Air-to-Water Heat Pumps – Getting it Right
We’re doing a major remodel of a two-level, 4400 square foot home built in 1982 in Steamboat Springs with a climate zone of 7B. It currently has in-floor hydronic heating via Quik Trak with zones for each room on both levels, powered by two Inteillihot propane boilers. The boilers are noisy, we can smell the combustion of propane in the house, and the house is cold in the winter – the current system doesn’t keep up with the heating demand. We will replace all the windows and address any ceiling or wall insulation, and are planning to remove the Quik Trak and replace it with WonderBoard throughout.
Based on what Yampa Valley Electric Association (our local utility) has sent us, we’re very interested in Cold-Climate Air to Water Heat Pumps, and estimate our design load to be somewhere in the 250,000 btus/hr. Based on what I’m gleaning from this site and others, we would need to have at least 5 SpacePak Solstice Extreme heat pumps to meet the demand, and likely an auxiliary propane boiler as backup with the outside temperatures drop below -5 to -10 degrees F.
We are planning to install photovoltaic and possibly batteries to power at least some of the necessary electricity for this set-up.
Our builder has never installed a system such as this, and there doesn’t seem to be anyone in the area who can provide the expert advice we need to create a warm, energy efficient home.
Are we crazy to consider this system? What’s are your thoughts on getting expert advice on how to design coupled with expert installation?
Many thanks for any support.
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Replies
Air-to-water heat pumps tend to be well matched to radiant floor systems, because the smaller the temperature gradient the more efficient heat pumps are. So if you can run your water at a modest temperature like 105F you should see good efficiency. Have you done a manual J? 250K BTU sounds really high for a 4400 SF house, even one built in 1982.
Thank you. I'll do a manual J because 250BTU seems high to me as well. The estimate was in part driven by average ceiling height of 13 ft in our great room (26x37) with a peak beam at 19 feet).
Ceiling height shouldn't matter. What might matter is having lots of wall area, if those walls are poorly insulated. And lots of windows matters a lot.
I am in climate zone 6 MSP. We run our radiant heat between 93 to 103F and use Warmboard which is similar to QuikTrack. Looking at doing a similar system in a retirement house, but using a air to water heat pump such as Artic Heat Pump or an Enertech Advantage Heat Pump. Both can shift over to a boiler (gas or electric) when temperature drops below -10F for multiple days. Your numbers sound high. My 2000 sq ft house come in around 30,000 BTUh.
My recommendations would be to go all out on improving the envelope first. Then go for a month with the present boilers, measuring their propane consumption, or maybe even their heat output through temperature difference and flow. That will give you an accurate measurement of your heating load. Then size your new heat pumps based on that data.
I'm dubious about the idea of replacing the quick track with wonderboard. What is the reason for doing that?
As far as who to hire, you could hire a system designer remotely, and then look for someone with commercial HVAC/refrigeration experience to do the install.
Yeah, taking 250,000 btus to 50,000 btus, and one heat pump instead of five, pays for a lot of air sealing and insulation. Even a few blower door tests.
If you could get down to 50,000 BTU you'd be at about 12 BTU/square foot, which could be achieved with a floor temperature around 80F, which you might be able to do with your existing emitters and a water temperature around 105.
Which saves even more money that can be used for upping the insulation.
Thanks, Charlie. We're definitely going to improve the envelope - we've got an infrared analysis and will get after wall and ceiling insulation, new windows.
We're concerned about Quik Trak because of it's diameter of 5/16 vs 1/2 in for Warmboard. We also watched this video, https://www.youtube.com/watch?v=kqlFML2QnQ0 which admittedly is produced by WarmBoard, but our thinking is that WarmBoard will allow for more even warming at lower circulating water temperatures.
With our current system the boilers are set at 145 degrees and like I said, it's not keeping up with demand. That may be mostly envelope, but I'm not sure we'll have the timing ability/luxury to address the envelope and then see how the existing boilers work.
Thanks as well for the suggestions on system design remotely - will any firm do, or do you have some specific recommendations?
The problem is with your emitters (hydronic speak) not with your water temp. Warmboard will do the panel layout. My carpenters installed the panels and plumber connected the pen to the manifold.
Every hydronic emitter will produce the same heat output at the same floor surface temperature. Warmboard may give a higher floor temperature at the same water temperature, but unless your current system is really whacked it's not going to be that significant. Definitely take the Warmboard marketing with a grain of salt.
What is your current floor temperature? You should get an infrared thermometer and measure it. Measure how consistent it is too. With 145F water I would expect it to be pretty high.
You're on the right track with infrared scanning. I would also recommend pressure door testing because air infiltration is a major source of heat loss.
With those numbers you can put them into a Manual J calculation and have a reasonably accurate model. For radiant floor you want to have room-by-room calculations. What you're experiencing shows the big drawback with radiant floor heat, which is that if your estimates are way off it's really hard to add more capacity after the system is installed.
At 250,000 BTU and 4400 square feet you're looking at almost 60 BTU per square foot. That is really high for radiant floors. The output of a radiant floor is directly proportional to the floor temperature. To get 60 BTU/sf you're going to have to have a floor temperature over 100F, which will be uncomfortable to walk on. This is true regardless of the type of radiant system.
So I would look at replacing your underfloor system only after every other alternative has been exhausted, and you've done the analysis and the difference in output between the Warmboard and what you have now is going to close the gap.
Oh, 5/16 is pretty tiny. Yeah, I can see wanting the change that.
I don't really know who to recommend you get to do the design work. John Siegenthaler is a leading expert who does training courses and writes books, and has recently been writing about air-to-water heat pumps. I don't know whether it would be possible to get his time, but you might be able to get him to tell you who to ask.
I've used the Quick Track with the 5/16" PEX tubing without issues. Makes great heat, I even had to turn it down a bit as it was causing the floor to get near uncomfortable temperatures.
Besides the questionable envelope issues, you might have a problem with the plumbing install. Most likely the system is not getting enough flow rate. If the system was not sized well and there are not enough parallel runs, this can easily happen. Getting this fixed is much simper than ripping out the setup + re-flooring the place.
Yeah, with water at 145F I'd think the floor would be too hot to walk on unless something is obstructing the flow.
We're also designing in Colorado, and had a very positive initial consultation with Hans Preiss, Brightsense LLC, based in Boulder. He designs and models high-efficiency HVAC systems for houses and will be familiar with at least most of your issues, including those related to high elevations (which can be significant at 8,000 + ft). He was recommended to us by someone with a lot of experience, but I can't speak (yet) about our outcomes.
Thank you all so much for the insights. I'll reach out to Hans at Brightsense, and John Siegenthaler regarding expertise. You were all correct on the BTUs - using Manual J and breaking it down by room, my design load is 95,200 BTUs. Even that seems high, but we have lots of large windows in the design to take advantage of the views.
I do have a couple of questions from the comments. All of the BTU calculations, including Manual J, include a ceiling height as part of the calculation. I don't understand the comment that ceiling height doesn't matter for BTU loads - doesn't it stand to reason that a larger volume of space with a higher ceiling will require a higher BTU load?
Second question is regarding Quik Trak vs Warmboard. It doesn't make sense to me that 5/16" tubing spaced at 7" would perform the same as 1/2" spaced at 12". For example, a 10ftx10ft space would require 170 ft of Quik Trak tubing vs 100 ft of Warmboard tubing. That's 70% longer distance, with smaller caliber tubing. Seems like to get the equivalent BTUs/square foot there would have to be a whole lot higher pressure at the pump with Quik Trak to achieve flows to get to the same BTUs/sf as WarmBoard. Or as in the video, if the same flow rate, it will simply take a lot more time for Quik Trak to get to desired temp. If the whole idea of hydronic systems is that the temperature in the room/floor is relatively constant, maybe Quik Trak vs Warmboard doesn't make a difference. But if there's any challenge to the load via external temperature, seems like there would be a preference for WarmBoard.
1. Don't think of it as heating space, think of it as replacing the heat that is lost. Manual J cares about wall area. A higher ceiling has more wall area for the same floor area. The manual J estimates heat loss for a given interior temperature and a given exterior temperature. The basic technique is to take the temperature difference, multiply by the area of exterior walls, and divide by the R-value. There's more to than that obviously but that's the gist of it. There's no reason that a taller room inherently takes more energy to heat.
2. The BTU transferred by fluid is determined by the temperature drop multiplied by the flow rate. So yes, to get the same BTUs with the same temperature drop you need the same flow rate, and with smaller pipe it's going to require higher pressure to achieve that. But it's a lot cheaper to replace the pump than to replace all the floors. That Warmboard is like $8 a square foot.
To get 92,000 BTU/hr, you need 1533 BTU/min. If you have a 10F temperature drop that's 153.3 pounds per minute, or 18.5 gallons per minute. That's a lot of flow. If the full 4400 square feet is used for coils that would be about 7500 feet of tubing. If loops were kept to 300 feet or less that should be at least 25 loops, so each loop is only moving about 3/4 of a gallon per minute. Even with 5/16 tubing that's a manageable amount.
If your system doesn't look like what I just described, then you have a problem. Specifically, if you don't have nearly 4400 square feet of heated floor, you don't have enough. People will argue about where the upper limit is, but getting more than 20 btu/sf out of a heated floor is difficult. And if it's not divided up into a whole lot of little loops -- like 25 of them -- you have a problem that your loops are too long. If the loops are too long the pump may not be able to move enough fluid to meet your heating needs.
If you just don't have enough tubing in the floor, the only way to fix that is to put more tubing in. But if the problem is the loops are too long or the pump is too weak, often you can fix that with a more powerful pump or replumbing the loops. It's important to figure out what the problem is before replacing equipment because otherwise you could just end up repeating the mistake.
And not to sound like a broken record, but if you can reduce the heat load all of these problems get easier. If you can cut your heat load in half you need half as much tubing, half as much fluid, half as much pump. You can run a lower floor temperature and use a smaller boiler. Hydronic systems really work best in tight houses.
The way the height factors in the area of the wall, not the volume of the space.
If you are replacing windows, that's a huge opportunity to get high-performance windows that cut your heat load and, more importantly, improve comfort.
The only way that warmboard and low water temps was possible on our project, was to spray 2 inches of closed cell foam on the rim board; insulate under the warmboard (R22); 2 inches of CCF on a vented attic; and 16 inches of fiberglass; and replacing a leaky window.
So yes you need to perform a heat loss and pay attention to BTUh per square foot. I would use 15 BTUh per sq ft as a rough guideline for design day.
If you are going to use carpet pay attention to the pad and the materials as that will act as an insulator.
In terms of zoning, we have separate thermostat for the bathrooms, which are kept at 70F. The bedrooms are at 65f. In -13f, our bathroom floors are warm. Life is to short to have cold feet !