Mini-split compressor in crawl space?

Icarus

Ok folks, another job, another question.

Changing from electric baseboard heating, to dual indoor unit minisplit, ~15-20k btu heating capacity. Pacific N.W. fairly mild climate, but can get quite cold (~10f) for many days a winter. We have little if any need for A/C, but it might be unused minimally.

This my question. This is a Condo unit with a very large joined crawl space, that has 6-8th of head room in most places (why they didn’t build a basement is beyond me!). This crawl space is minimally vented and the floor above the crawl is insulated. My thinking is...the crawl space is always much warmer than the OAT most days, but most especially when it is cold. It is also cooler in the summer. Both features make the basic efficiency curves are improved in both situations.

So, are there code issues with this? The volume of the crawl space is such that I don’t think I need to worry about it getting “too cold/winter” or “too warm/summer, given that the crawl space is vented.

Thoughts?

Thanks again in advance,

Icarus

pecmsg

A mini in a crawl space in heat mode will quickly lower the temperature cooler then outside.

Any water or drain pipes down there, they'll have to be protected from freezing.

I don't think it will work. Keep it outdoors!

ChrisJ

Request denied.

Keep it outside.

Icarus

Remember, this is a ~1000 sq ft crawl, with an 6-8’ head room, and it is vented. I don’t think it wold/could get to freezing, but I am not sure it would be of any net benefit.

Icarus

ChrisJ

Icarus said:

Remember, this is a ~1000 sq ft crawl, with an 6-8’ head room, and it is vented. I don’t think it wold/could get to freezing, but I am not sure it would be of any net benefit.

Icarus

Do a manual J on the space.
How much will it gain from whatever's around it, and how much does the minisplit need?

Btu's are btu's, it doesn't matter where you buy them.

If you prove you gain more than you need, you're there. I wouldn't include gain through the ceiling tho....

ethicalpaul

I like this idea. You are in effect using some of the temperature differential provided by the space. When/if the mini split equalizes this difference, then you are where you would have been from the beginning if the split was outside.

Also, the protection from wind is a nice benefit I think.

What does your local inspector think?

KC_Jones

Look at the CFM requirements for the unit you are thinking about. I did a really quick lookup on a 12k unit from Mitsubishi and the condenser fan will move 1229 CFM, so with the space you have 1000 sq ft x 8' ceiling it will turn over all the air in that space in 6.5 minutes, and it will be heating or cooling that air the whole time. You are talking about a unit even bigger.

If I was even contemplating doing this I would duct the fan output to the outdoors, but that probably negates much of the perceived benefits you are going for.

mattmia2

ChrisJ said:

Icarus said:

Remember, this is a ~1000 sq ft crawl, with an 6-8’ head room, and it is vented. I don’t think it wold/could get to freezing, but I am not sure it would be of any net benefit.

Icarus

Do a manual J on the space.
How much will it gain from whatever's around it, and how much does the minisplit need?

Btu's are btu's, it doesn't matter where you buy them.

If you prove you gain more than you need, you're there. I wouldn't include gain through the ceiling tho....

^^^ This. If it had enough heat to heat your living space you could just duct the air from the crawl space in to your living space. There isn't enough heat there to collect.

Icarus

^But, but, but...Heat pumps work on Delta T not just T. It is intuitive that the smaller the Delta the higher the efficiency. Since the requirement was (is) a vented crawl, the addition of moving air is probably net/net a good thing.

If you were really cleaver you could mount the condenser against the framed outside wall of the crawl, with the exhaust blowing directly out, while the intake is strictly from the crawl (being significantly cooler in the summer/warmer in the winter). If you could do this...I see no down side.

As for my local inspector...I haven’t spoken to him yet. I have found it always to ask an esoteric question of inspectors or building officials when you have your ducks in a row with some opinions based on people who have either done what I am asking or who have learned opinions.

Icarus

ChrisJ

> @Icarus said:
> ^But, but, but...Heat pumps work on Delta T not just T. It is intuitive that the smaller the Delta the higher the efficiency. Since the requirement was (is) a vented crawl, the addition of moving air is probably net/net a good thing.
>
> If you were really cleaver you could mount the condenser against the framed outside wall of the crawl, with the exhaust blowing directly out, while the intake is strictly from the crawl (being significantly cooler in the summer/warmer in the winter). If you could do this...I see no down side.
>
> As for my local inspector...I haven’t spoken to him yet. I have found it always to ask an esoteric question of inspectors or building officials when you have your ducks in a row with some opinions based on people who have either done what I am asking or who have learned opinions.
>
> Icarus

Youll be bringing outside air in....

There's no free ride.

ratio

I picture something like this:

jumper

If only all the building's heat loss was through the floor <<grin>>

mattmia2

i see lots of downsides. You are decreasing the pressure in the crawlspace so you are increasing infiltration in to it from the conditioned space. you are making the crawl space cooler so you are also increasing all the other losses through conduction and convection and radiation and such. You are trying to operate a fan designed to be in free space in an enclosed area with different static pressures than designed. You are bringing noise in to that space. Your efficiency drops dramatically as the outside temp drops so if you aren't running on outdoor air after a couple minutes when you've exhausted all the air in the space you're doing worse than putting it outside.

EBEBRATT-Ed

Your going to be cooling the floors above while running in the heating mode. Not a great idea

Icarus

What you guys are apparently missing is...in our neck of the woods we built (I never liked the regimen) non sealed, vented crawls, with a vent ratio of ~1-150 IIRC. Sealed crawls (conditioned spaces essentially are still the exception here.

Decreasing the pressure in the crawl is not an issue. In fact, adding a touch of pressure (assuming a reasonably tight floor structure) would be an advantage as the whole purpose of the vented crawl is to keep moisture at minimum levels.

Same as about “bringing outside air in”. We (mostly) don’t care about having “too much air” in the crawl.

All that said, we have had a number of stakeholders meetings with our local building departments about how to “properly” vent crawls during various parts of the years. The ideal is humidsatically controlled with exhaust fans/natural movement and dampers. Not likely to be adopted simply because it is complicated.

Also, assuming the crawl vents at about the same rate that the heat pump moves air, it is unlikely to cool the living space above.

Keep the ideas coming, certainly an interesting conversation.

Icarus

ChrisJ

> @Icarus said:
> What you guys are apparently missing is...in our neck of the woods we built (I never liked the regimen) non sealed, vented crawls, with a vent ratio of ~1-150 IIRC. Sealed crawls (conditioned spaces essentially are still the exception here.
>
> Decreasing the pressure in the crawl is not an issue. In fact, adding a touch of pressure (assuming a reasonably tight floor structure) would be an advantage as the whole purpose of the vented crawl is to keep moisture at minimum levels.
>
> Same as about “bringing outside air in”. We (mostly) don’t care about having “too much air” in the crawl.
>
> All that said, we have had a number of stakeholders meetings with our local building departments about how to “properly” vent crawls during various parts of the years. The ideal is humidsatically controlled with exhaust fans/natural movement and dampers. Not likely to be adopted simply because it is complicated.
>
> Also, assuming the crawl vents at about the same rate that the heat pump moves air, it is unlikely to cool the living space above.
>
> Keep the ideas coming, certainly an interesting conversation.
>
> Icarus

Then I'm missing the point of putting it in there?

What's keeping it warmer in there than outside?

Icarus

What is making it warmer is the earth... consider this for example. Lets assume that the OAT is 20f over night, and 35f during the day. The soil surface temp outside may have a inch or two (at most) of frost, below that it is above freezing, and 2’ down it is probably ~50f year round.

Now, the foundation wall (concrete portion) is at least 2’ tall, with the footing below that, so the soil under the house temp probably closely approximates that of the outside soil 24th below grade, continually.

If the heat pump extracts the heat, and the transfer air (colder) is exhausted from the crawl, the earth itself can keep supplying 50f virtually indefinitely. The crawl space soil never needs the sun to absorb heat, nor is it ever subjected to the brutal cold.

The key to making this work, is to adequately (if not directly) venting the cold (in winter) warm (in winter) from the crawl space. The earth will continue to be ~it’s average temp. I suppose eventually the soil will begin to warm up, but my guess is that over the heating season the temp rise would be tiny.

Icarus

Icarus

What I ended up doing is installing the HP outside, in front of a large foundation vent, such that the condenser fan draws much of it’s air from under the building, but exhausts it’s “cold” simply to the outdoors. I have not logged it in depth over the last year, but I am seeing COPs net net a bit higher than I might otherwise expect, simply because it is pulling warmer air in the winter,a nd cooler air in the summer. The only effect on the building is a slight increase in crawl space ventilation.

Icarus

Solid_Fuel_Man

Seems you would definatly be gaining from the earth's flywheel temps. Gaining usable heat during heating and cool during cooling. 

I know this is a much different climate than the near polar climate I'm used to!

k3nnyg

howd it go and can we get any pics??? im really curious on how this turned out??