Advice on Heating Approaches
I’m building a two-car garage with living space above in climate zone 6 (New Hampshire). I am insulating the exterior walls, including the garage, to about R-30 (2 inch polyiso exterior insulation and mineral wool in the wall cavities). Air sealing sills and rim joists. Well-taped Zip System sheathing. Matthews Brothers, Sanford Hills windows throughout (double glazed).
My original plan was to install a ductless mini-split for the upstairs living area and a hybrid hot water heat pump for domestic water service. But I have a golden opportunity to install radiant floor heat at a substantial discount and have been encouraged by many to take that opportunity given the advantages of radiant floor heat.
My quandary is that I do not want gas or any other fossil fuel as a heating source and I also want to minimize the number of heating sources and the amount of HVAC infrastructure to install given my limited space and budget. I also don’t have a good spot for a propane tank without ruining the landscape around the house as the area around the house is all open.
The latest plan I’ve come up with is to follow through with the plan on the mini-split and hybrid hot water heat pump, but install the PEX and manifold for radiant floor heat, but not connect it to anything. Perhaps technology will change before long, offering new options for the radiant floor heating source.
That feels like a good plan, but it also creates a risk that with the hybrid hot water heat pump in the garage that I won’t be able to keep the garage area above 50 degrees in the dead of winter. Hard to know what to expect for ambient air temp for an R-30 wall in the garage with cold garage doors. I could install an electric heater for those times when it gets really cold.
Any suggestions and advice is appreciated.
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Replies
You can switch heat pump water heaters to electric resistance heating only on the control panel. If needed you can do this in the winter, but take advantages of the hot garage in the summer.
Good point Kyle. I've been so focused on the heat pump part of the equation that I forgot that the electric resistance option is there for when it gets too cold to function as a heat pump. And, it would probably use about the same watts as an electric space heater to keep the ambient air temp warmer.
I would use an oil fired water heater for both DHW and heat. You could plumb an air handler directly to the heater or a radiant system must be separated by a heat exchanger. I note that there are frequent questions about how to manage the hvac for living space over a garage, cold floors, etc.
I don’t think you will find a cheaper way to heat your garage without combustion appliances than installing another mini split in the garage. Are you concerned with dust or debris clogging the filters? You could look for a cheap DIY model, just make sure the minimum heat set point is acceptable.
I did this exact same garage idea with an apartment on the second floor. Apartment was one of the best ideas we ever had. Kids, relatives, in-laws...handy to have.
We put in an Instant Hot Water Boiler/Heater which runs on propane that provides hydro air for the apartment and hot water as well. We also put in a commercial ceiling hanging hydro heater for the garage that runs on that as well. I keep the garage at 50 and the apartment at 50 too, unless I'm using it at which time I turn it up. This unit is extremely efficient and costs me maybe $150 a year to run. The building is spray foamed.
For A/C we have a separate Carrier unit that feeds the shared duct work. We only turn that on when we're using the apartment.
Maybe a minisplit is cheaper to run, but I can't imagine it being any cheaper than it already is.
I think you're on the right path: a minisplit gets you AC and heating while the hybrid heat pump water heater provides efficient water heating when the temperature is warm enough outside, and so-so heating when it's cold out, with the option of full resistance if needed. There's not an easier option available right now, but the in-floor tubing (you could skip the manifold for now if you wanted) lets you go that route later.
If you said how often you plan on heating the garage, I missed it.
To me how often you will heat the garage seems like the most important part of this question.
If the plan is to heat the garage every time your Toyota breaks down more or less never or if you plan is to restore a car working 10 hours a day 6 days a week in the garage, I would give you different answers.
I say a 33000 BTU electric unit will do the job quickly for a lowest cost to buy and install but you will go broke if you run it every day.
I you’re going to be laying on the floor for hours at a time then get the hydronic system but understand this tells everyone who looks at the plans that your budget is really unlimited regardless of the words coming out of your mouth.
I like my heat pump but is set the temp and forget it for months at a time. I think a heat pump is a bad idea if the plan it to keep the heat off or low and expect the heat pump to make it toasty with an hour’s notice.
I have to ask where is the staircase inside or outside. Seems to me every inside stair case ends up splitting the garage in 2 while permanently occupying the most useful real-estate in the garage
https://www.northerntool.com/shop/tools/product_200921653_200921653
Walta
At current electric rates in Maine (21¢ per kwh) and propane cost($3.50 per gallon) a heat pump with a COP of 3 will cost substantially less than half as much per million btus, to operate as anything burning propane. I suspect NH costs are similar.
I'd definitely insulate the garage floor with four inches of reclaimed foam.
Yeah. I was looking at efficiency Maines fuels costs comparison site. They estimate A heat pump would have cost $3200 less than propane this winter in Maine.
https://www.efficiencymaine.com/at-home/heating-cost-comparison/
The tech is already here for water-based heat pump heating. One example: https://www.daikin.com/products/ac/lineup/heat_pump
If you plan on running radiant, I'd suggest thinking about an air-to-water heat pump. You could run radiant in the garage and an air handler in the apartment off a single piece of equipment. Spacepak and Arctic are two brands to look at.
The benefit of going all electric is that you could plan to offset your energy use in the future with a solar array on the garage roof.
I remember this project from an earlier thread:
https://www.greenbuildingadvisor.com/question/looking-for-advice-on-wall-assembly
Have you figure out the boundary between conditioned and unconditioned space? I remember that was an issue.
From your earlier description I honestly don't see any benefit to radiant heat. It's a lot more expensive to install and won't give any efficiency benefits. The usual justification is comfort but I don't see any comfort benefits in this situation.