Optimizing Ductless Heat Pump in an Old House
Hi all,
We live in southeastern Ontario, climate zone 5-6, in an older home from 1895. Until recently, the home was heated with a forced air natural gas furnace, however it was never doing a great job – our ducting system was very poor, with very little airflow to and no return registers from the 2nd floor of the house. There were some recent government programs that enabled us to get a ductless heat pump system (Mitsubishi) with 5 heads at a very reasonable cost. The installers were, I think, not the greatest – e.g. they miswired the branch box, which led to a lot of trouble at first. However, over the summer, everything was working great in terms of cooling, and even into the shoulder season of the fall, things were ok.
A week ago, we had our first real cold snap, with overnight temperatures going down to -5C (23F). We got up in the morning and found that the thermostats were reading 17-18 degrees despite being set to 21, and it was hard to get the temperature to increase further over the course of the day without cranking up the setpoint tremendously. (We have the MHK2 wireless thermostat reading the temperature in the room, not at the head.)
A technician came by and checked everything out and says that the system is working fine, but the problem is that the unfinished, uninsulated basement is too cold, and that’s why the heat pump can’t keep up.
The basement and foundation are all limestone. There is spray foam on the headers but not elsewhere. There are some small inaccessible crawl spaces beneath part of the first floor.
The technician was recommending that we either run the furnace during the winter to heat the basement, run the furnace fan to circulate warmer air, or invest in another heat pump just to keep the basement warm.
It would have been nice if they had told us all this at the start, but c’est la vie.
Any thoughts on whether this explanation for poor heat pump performance sounds legit, and if so, the best way to try to compensate for the cold basement?
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Replies
The tech is feeding you garbage. While the cold basement doesn’t help, it is not your main problem most likely. Thermostat locations, air distribution, whole house insulation/ equipment sizing are- in that order.
Assuming when cranked up the equipment can maintain comfortable temp inside my guess is thermostat locations being the culprit.
This link tells how to use your fuel use history to estimate the size of a heating system:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Basically what the guy is saying is that your system is undersized. That link will tell you if that's true. If he didn't do a similar analysis he's just guessing.
In Ontario I would guess your design temperature is around -18c, so if it can't keep up at -5 c it's pretty extremely undersized.
If it's not undersized it's misconfigured or malfunctioning.
Check the temperature rise at each head when the unit is running at full speed if it is working correctly the air leaving each unit should be at least 20°f warmer than the air entering that unit. If you are getting a 20° Delta then you will have to chouse between upgrading the equipment or upgrading your thermal envelope with air sealing and insulation.
Note again multi split = unhappy owner, miny split = happy owner
Walta
" If you are getting a 20° Delta then you will have to chouse between upgrading the equipment or upgrading your thermal envelope with air sealing and insulation."
Or running the legacy gas furnace when it's really cold out.
Although if the heat pumps can only maintain 4C below setpoint when it's -5C out, that pencils out to the heat pumps only being able to maintain setpoint at 0C or so, which isn't a lot of the winter in Ontario.
Thanks all. I am also highly suspect about the basement being the culprit since it's usually 50-60F down there, which doesn't seem quite cold enough to affect the rest of the house tremendously.
I spent an afternoon caulking some obvious gaps and have turned on the furnace as a backup to get the temperature up when it falls below 20 C. I took a look around the edge of the foundation and in a spot that's hard to access (very close to the house next door) there's a ton of missing mortar and open spots where I suspect drafts are getting in, so I'm hoping that might help take care of things.
The thing is, a house is a system. Saying the basement is "the culprit" makes no more sense than saying the steerage passengers were the reason the Titanic didn't have enough lifeboats. When they sized the heating system, they should have calculated the heating load for the entire house and sized the system to meet that load. If they didn't include the basement in their calculations, well, that's on them.
If your system is undersized, you basically have two approaches: reduce the heating load by improving the sealing and insulation, or add more heating capacity. Which sounds like exactly what you're doing.
Update to the original issue:
We had someone come in for a second opinion on our system. He found that it was about 4 lbs short on refrigerant! When he added it back, and turned on all the indoor heads to 31C, it blasted hot air like there's no tomorrow. (He checked for leaks as well. It seems like the original installer either didn't add enough to begin with or removed some when they misdiagnosed another issue..) Great, as you can imagine, but we're left with a new problem...
There are two 12K units downstairs, and three 6K units upstairs. If I set the 2 downstairs units to 21-22C (70F), and leave the upstairs units off, the room temperature downstairs will drop to 18-19C despite the condenser working full time, and the air coming out of the units is about 85F. I only get warmer air from the downstairs units if I turn on 1-2 of the upstairs units in addition, in which case the air coming out of all them is 100-105F, and everything warms up really quickly.
Is this supposed to be how the system works? Can I not just use the 2 12K heads if they're all that's needed?
Final update: I asked the tech to add some more refrigerant and after putting another 1.5lbs (on top of the calculated required amount) everything seems to be working just fine. I guess it was just undercharged!
Consider that you have a leaky system and you can repair the leaks or pay to have the system topped off every 90 days or so.
My guess is at install the outdoor unit came charged from the factory to be within one pound of the perfect charge and being 4 pounds light seems impossible.
A responsible technician would not just add refrigerant because the customer wanted some.
Note over charging a system could damage the compressor and it will make less efficient and affective.
If you record the 3 temp reading now you will be able to tell when it is getting low and call for a recharge.
Walta