Air to water heat pump – current state of the technology?
My personal home in Squamish, BC built in 2008 is heated by hydronic radiant heated by gas boiler, which also heats our DHW. Back in 2008 my position was that houses in our climate didnt need AC, but recent years have me thinking otherwise.
Obvious option is to add a minisplit heat pump, but before going down that road wanted to flush out the idea or adding a air to water heat pump and use it to heat and cool the home. I searched the forum for radiant cooling but see mostly older posts before the technology came of age.
I would like any advice on current tech and if the idea of cooling using our in-floor radiant system is advisable. I somewhat understand issues around dew point and condensation. There is a Canadian company, Arctic Heat Pumps, that I have been aware of for a few years now and I am intrigued. I see there are control units to regulate temps above the dew point to address the condensation issue.
Anyone done this sort of conversion and have any advice? Would a system like this replace my gas boiler entirely or would it be good to have both?
Thanks in advance.
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The problem with underfloor cooling is there isn't any methodology for engineering it.
Traditional air conditioning provides dehumidification along with cooling, and the more humid the air the more dehumidification it provides. It's kind of a miracle that way. So the engineering is basically to make sure that you have enough cooling for the hottest days and assume the dehumidification will follow. In super humid climates that's not actually enough and you have to provide supplemental dehumidification.
Under-floor cooling provides no dehumidification. So unless you live in a climate where no dehumidification is necessary you need to provide some way of removing humidity. To be sure it's adequate you basically have to provide a convention air conditioning system.
One thing to look at is hydronic fan coil units that provide cooling and dehumidification. They're basically like minisplit heads and run off of cold water provided by the heat pump. The downside is they are a niche product so they're expensive.
A growing trend in cooler climate zones is to have a dedicated, whole-house dehumidifier, because air conditioning units are now so efficient at cooling that they don't always provide adequate dehumidification. Many of my clients are on the Maine coast where temperatures can be comfortable in summer but the indoor humidity is far from comfortable--to the point that they complain about "squishy" carpets and moldy furniture--so I'm using dehumidifiers for those. The same approach can be used for in-floor cooling, but the system has to be carefully engineered and even then it's too risky for me.
Houses are a lot tighter and better insulated than they were even 20 years ago. As the sensible load drops, managing the latent load becomes more important. In post #1 I wrote, "So the engineering is basically to make sure that you have enough cooling for the hottest days and assume the dehumidification will follow." If the latent load is high relative to the sensible load that assumption breaks down.
It's analogous in a way to the way that as houses have become tighter, under-floor heating has become less appealing because the required floor temperature gets so low it's not really noticeable.
Dehumidifiers are efficient space heaters, they have COP's of perhaps 4 or more. Running a dehumidifier so you can then run a cooler which doesn't provide latent cooling is, from an energy efficiency perspective, like opening your windows in the winter so you can run your under-floor heat warmer.
I'll add that there's not much out there on the engineering of humidity control. Like, if you were to supplement your HVAC with a dehumidifier, how big should it be?
You're the details guy. I just know that in many cases, air conditioners don't dehumidify enough, so people set them to very low temperatures to compensate. "Assume the dehumidification will follow" is simply bad advice.
I have very smart mechanical engineer and builder friends in the Hudson Valley where they have been doing this for a long time, for the same reason.
Also, all of my comments require moderator approval for a reason we can't figure out. There is often a delay of hours before my comments are posted. I wasn't responding to you, I was responding to the OP.
In addition to what DC brought up, keep in mind that if you get rid of your natural gas boiler and swap it out for a heat pump, your current radiant floor system may not be designed to work with the lower water temperature the heat pump will produce relative to your old boiler. Heat pumps tend to produce lower temperature water on average, but as long as you have enough surface area with your radiant floor system this can still work. Be sure to read the spec's and check out the efficiency of the heat pump unit at the water temperature your system requires.
Fan coil units can work, but you are basically talking about two independent systems at that point. You might as well leave your working boiler and just install minisplits.
The fan coils would run off of the same heat pump as the floors.
I guess I meant two systems as in in-floor radiant for heating and FCU's for cooling. Yes the outdoor heat pump unit would be the same, but it's still not exactly one unified system for both heating and cooling. Let's call it one and a half systems!
Great - thanks for information. Generally our climate is not very humid. Will read through info later and digest.
I think you might be in one of the few areas where you don't need latent cooling. In that case you can use your floor heat for cooling with an AWHP unit. The condensation control is not a big issue in that case, set the target temp on the unit to be a bit above outdoor dewpoint and you won't get condensation.
I'm in Toronto, summers much more humid but shoulder season is not bad and have tried this in the fall and works reasonably well. The one issue is cooled floors are not very pleasant to walk on, especially anything tiled. It definitely is better than no cooling though.
Your winter design temp is also high enough to any of the new EVI AWHP can also easily heat your place. Depending on the your local costs, it might be cheaper than your fuel burner. A rough estimate of your actual heat load is to take how much fuel you used during a heating season in Therms and multiply by 42. A more detailed calculation is here:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
If your loads are in the 35k to 50k range, an AWHP will easily heat your place. In your mild weather, the R32 units will also have no issues with running the indirect, just keep in mind that recovery when cold will be slower than with a fuel burner.
If the heat pump pencils out for heating, you can install it and see how it works for cooling, there's nothing lost if it doesn't work out. That's kind of what I did in my own house, when it didn't work out I just kept adding fan coil units until I had enough cooling.
In my climate -- Washington, DC -- I found I never had excess sensible cooling, just running fan coil units the house rarely gets below 60% RH in the summer. It's very common here for people to run dehumidifiers in addition to central air in the summer.
If you live in a place where big box stores have a big selection of dehumidifiers then under-floor cooling isn't going to work for you.