Heat pump fan--no more manual control

5 years ago

last modified: 5 years ago

We have a ducted Mitsubishi heat pump system, and I've always been able to manually set the fan to High, right up through last month when I had it on to help filter smoke from forest fires. I've tried to set it to high the last couple of evenings to help circulate warm air from the gas fireplace, which keeps the thermostat warm but other rooms cooler, but it won't go on high.

If I set the system on Cool, the fan cranks up to high, but on either Heat or Auto it won't. Any ideas as to why I can manually shift the fan to high in Cool but not in Heat or Auto? I'm 95% positive it would go to high on all three settings (Heat, Cool and Auto) in previous years. I changed the battery in the thermostat, but no improvement.

If it matters, the thermostat is a Mitsubishi MHK1, the indoor unit is PVA-A30AA4, and the outdoor unit PUZ-HA30NH4A.

Thanks for any ideas.

Featured Answer

AvatarWalt
Original Author

5 years ago

last modified: 5 years ago

Mike_home, that was it! Thanks so much for taking time to look the situation and point me in the right direction. I'd looked at the settings in the operating manual, but the additional ones in the installation manual were the key, once I'd done a little googling to find out what they mean. #125 was set so that once the heat had gotten to the set temperature the fan would go down to extra low and I couldn't change it manually. I've reset that to "Selectable Fan Speed" and it does what I tell it to. Now I just need to figure out whether it still goes to extra low if it's in Auto. And there are a couple of other settings to research too, just to satisfy my curiosity. Anyway, your good deed for the winter is done. Thank you!

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Comments (6)

HU-222354726
5 years ago
Interesting!!

Common question by many...Should I put the fan in the on position? Most thermostats have two settings for the fan - auto and on - and some well-meaning HVAC techs tell their customers to put the fan in the on the position to distribute the cool air in the house better.

Let me back up a bit here. For purposes of this discussion, your air conditioner has two components: the part that cools and the part that moves the air. You adjust the part the cools by changing the temperature setting on your thermostat. You can affect the air movement by setting the fan to auto or on.

When the fan is in the auto mode, the air conditioner moves air only when the cooling part is running. When the thermostat senses that the house has reached the setpoint, it shuts off both the cooling and the fan. When the fan's in the on mode, the fan keeps running continuously, even when the cooling part is turned off.

One of those settings is better than the other. The photo below shows how my thermostat is set, and that's how I recommend you set yours, too.

Get to know more in detail about the manual setting- HVAC contractors Essex NJ
AvatarWalt
Original Author
5 years ago
Thanks for the reply. We do indeed have a heating/cooling unit (outdoor heat pump) and air distribution unit (indoor heat exchanger). Both are variable speed, so ramp up and down smoothly to keep the temperature very even. My question arises because I now have the gas fireplace on in the evening making that room warm and, since the thermostat is nearby, the HVAC is basically off and the rest of the house cools. I'd like to set the heat exchanger's fan on HIGH when the fireplace is on to distribute that heat from the living room throughout the house. I'm sure I've done this in previous years, but now the fan won't go to HIGH unless I switch the Mode to Cool or Off. In Heat or Auto it just stays very low. Baffling.
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mike_home
5 years ago
I am not familiar with your thermostat. But I suspect your issue may be resolved by changing a setting. Here is the MHK1 installation manual. There are several set up functions which can be changed from the default value. See what the current out settings are for 108 (Auto Fan) and 125 (Thermal Fan Off). Changing one of these may allow the high fan speed in the heat mode.
AvatarWalt
Original Author
5 years ago
last modified: 5 years ago
Thanks Mike, I'll take a look at those. I haven't deliberately changed anything from when it worked before, but who knows what gremlins lurk. It sure seems like it's a settings thing, since the fan will go to high in other modes and will go up automatically in Heat when it needs to, just not when I manually tell it to.
mike_home
5 years ago
I am happy to hear you were able to resolve the problem. Enjoy the comfort your heat pump. Stay warm and safe this winter.
AvatarWalt thanked mike_home