duct versus in floor heating with ground source heat pump
My subcontract0r is installing a vertical loop ground source heat pump for heating and hot water in a new home here in northern Wisconsin, zone 6B-7A. The house will be new and very tight and well insulated. My question is should we heat then with in floor hydronic heating and minisplits for AC or a duct system for both? We have that in our current home and it seems fine. The house is a walkout with the main floor 1500 Sq ft and south facing. The second question is air purification and ventilation. He wants to vent a the ductal system to the outside with a 3 MERV filter all in one. I however think it needs a ERV unit separate. Which is best, keeping in mind we have a grandchild with a tracheostomy and several sons with bad allergies. I don’t think there are many installers in our area versed in this type of project. Thank you.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Minisplits seem to completely cancel out the advantages of ground source.
In a house that is tight and well insulated the heating load is going to be very low. Have you done a heat loss calculation? I would expect it would come in around 10 BTU/hr per square foot. That would require a floor temperature 5F above the room temperature, or 75F if your room temperature is 70F, which is barely going to be perceptible. And that's at design temperature, most of the winter it will only be a couple of degrees above room temperature. You won't get the toasty toes feeling. Heated floors worked best in old leaky houses with high heating loads.
In a tight house ventilation is important. You want a separate ERV, ideally controlled by air quality sensors.
Have you already committed to the ground source? Because it sounds like way overkill for a 1500 square foot house. A ducted minisplit like a Mitsubishi M-series could handle that. The energy savings will never pay back the installation cost.
From a dollars and cents point of view both ground source and floor heating are expensive luxuries unlikely to save enough fuel to repay their costs, add almost nothing to resale value, have very small if any comfort advantages.
Today almost everyone needs, want and expects air conditioning. Hanging a few mini splits on your walls will get you AC but it is far from the best comfort wise. If you put one in every room you end up with oversized equipment huge install costs and ugly bumps on your walls.
I say spend the money on good ductwork inside the conditioned space and if you want a quiet system double the size of the ducts and install some soft baffles.
Walta
Thank you so much for the comments . This helps. I am not married to a ground source as construction has not started. It does get cold up here with, excepting last winter, temperatures that can go 25 below zero in January. A 7.5 Kw solar array with battery back up is being planned as well. The house is a new construction so am not even sure how to size a ground source heat pump. I am as much concerned about CO2 emissions and longevity as cost. For that reason I am leaning to a ground heat pump with ducts. Further thoughts.
The most important step in construction planning is the heat loss calculations. You should start doing that as soon as you have a rough sketch and continue as an iterative process as the design evolves. It's a good way of identifying architectural features that turn out to be energy disasters, before they happen.
A starting point is to look up your county here: https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf
I suspect you'll find that for a house your size you won't be able to find ground source equipment that is anywhere close to appropriately sized.
You can have both ducts and floor heat without hydronics with a bit of design. I've inadvertently did this while trying to get heat into an enclosed porch. There was only 1.5" of height for ducts so I got some 1.5"x14" ducting made. Since the subfloor is almost directly touching the ducting, in the winter time you get a really nice heated floor area.
The typical way to install an ERV is to tie it into the HVAC return. This works reasonably well provided you have a modulating heat pump/air handler (these tend to run the fan 24/7 at some speed) and use an autobalance ERV. The better option is to only tie the ERV supply into the air handler return and run the ERV stale air pickups to the bathrooms. This extra bit of ducting cost can be offset by the bath fans and exterior wall caps you save.
Check how much filters will be for the ERV you choose, most cases it is cheaper in the long run to have an insulated in-line filter box on the outside air supply to the ERV. This would let you use off the shelf low cost high MERV filters.
P.S. I have hydornics + wallmounts + ERV with dedicated ducting at home. Since there is no large air handler with a good filter in the house, I get a fair bit of dust in the house.
From an economics basis, you’d install ducted because it can do both, so the in-floor heating is a luxury.
But ground source can do both so it’s a comfort decision. No boiler necessary here, but could be a add on.
Air source + backup of some variety is by far the cheapest and possibly most efficient option too. Ground source can’t always beat air source efficiency wise.
Again thank you for all your comments. This house is to be very tightly sealed and very insulated so it should be able to sized for a heat pump. It is also very confusing as to how to duct an ERV even on this advise blog. It does seem logical to keep separate as it will require the heating fans to be on continuous and other problems with combining them. I have a grandchild with a tracheostomy and severe allergies in other family members so air quality is important.
https://www.greenbuildingadvisor.com/article/integrating-an-erv-or-hrv-into-a-forced-air-system
https://www.greenbuildingadvisor.com/article/ducting-hrvs-and-ervs
Note the south facing part is a red herring. The passive solar simply just does not work in the real world not for lack of effort or wasted restores. When I computer modeled my house and spun it every direction It made almost no difference and more glass used more energy.
Note window will likely be the largest line item in your budget chousing smaller window goes a long way toward being on budget.
If you have great views, put in windows so you cand see them just limit all your total windows area to about 15% of the wall area.
When looking at air source heat pumps given your cold temps you will want a compressor with vapor injection. Most brands will have this technology in some models every brand had trademarked a different name for the same technology hyper heat, flash injection ECT. Make a plan that insures all of the ductworks will be inside the conditioned space and not in the attic.
Walta