HVAC options for addition above garage
We are adding a master suite above the garage in NJ. Our 2 story house has forced hot air which for a variety of reasons cannot be expanded to handle the additional 700 sq ft. An additional furnace cannot be placed in the basement. Our contractor suggested putting a 80% in the attic or in the attached garage. I am not happy with either given everything I have read. Attic- mold issue, ice dams, etc. garage – fumes, etc if the system is not sealed properly etc. We have read about the mini split systems to handle this space and wondering given where we are located if this system would handle heating and cooling without another source. Is it the better solution given the only other possible places for the furnace are the worst 2 locations? Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You can put any system in that's sized right. Mini splits are one option. PTHP's are a cheaper, easier and are all in one solution that doesn't require a back up heat if in a colder zone or a costly outside air introduction system. Always try to k.i.s.s. the problems in construction if you can.
Since its a master suite you could do a EPA wood insert fireplace/stove or even a gas fireplace for heat. Doesn't cover the a/c aspect, but better resale.
Christine,
Install a ductless minisplit.
And if you are worried about ice dams, make sure that you have an airtight ceiling without any recessed can lights, and that you have a thick, well-installed layer of insulation above the ceiling.
Thanks for the advice. I guess I found this site a little late on the recessed cans. I just read that Mitsubishi has a new H2i unit that will be available in January. It says it works down to minus 13 so the need for a secondary source given our climate probably isn't necessary. I was actually thinking of the ducted indoor unit so we could have supplies in all of the rooms (a bathroom, master bedroom and an adjoining office). Do you think we lose much of the efficiency of the unit? Now to find a knowledgable HVAC installer who agrees the mini split is the way to go.
Christine,
If the thermal envelope of your building is close to airtight and is well insulated, you will probably be comfortable with a single ductless minisplit -- as long as you don't have large unshaded west-facing windows that might cause overheating in the summer.
If you are worried about even distribution, you can choose a ducted minisplit -- although ducted minisplits aren't as efficient as ductless miniplits.
I'm sorry to hear that your ceiling has recessed can lights. If it's not too late, you might want to rip them out and put them in a dumpster. For more information, see Recessed Can Lights.
Before you decide what to put in there you need to run a heat load calculation for the space at your 99th percentile temperature bin (in the mid-teens F for most of NJ.)
A code-min 700 square feet is likely to come under 10,000 BTU/hr, and a high-R 700' with minimal window area could be under 7000 BTU/hr. DO run the numbers, and be aggressive, not conservative, since most heat load calculations will otherwise overestimate reality by a double-digit percentage.
A 3/4 ton Mitsubishi -FH09NA will deliver 12,200 BTU/hr all the way down to +5F. That's fine if your aggressively calculated +15F heat load comes out at 10,000 BTU/hr, but if it's under 8000BTU/hr it would be cycling (at lower efficiency) rather than modulating at the winter average heating load. Since it's a "suite", with multiple rooms, a mini-ducted SEZ/SUZ-KAxxNA might be a better choice, since you probably have multiple rooms with sub-5000 BTU/hr heat loads. http://usa.mylinkdrive.com/uploads/documents/4297/document/17_8_SEZ_Ducted_Heat_Pump_Systems.pdf (The extended temperature capacity ratings are on P.25.) It's likely that your heat load @ +15F would call for either the 3/4 ton or 1-ton version.
A PTHP can be a cheaper, if noisier & even less efficient point-source solution, but still way more efficient than a resistance electricity only solution. Most will cut over to restance-heating only at temps below ~25F, but at your average winter temp it would be running in heat pump mode.
There are no wood stove solutions that are sufficiently low output to be useful at your likely heat load, since it would have to either be dampered down to a high pollution firing rate or intermittently fired to keep it from cooking you out of the place. (Most would have to be run at better than 1/3 of the nameplate BTU/hr rating to keep the secondary burners going, which is necessary to keep the particulate emissions anywhere near the tested-rated levels.)