Heat Pump Thermostat

Forums › Home Workshop DIY › Heat Pump Thermostat

malamute5

16 posts

Geek

#299047 6-Aug-2022 10:22

We had our 10+ year old heat pump replaced with a new Mitsubishi heat pump. We had no issues with the old one except for when it was anything below 5 deg it simply could not heat the lounge. We set it on 20 and it heated the lounge to 20, was very much set and forget as they should be. This new heat pump seems to have a rather inaccurate thermostat. We set it to 20, on heat, auto fan, same settings as previous heat pump and were wondering why we were feeling cold. It has wifi so we could look to see what it was saying the temp was and it was reading 20 however, we have two thermometers in the lounge and both were reading around 17.5 deg. If you turn the heat pump up to 22 deg it heats the lounge to around 20 but being brand new and given they aren't exactly cheap, I was thinking we should go back to where we got it from and get them to look into this. Sounds to me like the thermostat is inaccurate.

We were wondering if anyone has experienced similar issues before?

1 | 2

607 posts

Ultimate Geek

#2951238 6-Aug-2022 10:43

I guess the first thing I think of is maybe the auto fan isn't blowing it far enough to mix the entire room. Is it a large room? Does it feel warmer if you walk over towards the heatpump?

The heat pump is only measuring temperature in the unit, so unless the thermometers in the room are close by there will be a differential.

Anyway other than it being annoying, ie setting it to 22 vs 20, is it causing you any problems?

Backblaze

Backblaze Unlimited Backup. World’s easiest cloud backup. Get peace of mind knowing your files are backed up securely in the cloud (affiliate link).

timmmay

19631 posts

Uber Geek

Trusted

Lifetime subscriber

#2951239 6-Aug-2022 10:43

Thermostats are all different. We set our kitchen Daikin to 21 but we set our lounge Daikin to 24 to get to about the same perceived temperature.

Just set whatever temperature is required to make you comfortable.

tweake

1043 posts

Uber Geek

#2951264 6-Aug-2022 11:41

Its rather unusual for it to under shoot the temps, especially by so much. Often they overshoot, especially if cold air leaks in around the sensor.

odds are its got a bad temp sensor. get it replaced under warranty.

tweake

1043 posts

Uber Geek

#2951265 6-Aug-2022 11:43

also check there is no packing, tape etc left around inside the unit. might be something stopping the air flow past one of the sensors.

Jase2985

12348 posts

Uber Geek

ID Verified

Lifetime subscriber

#2951430 6-Aug-2022 20:32

is your thermometer at the same location as the heat pump? that could explain the difference. there could be stratification in the room causing the difference you are seeing.

malamute5

16 posts

Geek

#2951576 7-Aug-2022 10:00

Jase2985:

is your thermometer at the same location as the heat pump? that could explain the difference. there could be stratification in the room causing the difference you are seeing.

It's within about a metre of the heat pump and only about 60cm lower than it. I guess my main question is should a brand new heat pump be less accurate than one that's a good 10 years older? The old one, 20 meant 20, not 17.. seems like something is not quite calibrated correctly.

malamute5

16 posts

Geek

#2951580 7-Aug-2022 10:04

Nate001:

I guess the first thing I think of is maybe the auto fan isn't blowing it far enough to mix the entire room. Is it a large room? Does it feel warmer if you walk over towards the heatpump?

The heat pump is only measuring temperature in the unit, so unless the thermometers in the room are close by there will be a differential.

Anyway other than it being annoying, ie setting it to 22 vs 20, is it causing you any problems?

The whole room seems to be the same temperature. The thermometer is right beside it and about 70cm below it. It's not causing any problems, just a bit frustrating buying something new that's less accurate than the old one!

Jase2985

12348 posts

Uber Geek

ID Verified

Lifetime subscriber

#2951587 7-Aug-2022 10:21

malamute5:

Jase2985:

is your thermometer at the same location as the heat pump? that could explain the difference. there could be stratification in the room causing the difference you are seeing.

It's within about a metre of the heat pump and only about 60cm lower than it. I guess my main question is should a brand new heat pump be less accurate than one that's a good 10 years older? The old one, 20 meant 20, not 17.. seems like something is not quite calibrated correctly.

there could be a lot of stratification in that distance, try measuring it at the roof to see. the new heat pump might not be as good at moving air around the room, and the auto fan speed might be running so slow its not actually mixing the warm outlet air with the cooler room air very well.

tweake

1043 posts

Uber Geek

#2951596 7-Aug-2022 11:08

Jase2985:

malamute5:

It's within about a metre of the heat pump and only about 60cm lower than it. I guess my main question is should a brand new heat pump be less accurate than one that's a good 10 years older? The old one, 20 meant 20, not 17.. seems like something is not quite calibrated correctly.

there could be a lot of stratification in that distance, try measuring it at the roof to see. the new heat pump might not be as good at moving air around the room, and the auto fan speed might be running so slow its not actually mixing the warm outlet air with the cooler room air very well.

no. stratification will have nothing to do with it, especially measuring so ridiculously close.

as its actually getting to temp when thermostat is set higher suggests its simply an incorrect thermostat, most likely just a bad sensor.

Yoban

417 posts

Ultimate Geek

#2951599 7-Aug-2022 11:20

Could always add an external sensor and mount it at a height more suitable for a representative temperature of the room/area assuming you could run cables down walls etc.

PAC-SE41TS-E Remote Sensor // Mitsubishi Electric (mitsubishi-electric.co.nz)

tdgeek

28613 posts

Uber Geek

Trusted

Lifetime subscriber

#2951604 7-Aug-2022 11:57

I assume you set the fan direction to pointing down? To rotate the warm air to displace the cool air, and keep inflow in the room so it equalises quicker?

As others have said, set it to the temp your skin likes, not a number. Treat the number as a number not an accurate temperature.

Nate001

607 posts

Ultimate Geek

#2951607 7-Aug-2022 12:15

malamute5:

It's not causing any problems, just a bit frustrating buying something new that's less accurate than the old one!

Honestly as others said, run it to what feels comfortable. The number is irrelevant end of the day.

I wouldn't get too fussed about it.

Jase2985

12348 posts

Uber Geek

ID Verified

Lifetime subscriber

#2951611 7-Aug-2022 12:25

tweake:

no. stratification will have nothing to do with it, especially measuring so ridiculously close.

as its actually getting to temp when thermostat is set higher suggests its simply an incorrect thermostat, most likely just a bad sensor.

1m is about 50% of a typical rooms height.

timmmay

19631 posts

Uber Geek

Trusted

Lifetime subscriber

#2951612 7-Aug-2022 12:25

In my opinion the first and second replies answer this question just fine. It doesn't matter what number is on the display, so long as the room is the temperature you want it.

tweake

1043 posts

Uber Geek

#2951614 7-Aug-2022 12:35

the heat pump should not be that far out, especially as many over shoot temps not under shoot.

the other question is how does it behave on cooling? if the cooling and heating are different then auto mode won't work properly.

the other issue is if you have a few heat pumps and you have to remember how much out each one is. even worse if you connect them by a common thermostat.

i have two high walls that over shoot. i suspect they are made for higher walls than what we have, so they can never be installed at the correct height. or far to big for the room. so they can be well over temp and still turn on. i can't run them on auto because it over shoots and keeps switching from heating to cooling and back again.

1 | 2