Do minisplits pull air from the crawl space?
Alabama. Old house (1892). Very little insulation. Air handlers (Daikin, 4 of them) mounted high on the walls. Ceilings are mostly 9 feet. But some are 11.5 feet. Original pine t&g flooring with no subfloor. Open cell spray foam under the flooring (from a previous owner).
Here is my thinking: move the compressor right up to a hole in the foundation wall and let it pull air from the crawl space. That would pull the warm inside air down in the rooms through the leaky floors causing the warm air to fall, then it would get “warmer” air to work with than ambient, making it more efficient in sub freezing weather.
Right now the air handlers cycle on and off as the outside ambient temp gets close to or goes below freezing. So I’m guessing it’s defrosting.
Ok, be gentle. How is this a terrible idea? 😊
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Very terrible. Just...don't.
This is kind of like plugging a power bar into itself. You're suggesting purposefully drawing conditioned air out of the house so that the air flowing over the outdoor unit is warmer? Whatever efficiency you might gain will never come close to making up for the additional load you've created by doing it. If the floors are leaky, fix that instead.
The amount of air in the crawl space is very very small compared to the amount of air the compressor pulls.
Some parts of this would work, other wouldn't.
Yes, slightly warmer air in the crawlspace would help your "outdoor" unit to work more efficiently in very cold weather. You'd get some gain from the slightly warmer air due to the ground around the crawlspace, like a simple version of a geothermal system. The downside is that you'd be making the crawlspace COLDER which would rob heat from the rest of the house. That "robbed heat" is just circulating around between the house and the crawlspace, doing nothing but wasting energy as the heatpump pushes it around the loop.
The colder crawlspace will result in colder floors in your house, and a need for the heat pump to do more work to keep the temperatures up, which would result in a colder crawlspace, which would result in the need for the heat pump to do more work, which would result in a colder crawlspace, which would ... There's the problem...
At some point you'd reach equilibrium, where the crawlspace would be some amount colder than it would be if the heatpump outdoor unit wasn't there, but still warmer than the outdoor air due to geothermal effects, but you'd be making the indoor air colder due to the colder crawlspace air temperatures. Would there be something to be gained overall in this setup? Possibly, but it's probably not worth the trouble. You'd also have much more difficulty doing any maintenance, and in the summer the issues would reverse when you're trying to run cooling.
My recommendation would be to NOT try this and leave the "outdoor" unit outdoors.
Bill
Bill,
I think the 2nd law of thermodynamics dictates that this is a net losing scenario. Given that every movement of heat energy incurs losses, I cannot fathom how taking heat from inside the house in order to improve the efficiency of a device that will in turn put heat back into the house could possibly end up on anything but the negative side of the ledger.
The “losses” usually end up as heat, so in this particular case the energy losses actually help you heat the space. The downside here is that those losses came from the input electrical energy, so you did pay for that heat — it wasn’t pumped in from outside.
Whether or not the crawl space actually helps you compared with having the “outdoor” unit actually outdoors depends entirely on how much heat you get through the geothermal effect from the ground surrounding the crawl space. I don’t know how you could accurately predict how much this would help you, which is why, as an engineer, I wouldn’t recommend spending the money to try it. If you were a researcher running a controlled experiment it might be worth a try, but not as an actually operating system in a regular home.
Bill
Most of the losses would also be outdoors, so not useful heat. I didn't really consider geothermal heat, since the premise he based this idea on was pulling conditioned air out of the house. The heat energy in those air losses would probably be the dominant contributor to the equation, dwarfing possible geothermal gains unless his house is on top of an active volcano or hot spring.
No question if most of the heat is being pulled from the house then this is a perpetual motion style “free energy” fallacy.
I don’t really think there would be any gain to do this though, which is why I’d recommend against trying it.
Bill
In fairness, this is essentially how ground-source heat pumps operate, except that the hole is deeper and the medium is water instead of air. It's not that it couldn't work, the question is one of scale, and I suspect what you'd find is that a typical crawlspace has nowhere near the heat-sourcing capacity to heat a typical house. Just thinking about the physical size of a typical geothermal installation, which has the advantage of warmer ground temperature at greater depths and where you still have to guard against exhausting the heat capacity of the hole.
If you wanted to get into experimental energy efficiency increases: Put everything in the sun, Make the split draw air through a plenum with an opaque surface, and a dark painted surface. Sun heats the dark surface, opaque sheet enclosed it, warming the air.
Energy usage would likely increase due to load on the fan, but larger size may overcome that.
A friend in Antarctica sent a picture of this in a warming hut they have, keeps it comfortable using just solar radiation, even when its way below freezing.
Enjoy my “iPad at the bar drinking beer” expert drawing.
Thanks guys. My mind was niggling at the perpetual motion machine aspect of this, but I couldn't articulate it. This is good info.
The problem I'm trying to solve is that below 35F the air handlers begin to turn off every 20 minutes or so to recover. We thought it may have been a leak, but it was leak tested and re-charged by weight by a Daikin trained pro. At this point it seems like it's just a matter of too much heat loss in our old uninsulated house. We are using electric space heaters to make up the difference during cold spells.
The multi-split system has saved me significant money so far (dropped power bill from $650 in summer to less than $400), so I'm happy with the decision. But, it's not at all what I would call "comfortable" when it gets down close to freezing or below. So, my thought was, how can I get warmer air to the outside unit so that it has more to draw from. The solar box is an interesting idea, but it wouldn't help at night when the temp really drops. If there were some sort of resistance heating unit that would attach to the suction side of the compressor to heat the in-flowing air that would be perfect. But I'm not aware of anything like that.
The focus is on insulation going forward, so hopefully we will make progress on the comfort front.
Heating the air prior to that air entering the outdoor compressor unit will actually increase the frost formation. This is a process known as “electric reheat”, and it’s used commercially in air conditioners to improve their ability to dehumidify the air. In your case, more dehumidification ability means more frost on the coil so it’s a “bad thing”.
If you want to reduce the defrost cycles, a heat lamp aimed at the coil and set to turn on when the compressor shuts off MIGHT help. The idea would be to improve the speed a which the coil can be cleared of frost. I don’t know what the systems brain would think of this, it’s just an idea I had that MIGHT help you a little.
Bill
"The problem I'm trying to solve is that below 35F the air handlers begin to turn off every 20 minutes or so to recover."
It shouldnt turn off below 35 unless its defrosting. Is that what your referring to as recovering?
I would install an electrical monitor on it. Tons of recent posts on this topic.
>"It shouldnt turn off below 35 unless its defrosting. Is that what your referring to as recovering?"
That's the most likely explanation. When it's +35F outside there is still a lot of potential moisture capacity to the air, and the outdoor coil is running well below freezing. That makes defrost cycles fairly frequent. When it's +15F outdoors there is about half the potential moisture per lb of air compared to 35F, and defrost cycles become less frequent. (Take a look at the right axis scale of a psychrometric chart sometime!)
The total volumes of air pulling through a multi-split compressor unit are enormous. Any temperature improvement on the intake air from pulling it out of the crawlspace rather than directly from the outdoors would be extremely brief. That configuration is basically using the floor & below-grade walls of the crawlspace as the heat exchanger to the subsoil, but there simply isn't enough surface area to be very effective, even if there weren't the additional problem of the low thermal conductivity of the soil.
Improving your building envelope is the main path to improved comfort and efficiency. Make sure you're plan for air sealing is complete before you proceed to your insulating plan. There's a lot of info on this website regarding air sealing and where to prioritize.