Cottage HVAC design approach
We are building a 1,300 sq.ft. cottage and would like expert advice on the best strategy to heat and cool It is a one story home with a low crawl space (max. 3ft head clearance) with 12 inch TJI (single slope) roof joists in Zone 6 and bedrock just below the surface. A few designers have recommended a dual system having an in-floor radiant system with a forced air system as a back-up should the boiler go. There is no propane currently on site and have concerns due to long driveway and access issues to fill in winter with no one around The place will no be used in the winter for the most part so thinking that maybe the water lines could be drained and possibly the heat tuned off? We are not fans of forced air heating and are thinking about mini-splits along with a wood stove, and some sort of in-floor heat in the the 2 bathrooms. That being said I don’t like the look of the mini splits with all the pipes running on exterior and a conditioned crawlspace provides plenty of opportunity (thinking combining HRV with heating/cooling?)
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Replies
What climate zone are you in?
Heat pump ductless split system(s) and a wood stove sounds like a good option if you have reasonable electricity and a reliable inexpensive wood source. Adding in a drain valve and designing the plumbing so that you can drain the whole system sounds like a simple and inexpensive feature to add at this juncture so that would be a good idea regardless if you ever use it or not. I would avoid in floor hydronic, especially for such small systems. Why not go with electric resistance floor heaters and just turn them on when you will use them kind of like space heaters?
Have you checked with your contractor to see which is less expensive to construct, a conditioned crawl space or insulating the flooring? If the conditioned crawl space is a wash or even less construction wise, I would go that route. A conditioned crawl space requires minimal airflow.
Often your biggest payback is by properly insulating and air sealing the entire building.
Thanks for taking the time to reply Krackadile
We are in Zone 6 and the hydro rates are 15.8Cents On-Peak and 7.6Cents Off-Peak
Wood is expensive; I'm leaning to a propane stove but the wife wants wood burning
Electric floor heaters would detract from the space aesthetically (wondering if there is a way to have flush with the floor.......)
oh, my apologies, when I said electric floor heaters I may have misspoken. Google "electric radiant floor heating systems". That was what I meant. They are more or less invisible similar to a hydronic radiant heating system.
It sounds like the stove whether it be propane or wood might be more for backup heating? If that is the case, I would size the ductless split system(s) for the entire heat load and just put in an electric or propane stove that has the appearance of a flame for ambiance. I guess if it is for a backup heat source which is wise, then either wood or propane would work.
BTW - Great design (except the lack of closet space) ...
I'm considering something similar
Thanks Begreener! It took many, many reiterations before we got there
The draftsman put in those "Rod and Hangers" in 2 bedroom but will actually be a built-in
Going to try and do this without a GC.....
amateurguy,
I may be misreading the drawings, but it looks like the only place where you have adequate depth of cover to keep to keep footings from freezing is at the bermed area near the main entry. This may play into your proposed strategy of leaving the building unheated during the winter, as that will increase the depth below grade or insulation you need to protect them.
The bedrock is just below the surface so my designer recommended the attached detail
Not sure that solves the freezing problem? if not and we do not heat place in the winter, wondering what the implications are for humidity inside and it's impact on the interior/
amateurguy,
It will be fine on rock. I was just going by the section.
Lots of summer houses don't get heated all year. Some of the contents suffer a little - magazine pages and bedding smell of mildew a bit - but the houses themselves seem okay.
A dehumidifier would solve the high relative humidity on surfaces, yes?
@amateurguy - I would consider rigid foam *under* the floor system if you don't want to condition the crawlspace.
Thanks JM
- Would the dehumidifier be for the crawlspace? (even if it is vented)
- Wondering about lifecycle cost of running that vs having a conditioned crawlspace (if that negates needing to dehumidify)?
- by "rigid" foam do you mean spray foam?
Jeffrey,
Unfortunately dehumidifiers get less efficient as the temperatures drop and stop working altogether at around 40F.
If you move the insulation from the crawlspace walls to the floor you will need a lot more than you can fit on the underside of the joists. You are looking at permeable insulation in the cavities too. Moving the enclosure boundary to the floor also means the services that transit the space will need to be kept warm to avoid freezing.
Malcolm --
But below about 50 mold won't grow anyway.
DC,
Which still begs the question as to why you would dehumidify an unheated house in the winter?
Malcolm --
I'm not sure how much condensation happens in an unheated building in cold weather because there's not much humidity in the air to begin with. What I see as a risk is when it warms up and outside humid air starts leaking in and the inside of the building is still cold with lots of surfaces for condensation to form on.
Most dehumidifiers will shut off around 50F, because the coil can't go below freezing or it will ice up, and it has to be about 20F below room temperature to work. And I think that's fine. Leave the dehumidifier running with the humidistat around 50%, don't worry if it's not running when it's cold inside the building.
DC,
That makes sense. It's probably during the shoulder seasons where any risk is.
We had a cottage in Ontario for 27 years. It is VERY common to drain water systems for the winter, along with antifreeze in the toilets and drain traps etc. Plumbers usually set these systems up to drain easily as a matter of course. To be sure, it's not a bad idea for them to plumb in a Schrader valve so you can blow out the pipes with compressed air before you leave. My wife and I had opening and closing down to a science.
We had zero heat as it was a summer only cottage (like you), but we did have a wood stove for the fall. In fact, almost every cottager I know had a wood stove, although one or two had propane. Trees were everywhere dropping wood, so it was easy to gather. Still, if you're not using it in the winter, a cord of wood will last you a long time. I love wood stoves.
Our gravel driveway was 1/3 of a mile long and the propane trucks had no problem delivering gas for the cook stove and hot water heater. They're used to cottage country.
If I had to do it again I'd add a mini-split, but still have the wood stove. Cottage country is lot warmer than it used to be and you'll need air conditioning to stay comfortable.
I miss the cottage, but we have something better now. Good luck.
I use a cold climate wall mount at the cottage. Great for cooling in the summer (needed even in northern Ontario now) and keeps the place heated down to -25. Wood stove for extra comfort. The reason for the wall mount was simple install and low cost.
If you are going with a ceiling unit, I would make a spot for it bellow the ceiling and box it in. Besides keeping it inside the house air barrier, this prevents attic dust from getting into the unit which will quickly gum it up. These are not fun to clean.
I'm with krackadile. Condition the crawl and get a slim ducted unit and put it there. This lets you get air to all the rooms plus you now have a real air filter.
Drainage is one thing to watch when building on bedrock. No matter how well you seal it you'll get water under the foundation and into the crawl space floor. Make sure there is proper drainage there to collect this and a solid vapor barrier on the floor to keep it from getting into the house.
Really solid advice Akos - thanks you!
Would really appreciate some clarification:
- ceiling cassettes would be in the 12" cavity of the TJI surrounded by insulation batts (would this negate the attic dust?)
- Do conditioned crawlspaces still require venting?
- "crawlspace conditioned with slim ducted unit" comment: what is this exactly, and is it in addition to the ceiling cassettes? Would you be referring to the fresh air (HRV?)
- If so could maybe be connected to the mini split condensor for using for cool air?
THANKS!
" ceiling cassettes would be in the 12" cavity of the TJI surrounded by insulation batts "
I'm assuming that means vented roof. If that is the case, the space where the insulation is would be outside your envelope since it is above your drywall (which is typically the ceiling air barrier).
This means any leaks around the unit will be pulling in outside air which is bad for efficiency and dust.
The typical 4 way cassette is also pretty big and regular rafter spacing can't accommodate it plus room for service. Everything is better if it is mounted bellow the ceiling into its own dedicated enclosure with nothing poking through the ceiling air barrier.
"- Do conditioned crawlspaces still require venting?"
Best way to think of a conditioned crawl is that it needs to be part of the house. That means about the same conditions as the living space above it. More info about it here:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-115-crawlspaces-either-or-out
Simplest in your case is a set of transfer ducts that allow air to circulate between the crawl and the house. If you have an air handler, you can also connect a supply to the space.
"slim ducted unit" comment: what is this exactly,"
This a different format heat pump that would replace your ceiling cassette. Since it is a ducted unit you can run supply registers to all the rooms which means you have cooling and heat in all the rooms. Something like this (they come in many sizes):
https://s3.amazonaws.com/s3.supplyhouse.com/product_files/LG-KNSLB241A-Submittal-Sheet.pdf
You can mount it in your utility room in a downflow configuration and run the ducting to each room through the crawl. Simple high return on the wall to the living space. Cost is comparable to a ceiling cassette but install is more due to the ducting.
Thanks Akos - VERY helpful!
So, if I'm understanding - you are saying that this AHU unit is an alternative to mini-splits (or a forced air furnace)? Has been mention in replies of a "cold climate wall mount" and a "slim ducted unit" - is the AHU the same thing?
Given the open concept of the floor plan would the ducts (wall transfer grille / high return, etc) work in a typical 2X4 stud wall - or do I need to have thicker joists the interior partition walls?
Been lots of chat about humidity as we may not be there in winters. Do you run the heat (set very low) or turn off?
There are all essentially similar systems but with a different indoor unit.
You can have:
-wall mount
-ceiling mount
-floor mount
-slim ducted unit
-multi position air handler
These all connect up to the same outdoor unit. For example you can look at the submitalls on the bottom for each configuration.
https://mylinkdrive.com/USA/M_Series/R410A_Systems-3/Outdoor_Equipment/R410A_Outdoor/SUZ_KA18NAHZTH?product&categoryName=R410A_Outdoor
"cold climate" means an outdoor unit with vapor injections compressor. In case of Mitsubishi, they call it hyper heat, but all manufacturers have a comparable unit. A cold climate unit will not loose output in the cold and generally deliver its nameplate rating down to 5F.
The supply needs to be ducted, no way around that if you want to be able to close doors at night.
The returns could be transfer ducts or door undercuts. Transfer duct for a return in a 2x4 wall work without issues.
The humidity issue is in the shoulder season and summer time. Have a bunky at the cottage and have to heat it for about 1/2 day to get the humidity out of the bedding.
The main cottage is full year and we keep it minimally heated. The area with plumbing gets curtained off and two space heaters keep it to 5C. This avoids having to drain stuff plus where we are, plumbing antifreeze can still freeze and split a toilet.
If designing from scratch for winter use, putting all the plumbing in a core area with doors and a heater will simplify your life. This would me redoing your layout a bit as you want all the bathrooms, utility room and kitchen close together. Not sure if worth it at this stage.
"Drainage is one thing to watch when building on bedrock. No matter how well you seal it you'll get water under the foundation and into the crawl space floor. Make sure there is proper drainage there to collect this and a solid vapor barrier on the floor to keep it from getting into the house."
BINGO!
This was a. major problem for all of us who built on the Canadian Shield. Water leaks in through the ledge and you have to deal with it. I'd be VERY wary of doing that again without a thorough game plan for excellent drainage, perfect vapor barrier and insulation.
Moisture in the crawlspaces was a problem in almost every cottage I visited, and it was worse in the shoulder seasons when nobody was around to deal with it. Mold and rotted wood are difficult to deal with once they set in. Tread very carefully with this issue.