering valve installations and frequent water heater replacements (140*F increases corrosion and scaling).

All that said, Legionella thrives on stagnation, so isolating any part of a potable water system for any appreciable time is an extremely bad idea - please don't do it.

I reiterate - the standby loss from a well insulated storage electric water heater during summer in a Knoxville home will be very low...on the order of a nickel per day. In fact, if the DSH supply to the finishing tank is more than 5 degrees above the finishing tank setpoint, and hot water is drawn normally, the finishing tank elements may not come on at all. In Florida, some report being able to open the circuit breakers to the finishing tank during 3-4 summer months, thereby accomplishing what you are after. At the first incident of a tepid shower in September, reclose the breaker. Knoxville may be steadily cold enough in deep winter to try this then, too. No harm in it...beyond possible domestic strife over said tepid showers.

To paraphrase an earlier question - "If one DSH meets 50% of hot water load, can two DSHs meet 100% of load? Answer - no. Why? Consider the seasonal characteristic of DSH recovery. DSH meets 50% or more of water heating load by meeting nearly 100% of the demand during summer and winter and much less of the demand, sometimes none, during spring and fall. If there is a week or more of mild weather needing no heating or cooling, DSH recovery drops to zero.

I can state with confidence approaching certainty that ROI on a $1500 investment in a 2nd DSH will be many decades, possibly centuries, given the facts of this case. If the house had 12 occupants my advice would be completely different - I'd advise both DSHs and 200 gallons of buffer capacity and 100 gallons of finishing capacity.

The pictured Marathon tank can be plumbed as Bergy depicts, but there is no need to spring for what I believe is a pricier tank designed for solar systems (extra fittings in place of elements) - just use a standard electric water heater (Marathon, ideally). An additional benefit of using a standard water heater as a buffer lies in its ability to be powered as an additional source of hot water if house guests wash up on your doorstep.

Some more background on DSH limits: When last I checked, Envision DSHs cut out at an inlet temperature of 135*F. This limits output to the mid 140s, providing a measure of scald protection. The buffering tank and then finishing tank will serve to temper this somewhat. In fact, the high efficiency of R410a systems combined with geo loops tend to limit compressor discharge so that the 135 limit is rarely reached. Under extreme circumstances a tempering valve arrangement might be necessary for scald safety.

Finally, I repeat my question - does this house really need 9.5 tons of HVAC?

Posted By engineer on 21 Nov 2010 10:45 AM
.....Sometimes I think the 140*F stance is a product of a plumbers union somewhere, seeking additional revenue from pricey tempering valve installations and frequent water heater replacements (140*F increases corrosion and scaling).

......

funny ..sad ...and true all at the same time

Curt,

good point with the stir up. Any idea at what GPM the flow will stir up the tank?

Posted By joe.ami on 21 Nov 2010 09:31 AM
I'm with Engineer, what is the heat loss/gain that requires 9 tons in a house described as follows:
"New construction (just finished framing), have selected two GSHPs for this project both with DSH option (WF Envision NDV049 & NDV064). They will both be on the same closed loop with the coils placed in lake. Location is west of Knoxville, TN so our heating/cooling is somewhat evenly divided. Expecting the house to be very tight and am using open cell spray foam to seal/insulate".
Making sure we don't miss the forest for the trees here......

RE DSH piping Bergy's is one I'd endorse.
Joe

Joe and Curt,

In response to your questions, the Heat Loss/Heat Gain for this project has not been completed. I have been waiting for these numbers for quite a while now. The sub has performed an initial calc for pricing purposes, but has not sat down to go through the Manual J calculation yet which is beginning to be a problem.

The house is about 7K SF. Another factor affecting the HVAC sizing is the attic - we have a 12/12 pitch and have decided to seal the attic area (vs ventilated attic) so that entire area as well will be part of the heating/cooling calculation. We have specific reasons for doing that with this house which are beyond the scope of this thread. No problem sharing that, just didn't want to stray off course of the DSH and DHW questions. Being a lake home we also have quite a bit of glass to take advantage of the views as well. All in all, I tend to agree with you - 9 tons of cooling is a lot - which is why I'm anxiously awaiting the Manual J numbers.

Posted By docjenser on 20 Nov 2010 01:09 PM

I am not big fan of throttling flow in order to achieve a higher delta. The amount of of BTU's transfered is directly proportional to the flow and the delta. You reduce the flow by increasing the delta T , but it will not impact the amount of BTUs transfered into the tank.

docjenser,

Thnx for the reply. I've read and re-read your post, and am having difficulty understanding this particular part of your response. Are you saying that the DSH flow rates or temps are adjustable?

Just a little unsure of what your mean there.

thnx again.

Posted By engineer on 21 Nov 2010 10:45 AM
At $2 per gallon consumed at 60% energy factor LP costs $36.23 per million Btu of delivered hot water

At $0.09 per kWh consumed at 90% energy factor, electricity costs $29.30 per million Btu of delivered hot water.

The same basic calculation applies to clothes drying.

Pricier gas fired water heaters can be had that bring that EF up a good bit, but they are very expensive, involve moving parts and, unless ducted to get combustion air from outside them home, pull conditioned air out of the house, imposing additional load on the HVAC system. Natural Gas at a buck or so per therm and / or 20 cent kWhrs might justify gas water heating and clothes drying. Trucked LP is an excellent fuel for grilling steaks and powering standby generators, but it is generally a poor deal for bulk heat.

Curt,

I'm gonna break this up into couple of different posts as I'm not really sure how to do it in the same post.

After reading your intial post I worked on the "LP vs electric" calculation and have come to the same conclusion - electric is the way to go. I've also been focusing on the heat pump water heaters that are out there. They seem to be an even better choice.

Thoughts?

Posted By engineer on 21 Nov 2010 10:45 AM

2) Nope - higher GPM requires more pump power, and circulating pump efficiency is quite poor ~30%. More importantly, higher than necessary flow into a buffer tank will stir it up. That is very bad, because successful (comfort, efficiency) operation of a domestic hot water tank depends on maintaining thermal stratification in that tank - simply put, we want the tank to be hot on top and cold down low whenever water is drawn from it. Bergy's diagram accomplishes this, though to convince yourself of that mentally add the dip tube that carries cold water inlet water deep down into the tank without disturbing the hot water on top. Incidentally, though thermodynamics would seem to favor doing so, we do NOT withdraw water from the boiler drain at the very bottom of the tank. That is to keep any sediment there out of the DSH system.

Curt,

Sorry for the confusion, but the lite is just not coming on for me regarding Bergy's plumbing schematic and reading you comments attached above. I understand and agree with your point regarding thermal stratification in a tank, hot on top and cold down low. So far so good. What I'm struggling with is that Bergy's schematic indicates (to me at least) that the return from the DSH (hot) is at the bottom of the buffer tank and appears to be the drain valve.

The feed to the DSH is the cold line and I can visualize the dip tube as you describe, but my guess is the bottom of the dip tube would be above the drain outlet (or inlet for the DSH return in this case), which would create the stirring or mixing of hot and cold.

Posted By engineer on 21 Nov 2010 10:45 AM

Finally, I expand and strengthen my recommendation against valving off any part of a domestic water system for any reason. First, some background: Mainstream institutions (Energy star, various conservation groups, utilities) recommend domestic hot water be set at 120 F for reasons of scald safety and energy efficiency. I agree, and run my own even lower at 110. That said, there is a small but vocal minority that suggests hot water be set at 140 to ward off Legionella. Though Legionella is an issue with cooling towers and other institutional water systems, I have yet to encounter convincing evidence of it being a problem in a domestic system. Sometimes I think the 140*F stance is a product of a plumbers union somewhere, seeking additional revenue from pricey tempering valve installations and frequent water heater replacements (140*F increases corrosion and scaling).

All that said, Legionella thrives on stagnation, so isolating any part of a potable water system for any appreciable time is an extremely bad idea - please don't do it.

I reiterate - the standby loss from a well insulated storage electric water heater during summer in a Knoxville home will be very low...on the order of a nickel per day. In fact, if the DSH supply to the finishing tank is more than 5 degrees above the finishing tank setpoint, and hot water is drawn normally, the finishing tank elements may not come on at all. In Florida, some report being able to open the circuit breakers to the finishing tank during 3-4 summer months, thereby accomplishing what you are after. At the first incident of a tepid shower in September, reclose the breaker. Knoxville may be steadily cold enough in deep winter to try this then, too. No harm in it...beyond possible domestic strife over said tepid showers.

To paraphrase an earlier question - "If one DSH meets 50% of hot water load, can two DSHs meet 100% of load? Answer - no. Why? Consider the seasonal characteristic of DSH recovery. DSH meets 50% or more of water heating load by meeting nearly 100% of the demand during summer and winter and much less of the demand, sometimes none, during spring and fall. If there is a week or more of mild weather needing no heating or cooling, DSH recovery drops to zero.

I can state with confidence approaching certainty that ROI on a $1500 investment in a 2nd DSH will be many decades, possibly centuries, given the facts of this case. If the house had 12 occupants my advice would be completely different - I'd advise both DSHs and 200 gallons of buffer capacity and 100 gallons of finishing capacity.

The pictured Marathon tank can be plumbed as Bergy depicts, but there is no need to spring for what I believe is a pricier tank designed for solar systems (extra fittings in place of elements) - just use a standard electric water heater (Marathon, ideally). An additional benefit of using a standard water heater as a buffer lies in its ability to be powered as an additional source of hot water if house guests wash up on your doorstep.

Some more background on DSH limits: When last I checked, Envision DSHs cut out at an inlet temperature of 135*F. This limits output to the mid 140s, providing a measure of scald protection. The buffering tank and then finishing tank will serve to temper this somewhat. In fact, the high efficiency of R410a systems combined with geo loops tend to limit compressor discharge so that the 135 limit is rarely reached. Under extreme circumstances a tempering valve arrangement might be necessary for scald safety.

Finally, I repeat my question - does this house really need 9.5 tons of HVAC?

Curt,

Regarding the "frankenvalve system" - I love that description btw - I see the wisdom in your advice. After thinking about it, I definitely do not want 50 gals of water sitting there stagnating for upwards of three months or more.

The Marathon Thermal Storage tank appears to be appr $100-$200 more than the same size Marathon electric hot water heater. The valve system incorporated in it may make sense and be worth the extra dollars - depending on the response I get to an earlier post I made regarding Bergy's schematic

Again, thnx for the thoughful responses. I'm not trying to "re-invent the wheel" here. Just posing questions and ideas to make this system the best that it can be given the technology and equipment available today.

Posted By Scewter on 21 Nov 2010 03:51 PM

Posted By docjenser on 20 Nov 2010 01:09 PM

I am not big fan of throttling flow in order to achieve a higher delta. The amount of of BTU's transfered is directly proportional to the flow and the delta. You reduce the flow by increasing the delta T , but it will not impact the amount of BTUs transfered into the tank.

docjenser,

Thnx for the reply. I've read and re-read your post, and am having difficulty understanding this particular part of your response. Are you saying that the DSH flow rates or temps are adjustable?

Just a little unsure of what your mean there.

thnx again.

You can increase the temperature delta by reducing the flow, that way the water gets heated up more, I thought that is what you meant by adjusting the temperature delta. Some manufactures spec the DSH temp Delta to be around 9-13 degrees, but I never found a good reason for it. Flow and delta T are almost indirectly proportional.

Posted By docjenser on 21 Nov 2010 05:24 PM

Posted By Scewter on 21 Nov 2010 03:51 PM

Posted By docjenser on 20 Nov 2010 01:09 PM

I am not big fan of throttling flow in order to achieve a higher delta. The amount of of BTU's transfered is directly proportional to the flow and the delta. You reduce the flow by increasing the delta T , but it will not impact the amount of BTUs transfered into the tank.

docjenser,

Thnx for the reply. I've read and re-read your post, and am having difficulty understanding this particular part of your response. Are you saying that the DSH flow rates or temps are adjustable?

Just a little unsure of what your mean there.

thnx again.

You can increase the temperature delta by reducing the flow, that way the water gets heated up more, I thought that is what you meant by adjusting the temperature delta. Some manufactures spec the DSH temp Delta to be around 9-13 degrees, but I never found a good reason for it. Flow and delta T are almost indirectly proportional.

Oh, okay. Actually, the temp delta issue started with a comment from Bergy where he wrote: "Make sure they are piped like the drawing and set up for a temp diff of 8~12 deg."

In responding to that, I wasn't sure what he meant, or how you'd achieve that.

Thnx for the reply.

Wow - lotta posts to sift through.

I honestly have no idea at what point flow into a water heater dramatically impacts stratification. I have performed an experiment - we have a Roman tub with dedicated 3/4 inch lines and a faucet selected to fill it fast. It flows about 7 GPM per side, hot or cold at the ~25 psig delivery pressure to that floor of the house. I deliberately emptied the 80 gallon finishing tank into it as fast as it would fill, and the temperature seemed to hold until near the end.

My DSH flows on the order of 1 GPM - the pump is truly dinky and the round trip is about 40' with several 90s each way, so I doubt it would appreciably impact stratification.

In the Bergy diagram water is pulled up the CW dip tube and returned via the boiler. That means all the mixing activity occurs near the bottom of the tank. After the water makes a couple passes through the DSH it warms enough for natural convection to take it up into the undisturbed upper volume of the tank. If no water is drawn off the buffer and DSH goes on for several hours, eventually the entire tank contents has made several passes through the DSH. The beauty of this piping configuration is that the finishing tank gets the warmest water from the top of the tank and the DSH gets to work on the coolest water near the bottom of the tank, and sediment is not drawn into the DSH.

You don't need the additional side inlets unless you are laying groundwork for solar hot water

I have an HPWH in addition to a DSH feeding a buffer and a separate finishing tank. I retrofitted a Geyser onto my finishing tank and killed its resistive elements. It works, but the ROI is on the order of 20 years since both being in Florida (lower water heating cost owing to warmer inlet temp than the rest of the country) and having a DSH had already cut my water heating cost to below $200 per year. I spent over $1k to save less than $100 per year. I knew that going into it, did it anyway, but want to make it clear that ROI alone won't justify an HPWH and a DSH together in nearly all circumstances. I particularly like the new GE HPWH - elegant design needs no water pump, but I repeat that its expense is not justified along with a DSH unless you simply decide you want cheapest possible hot water regardless of first cost. Solar panels are a way to spend even more money to heat water at even lower cost.

A sealed attic is absolutely the way to go especially if you'll have ductwork up there. The attic becomes indirectly conditioned space and duct losses fall way off. Having all the ductwork within the sealed envelope renders duct leak concerns virtually moot as well. No worries about air losses through ceiling penetrations My 6:12 hip roof has 3" of closed cell sprayfoam and that makes a major contribution to our ability to heat and cool 3400 SF with 2.25 nominal tons. The high pitch roof will increase cost of attic insulation.

Since you are going all out with this house be certain careful Manual J, D, and S calculations are done before any equipment is ordered or ductwork goes in. If J,D,and S calcs elicit a blank look from your HVAC contractor, find another. This is especially important for rooms with lots of glass - the load calc can let you run scenarios with different window specs (U-factor, Solar heat Gain Coeff.) You canidentify problem rooms and fix them in the design stage or at least make sure they get adequate airflow, summer and winter.