Posted By Dana1 on 17 Apr 2013 01:41 PM

Posted By docjenser on 17 Apr 2013 02:51 AM

Frank, Minneapolis 97% design temp is -16F. How much manual switch over do you want to do? Your suggestion to set the buffer target temp to 115F at 10F outside, which means entering water temp to the heatpump would be 115F and leaving temp at about 123F, will not work well. Capacity will be around 25 KBTU/H, and with an old Victorian with a heatloss around 70 KBTU, the balance point will be 30F for the 3 ton high temp. In reality, it will never reach 115 F buffer tank at 32F outside temperature. You are lucky if you get it up to 105F, at which temp the BTU output of the radiators will equal the heatloss and the BTU output of the 3 ton w-w.

ACCA Manual-J sez -11F, as the 99% outside design temp (and not -16F as a 97F outside design temp, thus). It's better number to use something in the -10F range or even -8F, as a 97% number, lest we oversize it even further.  

Whatever dataset places the 97th percentile outside design temp at -16F for Minneapolis may not be including the past 15 years(?).  The coldest temp logged in MN this winter was only -11F (19 January 2013), according to weatherspark.com data. The 2012 low point was -15F (20 January), in 2011 it was -15F on 21 January, in 2010 it got to -15F on the 2nd & -14F on the 9th of January, but you have to go back to 2009 to find a temp below -16F.  You have to go back the second & third weeks of January 2009 to find temps below -16F, which makes it a very unlikely number for a 97% design temp, or even a 99% design temp.

This feels like another  "...10% here, 10% there..." type of design load creep toward the high-side of reality, something to be jealously guarded against, since in this case it becomes  "... a half-ton here, half-ton there...", with a measurable effect on actual peak water temps & net efficiency.

Thanks Dana, for clarifying it. as you know, with radiant the design must be 100% since there is not strip heat which bails you out. So a radiant system must be designed for 100% of the load. Now you are adding a 40 mph wind to the -10F you have there on a normal end of January day and combine it with an old victorian, and all your theoretical calcs in term of heatloss don't really do much good anymore. Keep in mind, that system must cover the -16F in 2010, and also the -20F which might come up in 2014.

And what does tonnage has to do with peak water temps and efficiency? Is a 4 ton W-W heatpump less efficient than a 5 ton or 7 ton, or 10 ton? And no matter what tonnage you use, wouldn't you dial in the lowest possible supply temperature in the outdoor rest anyway? So why would a ton more capacity cost you efficiency or COP?

Designing a hydronic system is more complex than putting putting a Water -Air in, designing it for something between 95 and 99% capacity and let the supplement heat take care of the rest. Did you account for the drop in temperature from 10F to -10F and also the pick up in wind by 30 mph within 2-4 hours? Even a system designed for 110% of your 97% Manual J calculations needs more BTUS to increase the supply temperature in hundreds of gallons of water in the large, cast iron radiator system(e.g. heat up the water), battle the heatloss caused by the dropping temperature and compensate for the increased air intrusion into the old victorian at a higher than manual J accounted wind speed.

I know you like the mini splits and super tight houses, and you appear to know much about them. But they are the direct opposite from an old hydronic radiant system in an old house in Minnesota weather combined with w-w geo. Again, thanks for clarifying the 97% temp range versus the 99% temp range, but it is not really that relevant in this application where you need 110% to ensure that the customer has 100% comfort.

I designed the low temp hydronic heating system (albeit fossil-fired, not compressor-based) for my own antique house that I hard-limited to 125F AWT. I have more than a passing familiarity with how hydronic design is done, on both high and low mass radiation, even on 100 year old houses. I also have more than a passing familiarity with calculating heat loads on said houses.

The peak water temp requirements are still a direct function of the radiation, and what's necessary to cover the load- no rocket-science here. But it's important to at least start out with the correct outside design temps and an aggressive & accurate heat load calculation before diving into how much oversizing it takes to have any margin at the 99% condition. Designing for higher than necessary load conditions drives you to higher output water temps, and and higher temp output takes a toll on efficiency, increasing the size of the compressor needed to get there with any given set of radiation.

No system has to cover 100% of the 99.9% condition load (like the -22F low seen in Minneapolis on the 16 January 2009), or at least it's not worth spending much extra or run higher water temps to be able to be fully covered at the 25 year extremes, or even the 5 year extremes. The 99% condition almost always occurs when the occupants are still in bed, and spending even $500 in excess system to be able to get up to a 70F house instead of a 68F house on that one day in every 3 years is just silly for most people. The thermal mass time constant of even barely-insulated antiques is enough to not lose so much ground that it's an egregious hit to comfort. The 99.9% condition tends to be fleeting indeed, more so than even the 99% condition. But it's true that designing at the 97% condition makes more sense with tighter & higher R houses (like current code-min or better) than for leaky antiques.

But it's up to the client to decide how much they're willing to pay for the one day in a thousand comfort not ours. You still have to at least start from fairly accurate representation of what the 99% design condition load is before adding in fudge factors like higher-than-design wind speed or lower than design temperatures for those willing to spend real money up front for the one day in a thousand- perfect comfort solution. Starting with a presumed 97% condition that's fully 5F below the published 99% condition would be a lapse in due-diligence on the designer's part IMHO. (I would assume, or at least hope you would have caught that had you been the designer under contract.) Yet I've had local heating pros tell me with a straight face that my outside design temps were 10F below what's listed in Manual-J- whose numbers they were using are a mystery, but it's a temp we see only every 4-5 years here, and just over-the-top dead-wrong.

Spending the money on air sealing to cut both the average load and wind-speed related loss upticks buys more in total comfort than any amount of system up-sizing to cover the 40mph wind condition on the leaky version of the house. Having recently been involved in a project that reduced the air leakage of an 1890s vintage 3-story to under cfm/50, I'm pretty confident in that assessment- very-tight houses ROCK on comfort (at any R-value.)

Designing for higher than necessary load conditions drives you to higher output water temps, and and higher temp output takes a toll on efficiency

Over sized equipment wastes up-front money, but it most cases, it *increases* energy efficiency. Think over sized radiators (*lower* water temp), large loop fields (better delta-T), more first stage operation (higher COP), more ability to select the best source based on conditions (relevant in this case).

Plenty of power in this home to cover the load at whatever temp. They just need help to get the set-up and logic sorted so it runs as efficiently as possible using the w-w unit and gas boiler on the hydonic side and wether its useful to use the w-air unit in the mix during heating season.

Looks like what they have can be modified to give a good efficient system.

Posted By Dana1 on 17 Apr 2013 04:00 PM
I designed the low temp hydronic heating system (albeit fossil-fired, not compressor-based) for my own antique house that I hard-limited to 125F AWT.

I don't consider a system with 125F AWT low temp. 85F supply temperature is low temp. Great you designed a system which works, but this is a system which is fossil fuel fired, where you can always dial up the capacity and the temperature. Please don't tell me that your fossil fuel boiler is exactly sized to your 99% manual J heatloss calculation...

Designing for higher than necessary load conditions drives you to higher output water temps, and and higher temp output takes a toll on efficiency, increasing the size of the compressor needed to get there with any given set of radiation.

This is simply not the case. The house will need a certain supply temp to satisfy the BTU needs to keep a certain temp. That does not mean that is needs now higher output water temps. A larger compressor also does not mean it runs less efficient. Nor does it mean that it runs at higher temps. Please show me how a w-w heatpump run at a lower COP because it is 5 tons instead of 4, or a 8 tons instead of a 6 ton.

spending even $500 in excess system to be able to get up to a 70F house instead of a 68F house on that one day in every 3 years is just silly for most people. The thermal mass time constant of even barely-insulated antiques is enough to not lose so much ground that it's an egregious hit to comfort. .

I guess you never experienced a cold span over a couple days in Minnesota with some windchill. The (almost non existing) thermal mass of an old victorian disappears pretty quickly. If I would have to tell my customers that the geosystem can not keep up with 70F inside on the coldest days, I would not sell many systems. Again, difference between theory and practice.

I cannot tell you how tremendously helpful all of these comments and recommendations have been. I had a 2 hour meeting with our architect this evening, and he took the highlights of this very long thread home with him to consider and to help us figure out what's fair, reasonable and most importantly, what will make the system efficient. While we were going over the collected information, the SpacePak ran continuously and the gas boiler came on also. The radiators warmed up. Not good considerng the outside temp was 39 degrees and the gas is supposed to be auxiliary. The 5 ton WtA should be able to handle the load when outside temp is 39. As you've all been saying, we have some design/contorl issues, and there's still work to be done. Your recommendations and insights have been invaluable! Thanks to all, and of course additional comments/recommendations are also appreciated!!! You guys are great; I never expected this much insight when I first posted!

"A larger compressor also does not mean it runs less efficient."
Yes but, in heat dominated installations a "larger" compressor can cost more to run for heating and a lot more for cooling. Bigger can cost more to operate with refrigeration. Generally not so with fossil systems.

Just an aside Doc, Tamar is lucky you are available to her.

The only "higher temp->lower efficiency from load overestimation" case I can think of is in setting up outdoor reset. If the controller has fixed curves and you operate with the heat pump parallel to a buffer tank and you overestimate the temperature needed for a given outdoor temp. But maybe some controllers are smarter than that.

Freud would have a field day with interpreting my dreams these days. A few nights ago I dreamt I was walking down a freeway on-ramp holding a green bowling ball.

Last night I was on a bus in familiar territory and thought the "next stop" would get me where I wanted to be, but I ended up lost and then my cell phone fell apart so I couldn't call my husband to tell him where I was....however, there were some wonderful strangers who gave me directions on how to get where I wanted to be.

Posted By docjenser on 17 Apr 2013 11:04 PM


I don't consider a system with 125F AWT low temp. 85F supply temperature is low temp. Great you designed a system which works, but this is a system which is fossil fuel fired, where you can always dial up the capacity and the temperature. Please don't tell me that your fossil fuel boiler is exactly sized to your 99% manual J heatloss calculation...



I guess you never experienced a cold span over a couple days in Minnesota with some windchill. The (almost non existing) thermal mass of an old victorian disappears pretty quickly. If I would have to tell my customers that the geosystem can not keep up with 70F inside on the coldest days, I would not sell many systems. Again, difference between theory and practice.

In most of the hydronic design world anything under 140F AWT is consider a low-temp system, but clearly you can't run geo there with any efficiency.  Maybe there needs to be another "very low temperature" category to address the sub- 100F situations.  (And of course the gas burner isn't sized exactly to the load- modulating gas burners don't actually come with peak outputs as low as my load.) 

The radiation is maybe 15% over Manual-J at the peak temps I have available- in one zone it's probably only sized to about the 95% condition, but it's pretty cozy sitting on the radiant floor at 10F below the 99% condition in there.  But it started out with much less (margin, not radiation.) I could always hack the system in a redesign to get more, but I won't.  Instead I keep picking away at the load by envelope upgrades, and recently installed  a "hail Mary" woodstove for when the grid goes down for extended periods (which it has more than once in the past 5 years- it was out for 10 days in mid-December 2008.)  If the cold snap of the century arrives any potential shortcomings of the heating system can be made up in other means. But since I'm lowering the load with every construction/maintenance project on the house I have plenty of margin already, and it's increasing over time. Were I to have gone geo on this place I'd have addressed the load much more aggressively up front rather than picking at it at the current plodding pace.

I dunno if -5F for the daily highs with 50mph peak gusts over a coupla days meets your cold span standard, but I've been there, done that (yes, in MN, back in the 1970s), so guess again!    I've seen nearly comparable conditions in my MA house too during the quarter-century cold snap in the 1990s, but that was well before any of the rehab on the envelope & heating system got started (hadn't married her yet, and it was her house, eh? ;-) .)  Yes there were some cold rooms in the house then (despite a 3x oversized heating plant), but it would be more comfortable under the current state of the house & system despite the much skinnier margins on the heating end.

I'm not saying there aren't sometimes reasons to upsize modestly over a calculated heat load, only that you at least need to start out with the right numbers before making those rational decisions, not starting with unrealistic design temps such a stated 97% condition that is inexplicably 5F warmer than the published 99% condition, or padding the Manual-J in other ways, or adding an arbitrary offset when using a fuel-use calculated linear approximation for analysis. There is some built-in margin in Manual-J methods, and it's fairly cheap & pretty easy to add to plug loads during the cold snap of the century (if your grid goes down, your toast anyway.)  You've stated on previous threads (and maybe this one?) that you don't believe in oversizing...  except for maybe when you do!?

I get that on geo-air you automatically have some safety margin available with auxiliary strips where you won't in an all-hydronic show, but how much margin DO you normally design for on hydronic-output geo?

Posted By joe.ami on 18 Apr 2013 07:13 AM
"A larger compressor also does not mean it runs less efficient."
Yes but, in heat dominated installations a "larger" compressor can cost more to run for heating and a lot more for cooling. Bigger can cost more to operate with refrigeration. Generally not so with fossil systems.

I am not sure I follow you here. The BTU output of a larger compressor is higher, it consumes more while running, but will run proportionally less and thus cost you the same to operate. W-A heatpumps are different, since their design results in larger units (4-6 tons) being less efficient than smaller units (2-3 tons). But the COP of W-W heatpumps is very similar. You could argue that the startup until it builds up pressure has a higher proportion of the run time, since it cycles more.To small to even measure. The larger, two compressor units are running more efficient now since they have 1 plate heat exchangers serving both copressors, meaning that if only one compressor is running, the heat exchanger is much oversized, meaning more efficient. That is why the COP is around 3.5 in first and 3.1 in second stage.

Doesn't the COP of the W-W compressor normally drop with higher output water temps, since it's working against a higher delta-T? (Which is why I figured you'd need more compressor to get there?)

Posted By jonr on 18 Apr 2013 09:03 AM
The only "higher temp->lower efficiency from load overestimation" case I can think of is in setting up outdoor reset. If the controller has fixed curves and you operate with the heat pump parallel to a buffer tank and you overestimate the temperature needed for a given outdoor temp. But maybe some controllers are smarter than that.

That is exactly the point. The only way it it could be less efficient in a w-w application would be if the outdoor reset is not set correctly, but then even a smaller sized unit would be less efficient. Or if you have no reset controller at all, and always run it at a high temp.

Posted By Dana1 on 18 Apr 2013 12:25 PM
Doesn't the COP of the W-W compressor normally drop with higher output water temps, since it's working against a higher delta-T? (Which is why I figured you'd need more compressor to get there?)

Sure, higher EWT will result in a lower, less efficient COP. However, this has nothing to do with a higher (or oversized) capacity of the geo system.
The required output temperature for the radiators remains the same, so does then the buffer tank temperature. An undersized unit might never get to the required supply temperature when it is cold outside and the load requirements are higher, which is the case here with Tamar's system. A unit sized for 100% load will get to that temperature, and an oversized unit will also get to that temperature, however, due to the higher capacity, it will cycle more. However, it can respond quicker to temperature swings.

So the supply temp to the radiators is not different, the larger unit is simple running less, but using more power when running, but the BTUs delivered are the same, so it is a wash.
The problem with older houses, in contrast with new houses where you know very well the insulation material going in, and the R-value etc, is that a manual J is pretty much a guestimate. You are standing in front of a wall and have to guess the R-value of that 150 year old wall. So now this an owner who bought the house 5 years ago, and the previous owner told him that "he had done some insulation in the walls", but no details are available. Or they blew cellulose into the walls, but did they nicely fill all the cavities? So a manual J is tricky at older homes.
Some have a tree line as a wind barrier, some have nothing in front of them. Difference between wind and no wind scenario can increase the load by 40%, same outdoor temp. I admit, I have been off by over 50% with manual Js, in each direction. So what do you do when sizing your system, knowing that you cannot undersize a system at all?
You put in a higher capacity, and sometimes all you can do is rely on your experience.
There is no other choice, if you want this to work well. Your customers understand that and actually appreciate this. What they don't understand and not appreciate is the house not getting up to temperature on the colder days, or responding to changing weather conditions because you sized it for 97% of the load, after investing $30,000 in a geo system.
W-A heatpumps are different, easier to predict, and even if you should ever been off too much, you cost the customer $50 of supplement more per year, not a big deal in the big scope of things.
The best input have been the monitoring systems, since you get the feedback on how well you design decisions were on target.
What you call "...a lapse in due-diligence on the designer's part..." I call taking care of my customers and ensuring that it will work well for them. If they are not happy, I am not happy! I want to be happy! So I am simply selfish here!

Posted By docjenser on 18 Apr 2013 12:28 PM

Posted By jonr on 18 Apr 2013 09:03 AM
The only "higher temp->lower efficiency from load overestimation" case I can think of is in setting up outdoor reset. If the controller has fixed curves and you operate with the heat pump parallel to a buffer tank and you overestimate the temperature needed for a given outdoor temp. But maybe some controllers are smarter than that.

That is exactly the point. The only way it it could be less efficient in a w-w application would be if the outdoor reset is not set correctly, but then even a smaller sized unit would be less efficient. Or if you have no reset controller at all, and always run it at a high temp.

And this is what we currently have (no reset controller at all).

And this is what we currently have (no reset controller at all).

That's why the boiler comes on even when a low load could be efficiently serviced by just the heat pumps. The summary seems to be that you need a smarter controller/thermostat, some fixes to the ground loop and someone who knows what they are doing to do the work. But don't expect a huge difference in operating cost (all sources are similar in $/btu).

Posted By jonr on 18 Apr 2013 03:59 PM

And this is what we currently have (no reset controller at all).

That's why the boiler comes on even when a low load is being efficiently serviced by the heat pumps. The summary seems to be that you need a smarter controller/thermostat, some fixes to the ground loop and someone who knows what they are doing to do the work. But don't expect a huge difference in operating cost (all sources are similar in $/btu).

The operating costs are calculated with the system running at 2.47 COP. That could be improved by lowering the supply temp by lets say 30%. Then they have 3 circulation pumps running, costing $80 each, when the 3 ton high temp only needs one. W-W could be running at COP of 4 at warmer outside temps and lower buffer tank temps. I would say a 30-40% improvement could be achieved. Just a guess.

I think that total systems (all 3) annual operating cost ($/year) is the important number. Then compare it to a simpler (but correctly done) system using only two sources. "30-40% improvement" could be misleading when you mean "30-40% improvement for part of the system part of the time".

Staring at a 150 year old wall and taking a WAG at it's construction for a U-factor is never going to cut it, but shooting in the dark just isn't necessary anymore. I'm a believer in infra-red imaging under blower-door test to ferret out (and fix) any egregious insulation gaps & air leaks before embarking on anything as serious as specifying GSHP system (or even a low-temp mod-con boiler based system), at which point the heat load calculation error-window narrows considerably, and the deletory effects of wind-washing are much diminished, and the need for building in "just in case" margin melts along with it. Maybe that's not the geo designer's job (or maybe it is?), but it's pretty silly to drop that kind of coin with such huge error bars on the fundamental load numbers. Several grand up front nailing down (and lowering) the heat load is going to be money well spent, and more than likely largely offset on the back side in downsized mechanical systems and operating cost, with a real up side on comfort worth paying for even if the mechanicals stayed the same.

Running a GSHP without outdoor reset control on a hydronic output system in a climate like MN seems like a fundamental error on the original system designer's part too, but that's probably not a very expensive fix. Outdoor temperatures don't track the increase in load with wind at ALL, and if you're leaky enough to be suffering docjenser's stated 40% increase in actual load in a wind vs. no-wind situation, you'd still have to run water temps much higher than optimal to cover that potential. A 40% hit with 40mph wind would have to be a MIGHTY leaky house though (yet I'm sure they exist- I've lived in one of those).

My bottom line is that it's always going to be better to start with a tight house, and the notion that you can't get there with a classic Victorian is just plain wrong. It isn't always super-easy or super cheap- it might take 3 guys 3 days to really tame the beast whereas in a simpler-newer house it can be a 1 day 2 guy sort of deal. But even a week's worth of air sealing & spot insulation would usually cheaper than a 40% overdesign on the GSHP to be able handle wind factors on the coldest night of the decade, and it's a bigger upgrade in comfort all the time than any heating system is capable of delivering.

Tight is right- always. I like Curt Kinder's theatrical smoke & blower door pressurization approach to making sure his foam contractors don't leave the job site with gaping holes remaining on attic sealing & insulation jobs as part of his take on geo designs. I wish more GSHP contractors had a more whole-systems approach, taking a truly serious stab at the envelope deficiencies as part of the process, not just the dead-obvious, lowest hanging fruit. High efficiency heating systems don't deliver comfort, higher efficiency buildings do.

---------------end of soap box speech------------------------------ (at least for now. ;-) )

Dana, in one way you are preaching to the choir here, I emphasize to just about every customer that the best money they can spend is on insulation, so I can make the system smaller for lesser upfront money. Most of our customers with new built houses listen, about 50% of our jobs are new built, almost all of those have foam insulation, many are net zero houses. They run ultra efficient.


http://welserver.com/WEL0712/ is a good example.


The problem are the retrofits. You live in your ideal world where you improve your envelope more and more, and worst case, your boiler bails you out with more capacity and higher temps. My customers have a failing fossil fuel system on life support, need a new heat system, and look into geo. Or are on propane or oil, existing system still is fine, but the want to save money. So they want a quote for a geo system. The geo system will cut their costs down by 80% (from an 65% 25 year old oil burner to a dual stage running at a COP of 4.5), from $5000 on oil to $1000 on geo. The savings are extreme for them. They tell you they look into further improving the envelope, but they are on a budget, and they end up not doing it. But when I come out to them, they want a quote and move forward with a geo system. So what do you want me to do, refuse to put a geo system in unless they tighten up their house?


You suggest that it should be the geo designers job to design a better house, and call it silly to calculate with high error margins in those situations. Forced air you have the strip heat bail out. Old house with radiant, those are the once you are getting burned with if they are not designed for 110% or more of the load. Plus the system usually don't run as efficient as they could, dues to the higher load temps needed with old radiant systems. I agree with you 100%, the best money and effort is spend in reducing the load, but you live in a theoretical (almost academic) world. In the real world, customers don't always do what you tell them to do, and they are unhappy if their house is not warm when they want it warm on the colder days, especially after they dropped a lot of money for a system. It does not help if you have all kinds disclaimers in your contract, or remind them that they wanted to do all the efficiency upgrades. Old houses with inefficient radiant I learned to treat different, it is not over sizing, it is correctly sizing to their current load. If they do the improvements later, great, the geo will run more efficient and turn on less.


BTW, 40 % higher output of the radiators load does not mean 40% overdesign in GSHP. For example, going from 120F supply to 140F supply temp will increase the BTU output of a radiator by almost 50%. 10F get you about 20% more output, but render your geosystem about 15% less efficient, for a short period of time. The outdoor reset cannot be set just for 15 mph wind, you need to set it up higher and adjust a few time when you find sweet spot, which is a higher temp than just to hold the house temp, since you want some responsiveness of the system as well.