Long down time on ducted heat pump
Hi everyone. Question about run times on my heat pump. I’ve seen lots of questions on either always running and not getting warm enough or short cycling due to oversizing. But my situation seems a bit different and I’m not sure if it’s bad or just different.
First some details
– Northeastern Massachusetts – 2,600 heating area.
– mitsubishi hyper heat 4 ton outdoor condenser
– Branch box in basement
– 3 ton air handler in basement hooked up to existing ducting from old whole house AC
– 3 wall units (6k each) in bedrooms.
– Air handler serves the first floor and some rooms upstairs not serviced by the wall units. Their registers have been closed / blocked but can be reopened.
– Ducted air handler is run off MHK2 thermostat located downstairs in a central location.
The system has had no trouble keeping us warm. We’ve been primarily running it just off the air handler with the wall units turned off. The wall units upstairs were more to help with cooling so we haven’t really needed them.
What I am seeing is that the air handler will only run a few times a day usually for about an hour and a half to two hours at a time. So definitely not short cycling. However, the MHK2 seems to react to temperature changes slower than my Honeywell thermostat for the backup oil boiler that sits right next to it.
I have the heat pump set to a constant 65F. When it turns on it will stay at 65 for a good amount of time, while the real air temperature is a few degrees higher. Then the minimum deadband is 3F. So it won’t turn off until the MHK2 thinks it’s 68. By this time the Honeywell may think it’s 70 or more.
So I get a lot more heat that is banked once hitting the off point. It then takes the house a few hours to cool down again.
Is the slow response time to read the temperature a way to avoid short cycling for a system that may be on the large side for this time of year? Today started in the upper 20s and got up to the upper 30s. The heat pump came on twice during the day using 13 kWh over a 10 hour period but it was only on for 3.75 hours. On average it’s running less than 2 kWh per hour but it’s not the low steady output heat pumps are known for.
Thanks!
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Replies
How often did the oil burner run?
3° dead band seems very wrong 0.3° seems much more likely.
Sound to me like the system is hugely oversized Did you do a fuel use or manual J calculation before installing the system?
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Is the thermostat cloud connected? As doing so seems to be associated with cycling.
Walta
Thanks Walta.
The MHK2 thermostat is not connected to wifi / kuomo cloud.
Prior to the heat pump we used a wood stove with the oil kicking in as a backup. If we just used oil it would be probably be around 600 gallons per year.
The installers had to do a manual j for our local utility rebate, though I don't know off hand the btu indicated. The house is pretty well air sealed with R-49 in the attic. I used a Mitsubishi Diamond installer and got quotes from others as well. They all recommended 4 ton units. 2 separate smaller systems would have been more efficient but the extra labor made a single outdoor unit connected to multiple heads the best option for us.
Last night was in the mid upper 20s. The woodstove covered the overnight and the heat pump came on at 5:45 AM. The MHK2 was set for a steady 63 the whole time.
- 10 minutes - steady at ~ 1.8kw
- 20 minutes - slow ramp up to 5.2 kw
- 30 minutes - held steady at 3.8kw
-10 minutes - steady at 2.2kw
- 5 minute rise to 4.5kw
-20 minutes steady at 4.5kw
- Total continuous run time ~ 90 minutes
The system shut down when it hit 65 on the mhk2 though the oil thermometer next to it was registering up to 71. Eventually the two thermostats do come in line with each other. The MHK2 seems to take a rolling average where the other thermostat is real time. The MHK2 may drift up past 65 while the other is drifting down from 71. It seems the the MHK2 goes on at set point -1F and off at set point +2F. The manual is a little unclear but in the settings the 3F min dead band may only apply when using to auto switch between heating ad cooling (which I am not doing).
Later in the morning I bumped the thermostat to 65 (it was reading 64 room temp at the time). It went into a cycle for 1.5 hours where its steady state was at 5Kw.
The minimum rated output for the outdoor unit is 27k btu which I believe the rule of thumb would be that it would run around 2-3kw. I just wonder if there is something that is preventing it from running at its minimum capacity - either the thermostat or being connected to a ducted air handler?
From what I've read of short cycling, at least the type that is hard on equipment is something like 10 minutes 5 minutes off, not 2 hours on 3 hours off. Even if I am oversized I wouldn't say that I'm short cycling. Its more like it always runs like its the shoulder season even when its in the upper 20s.
The current behavior is not necessarily a terrible thing for me. Our solar net metering encourages real time use so its not bad to run the heat pump a little higher instead of dumping excess to the grid during our peak solar times to carry us into the darker hours. I'm just trying to understand it better.
Thanks.
Running a remote thermostat does not mean necessarily that the temp is measured at it. If not specifically configured the temp is measured at the intake side of the air handler…. Just a random thought
And when connected to air handler the outdoor will not turn down as low as it would when connected to a smaller single head. Generally those non- vrf outdoors will turn down to 1/3 of rated power at most.
Right - the MHK2 allows you to pick the source temperature (incoming air or at the thermostat). I’ve tried it both ways and didn’t really notice much of a difference. It’s currently set to read from the thermostat.
I’m also experimenting with opening some vents upstairs to see if having a larger space will lengthen the run times. The natural convection so far has been enough to keep it warm up there (we like it a little cooler in the bedrooms).
The only thing that really seems consistent is that it will run until it is set point plus 2. Today was warmer and it ran for an hour straight at a little less than 3kw to get to set point +2. On colder days it will run at a higher wattage for about the same amount of time and still get to set point +2. I’d think that when it’s colder if it ran at 3kw it would probably be a longer maybe constant run. It seems to ramp up or down based on the outdoor temperature more so than what is going on inside which seems odd to me.
I agree the 3° dead band is for the auto change over from heating to cooling.
Heat pumps and especially variable speed heat pumps are best operated in a set it and forget it kind of way.
Consider on a cold day not using the woodstove or boiler and not adjusting the HP thermostat for 36 hours. My guess is the HP thermostat will settle in and the unit will stop cycling.
Walta