Minisplit as Band-Aid Cooling Solution
Climate zone 4a- NJ. I have a brick Cape built in the early 1940s that has no insulation in the walls. A couple of years ago we had an energy company come and spray foam the knee walls, dense pack the roof slopes with cellulose, and fill the tiny attic space with loose cellulose. It helped make the upstairs a bit more comfortable, but only a little. We use a 8000btu window AC almost daily during the summer and run the central air only on really hot (85+) days or when it’s really humid. During the summer the downstairs is very uncomfortable in the evening; the house holds a ton of heat even after the sun goes down.
The central air cannot keep up and runs almost continuously; if it’s in the mid 90s I can’t get the house to any lower than about 78. I had a HVAC tech clean it and check the pressure; he said everything is working as it should and it is sized appropriately for the house square footage (about 1300). He said with poor insulation and the trend of hotter summers it could probably be replaced with a larger unit.
I’ve had several spray foam and other insulation companies out and all have said it would be too expensive to spray foam the walls. Cellulose fill isn’t an option due to moisture retention. This means either filling the walls with foam, or tearing down the plaster, doing spray foam, and then new drywall. Both are prohibitively expensive, especially for a house we don’t plan on being in more than 5 years from now.
Would a mini split be a good option in the downstairs? The upstairs is stuffy (naturally) but the only bedroom we use is well-handled by the window AC. The majority of the time we stay downstairs.
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I'd hesitate to add central AC capacity as AC is usually oversized, which seems to be the case if it's cycling like you say. Another concern is how you're using the AC - consider just turning it on and forgetting about it. All systems will struggle if you leave them off until the hottest days of the year. Is the ductwork sized correctly? It doesn't seem to be if the unit turns off before the thermostat setpoint is reached.
I think you misunderstood- the central AC isn't keeping up with the cooling load. If I set the thermostat to 75 the AC will run continuously and never stop, and still never get the temperature below 78 degrees until the next morning. I know $300 a month is not normal to cool a 1300 sq ft house in the northeast.
Part of the problem with the ductwork is there's no return for the upstairs; just a 6 or 8" flexible pipe that runs up to the second floor and comes out 3 small vents. There is no room to run a return for the upstairs.
It's all good: I read "runs almost continuously" and interpreted that as cycling. What's the current size of the central AC?
I’m not sure it’s realistic to expect the minisplit to decrease your monthly cooling bill, but it could help with cooling itself, assuming the distribution is correct. There’s nothing to do about the ductwork? 32,000 btu is a lot for a house than size in that climate, so it being undersized significantly is extra surprising.
2 tons is probably undersized for that sort of house. 2 tons is probably about right for the downstairs alone. So, a minisplit upstairs might be a good option if the upstairs is one large space, or you can leave the doors open. The lack of a second floor return is definitely hurting your distribution and overall performance upstairs, but I'm unsure about why the system is not keeping up with the necessary cooling downstairs. Something may still be "off" there.
One possibility is that you mentioned only running the central AC when it gets above 85F or so. When you turn on AC in hot/humid weather, it has to remove both sensible heat (temperature) and latent heat (humidity). In swampy NJ, the building materials will soak up a ton (literally) of moisture, and you can't effectively lower room temperature without dealing with the moisture at the same time. At the beginning of the cycle (sometimes a day or two), the AC is working at its hardest just to remove the humidity and you get little reduction in temperature. You might try turning on the AC a day or so ahead of predictions for very hot/humid weather. If the house is preconditioned, the AC doesn't have to work as hard removing structural humidity, only the humidity that leaks in with outdoor air.
Is the furnace/air handler in the basement? And is it a single zone serving the whole house?
What about solar gains? Do have unshaded exposed windows? Solar gains can make a massive difference.
Is there any low hanging fruit to air seal exposed ductwork joints with mastic?
Maybe pull the cover on your plenum and have a look at the coil. Is it clean? is there significant air bypassing it?
If the return is undersized or restricted can you try opening up the return close to the furnace but removing a duct cleaning cover or two?
What is the supply temp? and return temp and humidity?
Yes it is in the basement. There is unfortunately a lot of southern exposure without a way to put a big tree in, thanks to the tiny lot sizes in NJ. We keep shades on the windows where the sun shines in on the south side but it's such a tiny area in comparison to the entire size of the house.
Nothing I can do about the lack of return in the upstairs, there's just not enough room for another HVAC pipe to the basement. There's a small corridor that has the heat/AC pipe, plumbing, and electrical for the upstairs and it's packed full.
And yep, everything is clean, HVAC guy checked the coil and supply temps. Everything was within normal limits.
Band-Aid solutions generally have side effects.
You could throw equipment at your problem but buying installing and operating that equipment is not a low-cost solution. Yes, you could add 2 more tons of equipment and the house would likely be quite comfortable but your electric bill would likely double.
Have you had your house blower door tested?
Air sealing generally will make the biggest comfort improvement for lowest cost if done as a DIY project. Consider fitting a box fan in a window and using incense sticks to locate the leaks and plug them with caulk.
"I had a HVAC tech clean it and check the pressure; he said everything is working as it should and it is sized appropriately for the house square footage (about 1300)."
This sentence set off alarm bells in my head. There is a process for sizing HVAC equipment, and it has nothing to do with square footage.
Empirically, your system is too small, it's not cooling the house.
You're probably better off trying to insulate the house better than installing more cooling.
Normally I would agree, but the tech was there just to check the function of the AC. Without doing a full load calculation I don't know how else he could determine the size.
And yes, I know insulation would be the best solution, but we've hit the point where what's left isn't economical feasible. I'm not going to tear down drywall and get in to a crazy expensive remodel.
I would agree not to tear down drywall and destroy the house. But, this does not mean that you should not air seal. The info that I've learned on this site has really benefitted my house comfort and energy use. A lot can be done from the attic and basement. All those cracks add up to a lot of space for air to travel. By reducing those, you can "air seal" without tearing apart the house. In your case, it is possible that the energy company missed some spots.