Hybrid Hot Water Tank - how much cools basement??
Last Post 20 Oct 2013 10:18 AM by engineer. 19 Replies.
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ANdadUser is Offline
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08 Aug 2013 10:01 AM
Thanks to those on the forum for all their past help! Situation - 3700 sq ft home - ICF walls - Geothermal with DSH (4 ton - 4 150 foot deep pipes) - Simple electric resistance tank - in Cleveland Ohio. I would like to lower hot water bill which is the low hanging fruit at this point. DSH only contributes a bit. I am considering hooking a hybrid hot water tank (Rheem 2.45 Efficiency factor - 50 gallons) into the basement. How much will this cool the basement? I read a bunch of posts rating a hybrid tank on the GE website and people there loved it and did not complain too much about the temp drop. However I have spoken to 2 contractors at this point who say it will drop it 10-15 degrees. (The first said 10-15 degrees and the second 10) My marriage will drop 10-15 degrees if that happens. I figure it will be less because I have a DSH which will do some of the lifting. Also my basement is 1900 sq feet. And we have all low flow shower heads and don't use a ton of hot water. But I need a better estimate on how much this will chill things off in the basement and my love life. Thanks!
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08 Aug 2013 12:03 PM
How much it drops the temp in the basement has many factors, not the least of which is how much hot water you use (which determines the duty cycle of the HPWH), and whether the space is actively heated. If the basement is it's own zone the geo can certainly keep up with the average load, and the net efficiency is still pretty good.

With the hot water heater in heat-pump-only mode about 40% the heat going into the water is effectively resistance electricity at a COP of 1, and the other 60% is being drawn indirectly from the geo heating system, which probably has a net efficiency of 3.5.

The crayon on napkin math estimate of how much it drops the temp goes something like this:

If your hot water use is low (and it probably is) the hybrid water heater won't cool your basement much, but it also won't be saving a whole lot. To raise 50 gallons of water from 40F to 130F takes 37,530 BTUs. With an EF of 2.45 that means only 60% of that heat is extracted from the room, or 22,518 BTU. If you have 1900 square feet of barely-legal 3" slab you have 475 cubic feet of concrete that weighs ~63,175 lbs, at a specific heat of 0.20 BTU/degree-lb, that's 12,635 BTU per degree. So heating a whole 50 gallons drops the temp in the basement about 22,518/12,635 = 1.8F.

If you used the full EF test volume of 64.3 gallons in a day, that's about 1.8F x (64.3/50)= 2.3F, imparting an average heat load of 22,518 x (64.3/50)/24= 1200 BTU/hr to the basement. (Think your GSHP can keep up with that? :-) )

If your basement is not directly heated by the geo it will cool off over time to where the combined convection, plug loads, and heat radiated off the ceiling matches the heat being drawn by the HPWH. With 1900' of ceiling that's about 0.63 BTU per foot, which takes a ceiling temp about 0.3F above the average basement temp to deliver. Assuming no insulation in the ceiling, the combined R value of say a hardwood floor, subfloor, air films and basement ceiling gypsum could be as high as R3, and with a heat flux of 0.63 BTU/ft the basement ceiling will run about 3 x 0.63= ~2F colder than the first-floor floor, which is probably 2-3F below the average room temp due to stratification, so you're looking at a basement temp 5-7F colder than the average temp on the first floor, at the full 64.3 gallons/day, running heat-pump-only-mode, with 40F incoming water, assuming there are no plug loads or other heat sources in the basement. (Got a freezer or spare refrigerator down there?). If your water use volumes are lower, an the DSH is delivering 60F+ water, the impact is a lot less.
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08 Aug 2013 03:05 PM
So heating a whole 50 gallons drops the temp in the basement about 22,518/12,635 = 1.8F.


It's important to note that this is an average value. Depending on how fast your slab transfers heat to the air, short term air temp drop could be much larger. Maybe 10F (a guess, not a calculation).

My basement (with the gas water heater adding, not subtracting heat) runs about 6F lower than upstairs. Any cooler would be irritating. If I wanted to use a HPWH, I'd put a box around it and vent the box to the upstairs. More efficient and less irritating. Or put it in an upstairs hallway closet with louvered doors.
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09 Aug 2013 01:52 PM
Is this a one tank set-up for the DSH?

If so save the electric tank as a storage tank for the DSH, then add the HPWH. Two tank set-up's work better.

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09 Aug 2013 03:17 PM
Wow - great answers. Chris - This is definitely going to be a 2 tank setup. Dana I'm going to assume your estimate of 5-7 is a little high just because the incoming water will be warmer what with the DSH working on the preheat tank. So lets call it an average of 4 degrees. There may be spikes of up to 10 degrees of cooling off though as Jon has pointed out. Since this is in basement on concrete - any thoughts about using thermal storage tubes next to the unit to soften out the spikes? http://www.solar-components.com/tubes.htm
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09 Aug 2013 04:13 PM
One other point worth mentioning - my other option is a Waterfurnace water to water heat pump. Since my Geo unit (and presumably loop) are a little oversized I may be able to tie into that. My concern there is that my loop temp would drop. It is currently about 37 degrees at the end of winter. Is there any chance I will freeze my loop if I go the other route instead of the HPWH? Maybe I should post that on the other forum - but please let me know your thoughts.
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09 Aug 2013 04:41 PM
Posted By ANdad on 09 Aug 2013 03:17 PM
Wow - great answers. Chris - This is definitely going to be a 2 tank setup. Dana I'm going to assume your estimate of 5-7 is a little high just because the incoming water will be warmer what with the DSH working on the preheat tank. So lets call it an average of 4 degrees. There may be spikes of up to 10 degrees of cooling off though as Jon has pointed out. Since this is in basement on concrete - any thoughts about using thermal storage tubes next to the unit to soften out the spikes? http://www.solar-components.com/tubes.htm

With 1900 square feet of bare concrete the thermal transfer rates between the air and concrete is pretty good. I doubt adding water in tubes of substantially lower surface area would make an appreciable difference. 

If you want to even the coolth-spikes out, partition a room around the HPWH, and let it super-cool the room, using the ~R1-R2 of the partition wall be the heat exchanger between the water heater room and the greater basement space.  It can't be literally a closet or it will impact function, but IIRC a 100-120' room (~1000 cubic feet of volume) is big enough for most of them, if you don't mind the letting that room take the intermittent super-chill.  The short term spikes will be more severe in the water-heater room, but you'd still be able to store stuff there- it'll be the driest room in the basement.

And yes, I was worst-casing it using two assumptions: 40F incoming water temperature, and ONLY heating the tank with the heat pump, never the resistance elements.
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14 Aug 2013 03:13 PM
I have this same setup. 3000sqft ICF with basement, radiant heat, Rheem located in a basement mechanical area.

We've chosen to run the unit in "energy saver" mode during the summer, and use "electric heat only" during the winter. There are enough other heat producers in the area, central vac, septic air pump, freezer, networking, etc than we haven't noticed any real temperature drop from the Hybrid heater. It does however take a bit of humidity out of the air.

During the heating season it makes no sense to pull heat out of the living space to heat the water, so use electric.

In hindsight, The payback will be too long on this unit, and I would have stayed with an 80gal electric and used a good blanket on it. I don't expect the Rheem to last more than 10-15 years.
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14 Aug 2013 04:15 PM
During the heating season it makes no sense to pull heat out of the living space to heat the water, so use electric.


That's correct only if your house heat is electric resistance. Otherwise, use the heat pump in the water heater.
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14 Aug 2013 04:41 PM
Posted By jonr on 14 Aug 2013 04:15 PM
During the heating season it makes no sense to pull heat out of the living space to heat the water, so use electric.


That's correct only if your house heat is electric resistance. Otherwise, use the heat pump in the water heater.

In some markets oil or propane heat is more expensive than resistance electricity. Without knowing the heating source & efficiency and the fuel & electric rates, blanket statements will be rife with exceptions:

At 15 cents/kwh electricity is ~$44/MMBTU, at 10 cents it's $29/MMBTU. At $4/gallon in an 85% burner with typical 10% system & distribution losses, oil heating costs run  ~$41/MMBTU, a bit more when air handler or pumping power is factored in, making $4 oil pretty much a wash against 15 cent electricity, and significantly more expensive than 10 cent electricity.  In cheap electricity markets it makes sense to change modes if you're heating with oil, but in those markets heating with heat pumps is extremely cost effective against heating with oil or propane. (Spending the money on a mini-split rather than a heat pump water heater would be a better investment.)

That said, in most markets if you're heating with natural gas, running the water heater in heat pump mode still makes sense in the winter.
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14 Aug 2013 05:10 PM
Well yes, if you are using something more expensive than electrical resistance heat, fix that first and then worry about water heating.
engineerUser is Offline
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15 Aug 2013 10:54 PM
Getting back to OP's original concern - I echo Dana's remarks that a HPWH (avoid the Rheem in favor of the GE, IMO) would have very minimal impact on a 1900 SF basement's operating temperatures. Area near the HPWH might feel 2-3*F cooler when the HPWH was been running awhile. That the HPWH is fed by a geothermal DSH greatly reduces the load on the HPWH, probably to the point where the investment in the HPWH is hard to justify.

I have a two 80 gallon tank system in a 1000 CuFt mechanical room. Tank one is fed by the DSH on my WaterFurnace heat pump and averages 100+*F all summer. Tank two is topped off using a Geyser external HPWH. Typical monthly consumption is 20-30 kwH in summer and 100 during spring and fall. Mech room cools by no more than 2-4*F when Geyser runs.
Curt Kinder <br><br>

The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
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15 Aug 2013 10:54 PM
Getting back to OP's original concern - I echo Dana's remarks that a HPWH (avoid the Rheem in favor of the GE, IMO) would have very minimal impact on a 1900 SF basement's operating temperatures. Area near the HPWH might feel 2-3*F cooler when the HPWH was been running awhile. That the HPWH is fed by a geothermal DSH greatly reduces the load on the HPWH, probably to the point where the investment in the HPWH is hard to justify.

I have a two 80 gallon tank system in a 1000 CuFt mechanical room. Tank one is fed by the DSH on my WaterFurnace heat pump and averages 100+*F all summer. Tank two is topped off using a Geyser external HPWH. Typical monthly consumption is 20-30 kwH in summer and 100 during spring and fall. Mech room cools by no more than 2-4*F when Geyser runs.
Curt Kinder <br><br>

The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
ANdadUser is Offline
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20 Aug 2013 06:57 AM
Thanks all for the help.

Curt - why would you favor the GE? The Rheem has a better EF (2.45 vs 2.4). Have their been reliability issues with the Rheem?

http://www.energystar.gov/productfinder/product/certified-water-heaters/compare/2184476/2174050/
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20 Aug 2013 11:24 AM
Without direct experience I tend to like approach & construction (and warranty) of the AirTap Ati-xx series. Ducting the cold output to a location where you might make better use of the cooling has some value beyond any that merely cool their surroundings. Ducting the cold output directly into a cooling system return duct may be the right solution in some homes. This installation ducted it to a hot-spot:

https://www.youtube.com/watch?v=5EzfmLFCYy4

Of those only capable of cooling their immediate surrounding the Stiebel Eltron units are pretty solid, if your wallet can stand the upcharge.
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20 Aug 2013 12:41 PM
The AirTap hybrid allows operation down to 20F. During mildly cold weather it could use outside air, creating some efficiency increase over inside air only models.
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01 Sep 2013 09:56 AM
I have a family of four using hot water, and a HPWH in a 2 car attached garage that is decently insulated. It drops the temp ~2°F or so on average. Note that I have an AOSmith unit that moves a lot of air, so more sensible cooling and less latent. IF you are using a GE unit in a damp basement, a lot of the BTUs could come from latent heat, and the cooling could be quite small.

Not that there's anything wrong with that!
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04 Sep 2013 04:56 PM

We live in West Michigan, have 3 ton wshp DSH piped to std elec buffer tank and GE HPWH as our powered tank.  The setup has worked fabulous so far... we are coming up on completing the 2nd year since we added DSH and HPWH. 

Basement/surrounding temperature....  As stated by others impact on temperatuer will vary on volume of the room, envelope, DHW usage, and your climate.  For us we have ~1600 sqft basement (mech room is open to only about 1200 of that)... some heating vents in the basement so during active heating or cooling season we don't notice any temp difference..... during shoulder months or light heating/cooling seasons there are few degress difference max...(the freezer sitting next to it might help a bit, but it is new and efficient so it can't help that much!)... durign the summer its my "first-stage of cooling" :).   Unless you have no heat runs down in the basement AND have insulation between the basement and upstairs AND consume a lot of hot water (I can't image there are alot of people who use more than our household - 8) ... I doubt you would have an issue espeically if you have an open basement that is 1900 sqft. 

I don't know the exact incremental contribution of the HPWH as I added both the HPWH and the DSH at the same time, but there are several factors to consider.

1. Humidity.... I used to see condensate on pipes and well expansion tank, and even had some rusting going on... not any more and the rusting has stopped. Basement feels more comfortable and people comment on it... without knowing I have the heat pump water heater.

2. Cooling benefit... works like first stage cooling and if you turn on furnace fan can be distributed to the upstairs.

3. The Reem offering 2 years ago was not as efficient compared to the GE (per DOE listings) and used a different style HX that was supposedly "inferior"... also there were lots of bad review on issues /failures. If the current models show that the Reem is higher efficiency it may be a completely different design that has been improved.... My GE unit is the old blue and gray model not the red and white ones built in Kentucky... I bought it at Lowes on sale and then used 10% off and was able to get a very reasonable 10 year labor warranty to go with the 10 year standard parts warranty required by Energy Star.

4. Energy efficiency of the HPWH during the heating season is a funtion of the efficiency/cost of your space heating equipment... when I was in the decision making stage I came up with the following worst case, heatin season effective COP for a HPWH in a space heated by electirc (or GSHP)......   

Effective Overall HPWH COP = HPWH Rated COP / (1 + (HPWH Rated COP / Space Heat COP) )

Example… Hot Water: GE Model HPWH COP 2.35 has one of the highest published/rated COP.
Space Heat: ClimateMaster Heat Pump has rated heating COP of 3.5 (adjusted to include pumping penalty)

Effective HPWH COP = 2.35 / (1+ (2.35 / 3.5 ) ) = ~ 1.4

Note: this is worst case analysis, ignores tank loses, ignores efficiency pickup due to longer run times on space heating equipment on milder days, and is only relevant for periods of time when your space heating is operating reguarly. Any time you are not close to needing heating and the amount of space temperature depression due to the HPWH operation is not enough to require space heating, the efficiency increases dramatically. Hot water COP is then around 2.3 and 2.75 times cheaper than a standard elec. tank (depending on ambient temperature, tank setpoint, operating mode, and how fast you are using hot water). Any time it's hot and you have your A/C cooling your house, the GE HPWH is essentially making both hot water and cool air, which makes the effective COP go up much higher... don't have the numbers handy now... but I thought it was around 4.0.


Here's our experience for what its worth. (following is a re-post from another thread)
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Abstract...   Based on before and after tracking we are saving more than $400 per year on hot water costs due to the addition of the DSH and the GE HPWH (was elec resistance before). Payback (for unit, extended 10 year labor warranty, and piping materials) should be right around 3 years.

Intro...
In December of 2011, I connected/piped and enabled the desuperheater on my Climatemaster TTV038 (open loop pump and dump, source well has about 45' lift, and consumes ~800 watts at 1st stage flow where it runs a vast majority of the time). I installed a new GE Heat Pump Water Heater as my powered tank, and repurposed my existing dual element electric tank (50 gal) as the buffer tank. At about the same time I replaced my old upright frost-free freezer with a new energy star (manual defrost) freezer.

There are several relevant points that took place since the desuperheater was enabled and HPWH was installed:
1. A record or near record warm summer 2012

2. A colder than average winter 2012-2013 (based on local HDD data)

3. Our family grew in size. Based on data logger information from my original standard electric tank heater (prior to the DSH/HPWH tank install) and total cold water consumption read from my water softener…. I’ve assumed the same proportion of hot to cold and estimated that my domestic HW load has gone up by 15-20%

4. Our family’s comfortable winter t-stat setpoint went from 67 to 68F

5. Added (2) small noise maker fans that run at least 10 hrs per night (sometimes they are left on during the day! :))

6. We stopped using our outdoor clothesline to dry most of our clothes, alternate method is electric dryer (we are off the NG grid of course).

After subtracting out the effect of the new freezer (based on before and after KiloWatt meter readings) AND eliminating the data from the record warm weather from 2012 (Jan, Feb, March, April)…. the combined effect of the above changes has been an overall average decrease in energy consumption of 152kWh per month or about $313 per year at current rates (actual monthly savings is greater in the summer months, but there is noticeable savings in the winter/heating months too).

Factoring in the increased HW use over that period, shows that the new equipment DSH/HPWH is actually saving us an additional ~$100 per year (assuming 10.5 gal/day increase, 68 water DT, 0.89 de-rated electric efficiency including average measured stand-by losses).

So for me the combined savings for the GE HPWH & DSH is more than $400 per year or about (17% of my total energy at the time when I installed the units). Total cost for me - ~ $1,200 (including 10 year labor warranty on HPWH, piping, fittings, insulation, etc.) Simple payback - ~3 years (less as we continue to use more HW, and local electric rates go up).

This was a pleasant surprise being a more northerly location (West Michigan), where in general the HPWH concept isn’t as favorable. Based on COP data for the GE HPWH and knowing approximately how much HW we use, it is clear that you can’t get to the savings I’m seeing with just cheaper HW. So it would seem that a good amount of the savings I’m seeing is coming from both the longer more efficient run times in the heating mode (as the DSH and HPWH move almost the entire HW load onto the GSHP during the heating season), and the cooling side effect of the HPWH in the hot summer.

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15 Oct 2013 06:46 AM
Thanks to all for the feedback.

I ended up putting in the A.O. Smith 80 gallon model. The installer put 4 small vents in the drywall (per A.O. Smith instruction manual's recs) of the mechanical room to help circulate the air.

The upshot is that in the mechanical room the temp drops about 4-6 degrees during the run cycle if the doors to the room are closed. The whole basement not too much - maybe 1 degree.

I think part of the smallish drop is that the mechanical room where the tank sits has sheetmetal ductwork running in there. Since we have an ICF home we leave the HRV and air circulation going almost all the time so there is air passing through the ductwork there even when the geothermal system is not on. We are in the non heating season now so the drop should be less in December with the DSH running.

Overall I'm very happy and might not have done it without the help here to reassure me that the temps would be OK.
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20 Oct 2013 10:18 AM
Update on one of my earlier remarks:

Rheem appears to have improved its HPWH - older model had EF 2.0, had water pump, external heat exchanger, and poor performance per a lab test I read comparing several models. Newer Rheem is said to have fixed some or all of these issues, but I have no direct experience. I'm looking forward to seeing test results / hearing user experiences.
Curt Kinder <br><br>

The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
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