Is my Multi Split working properly?
I have a ASHP multisplit installed in 2021– one outdoor unit running two indoor units in two separate rooms. I am in climate zone 5A. My understanding has been that ASHPs are most efficient when the compressor runs continuously, but ever since we have had this system the indoor units have always cycled on and off, even on the very coldest nights when the load is highest. I am disappointed in the amount of electricity that we use when this unit is turned on.
Specifics of the units are Daikin 3MXS24RMVJUA Outdoor unit running a FTXS18LVJU which is in a 600 sq ft sunroom/greenhouse, which heats up from the sun midday on a sunny day so the minisplit does not run during these hours. The other indoor unit FVXS12NVJU is heating a 288 sq foot second floor bedroom. I got a Manual J calculation done (online) after the system was installed, and the calculated load for the sunroom/greenhouse was 15,500 Btuh and 3776 btuh for the bedroom, with a design temp of 9oF.
I have attached a single day whole house electric usage graph from our Tesla app (we have solar panels and batteries). This was a cold sunny day (16-26oF) and only the unit in the sunroom was turned on.
I think that the continuous small spikes in electric use are from the compressor, since they stop at midday when the sunroom warmed up and stopped calling for heat. Also in this midday period you can see what the base electric load in the house is pretty low – probably less than 2KW/hr. The big spikes in electricity use (up to the max 9 Kwh) are I think the hot water heater coming on.
My understanding has been that ASHPs are most efficient when the compressor runs continuously, but ever since we have had this system the indoor units have always cycled on and off, even on the very coldest nights when the load is highest. The unit in the bedroom cycles on for 2 minutes even on cold nights. According to the NEEP website, this unit should be able to modulate down to 5500 btu/hr at 17oF.
Any thoughts on if this unit is working properly and if there is anything I can do to improve the efficiency? I realize that I am losing some efficiency with a multisplit, but I was hoping for a better outcome. Many thanks in advance, and I really appreciate all the great information on this site.
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Replies
Since your unit is cycling off when it is above the design temp you units minimum run speed is oversized for your load. The question is what percentage of the time does it run at both design temps? As much as I dislike rules of thumb 2 tons of cooling for 900 square feet is almost double my local rule of thumb. If you room is more like a greenhouse the thumb rules don’t apply.
The way I see it cycling is hard on the equipment and will shorten its life. I think the difference in electrical usage is pretty small and you would never save enough energy to recover what new smaller equipment would cost. The first 15-30 seconds of the cycle is what could be saved with continuous operation and it seems likely it was off longer than that.
It seems your equipment is running at min speed even on the design days and it seems likely your home is larger than 900 sqf consider adding another head or heads and put more of a load on the system.
The spikes you are seeing are when the system runs at full speed. This will happen when the unit enters a defrost cycle in heating mode. The unit also need to run at full speed to insure the oil in the system get returned to the compressor and you will see this in both modes.
Walta
Walta, thank you. I am sure you are right that the system is oversized - the guy that put it in was still new to heat pumps. I can increase the load by leaving to the two rooms open, and that does increase the time that they run. But will this just increase the electric use even more, although increasing the efficiency?
Is it possible to add more heads to an existing compressor? That could be an option.
“I can increase the load by leaving to the two rooms open, and that does increase the time that they run. But will this just increase the electric use even more, although increasing the efficiency? "
Yes longer run times at higher speed will use more power but I assume the other parts of the home are heated by some other system that you could turn down or off.
“Is it possible to add more heads to an existing compressor?”
I have not read the instructions for your model but most seem to support 5 or so heads.
Walta
Higher resolution power trace would help.
By the looks of it the unit is running well initially, slowly ramping up then shutting down. Sometimes what happens especially in older houses you can get a lot of air leaks which move hot air right up the ceiling. What could happen is the unit starts, which makes a bit of heat that rises which go right into the intake and the controls see as the room is now hot causing it to shut off.
Quick check if this is the case is run a ceiling fan in the room with the wall mount. Usually the fixt to this is to get a wired thermostat with a remote sensor placed away from the unit so it picks up ambient temperature properly.
If you are short cycling run the indoors at the lowest fan speed. Do install wall/ remote thermostats as the ones in the heads will contribute to the short cycling.
Install/ run ceiling fans as Akos suggested
Thanks so much for the suggestions. This is an old house, and for sure there are air leaks despite the tyvek we put on at the last renovation. I’m arranging to have the remote thermostats put in, this makes a lot of sense. There is a ceiling fan in the greenhouse, I’ll try that as soon as we turn the heat on in the next couple of weeks.
A generic thermostat will only have the ability to command the system run at 1 or maybe 2 speeds and you are likely to get higher speed compressor speeds when it runs making the on cycles shorter and the off cycles longer.
If the complaint is the room temp is different than the set point. Then you would want Daikin’s branded thermostat or a Daiken remote temp sensors and not third party remote generic thermostats.
Walta