Control Heat Pump With Wall Thermostat

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Graycenphil

Well-known member
Joined
Jan 19, 2014
Messages
209
Location
CT USA
This is totally non-Leaf, but I have been so impressed by the knowledge and expertise on this forum, I though it was worth a try.

I recently installed an air source heat pump (basically a split system-type air conditioner, with a reverse cycle heat pump) in a house in central Vermont It is supposed to be much more efficient than the regular electric baseboard resistance heating. It works fine. It's very quiet and it makes the house nice and warm.

The problem is, it can only be set by a hand held remote control, and the temp only goes down to 60 degrees. When I'm away from the house, I normally leave it on 45. So I'm afraid I'll lose all my savings by keeping the house much warmer than it needs to be.

I have an idea for fixing this, but am not sure it's a good idea. The unit is 240 volt, just like the baseboard heating in the house. So I could run the heat pump through a regular wall thermostat. When the thermostat senses 45 degrees, it cuts the power to the heat pump. Simple.

My question is, will this be bad for the heat pump? Will it not like being turned off and on by killing the power to it? What do you think?

Thanks.
 
Does your system not have an external input that you can use to control it? I think most do, check the technical manuals. It could be as simple as two pins on the control board that you short to force the system to run, coupled with a programming change for the external input to take precedence over the usual controls.

Cheers, Wayne
 
Thanks Wayne. If it does, I'm not aware of it. There is no technical manual, and the company has not been very helpful. I wonder if I opened it up if I could find those two pins...
 
I know that you can not use a baseboard type thermostat in place of a heat pump thermostat under normal conditions. This partly has to do with the reversing (valve?), thermostats designed for heat pumps have a delay, so you do not cause damage to the heat pump (valve?). Also a baseboard thermostat is designed to have the AC voltage run through it, while as far as I am aware, a heatpump thermostat is low voltage and sends a signal to the split system to call for heat/cooling. I would not simply use a 240v thermostat in the way you describe, without more research.
 
Graycenphil said:
My question is, will this be bad for the heat pump? Will it not like being turned off and on by killing the power to it? What do you think?
It's not different than a power outage, so it likely is designed to handled such outages. Whether it would shorten the life of the unit, I cannot say.

Here are a couple of things I would wonder about:

1) What does the unit do when it is turned back on? Does it come back on with all the same settings or does it come up "off" until commanded on by the remote? You could test this by unplugging the unit and then replugging it. Or trip and reset the breaker.

2) Perhaps there is a reason the unit does not operate below 60F room air temperature. For instance, the components chosen in the indoor unit may not be specified to operate reliably below that temperature.

I must say that its a shame when engineers don't manage to include the firmware to address all of the use models which their units likely will be expected to handle.

Good luck getting it to do what you want!
 
Thanks all. I guess controlling it with a line voltage thermostat doesn't sound like a good idea. Maybe I'll carefully open it up and see if anyting inside makes sense. Maybe I can find the internal thermostat and do something with that.
 
Okay, here's my new thought, kind of a Rube Goldberg solution, but it might work:

Heating pad on the bottom of the heat pump, connected through a relay to a thermostat. Thermostat set at 45 degrees. When the thermostat drops below, 45 degrees, it turns OFF the heating pad. The heat pump cools off, turns on and warms the house up. When the temperature gets back above 45, the heating pad turns back on and warms up the heat pump. Heat pump gets over 60, turns off.

This would also allow me to use a wifi thermostat, so I could turn the heat up before I get to the house, so it's nice and warm. And I could monitor it from away.

Make sense? Think it will work?
 
Graycenphil said:
Make sense? Think it will work?
If you can program the indoor unit to use a temperature sensor in the remote control instead of the indoor unit, then I could imagine doing that with the remote control temperature sensor, but it is not very elegant.

The unit apparently the Caribou CB-50MSDC, the website is http://www.caribouac.com/our-products/18000-btu-inverter/

I couldn't find a technical manual for it, but the basic manual is http://www.caribouac.com/uploadfiles/2013/08/manual-for-inverter-final-english.docx

The manuals shows an exploded parts diagram for the indoor unit, I suggest taking the cover off the indoor unit and scrutinizing the PCB for some pins conveniently marked "external control".

Failing that, the list of troubleshooting codes implies that the indoor unit has an ambient temperature sensor that is field replaceable. So you should be able to modify it by adding a switched resistor controlled by an external thermostat to fool the indoor unit into doing what you want.

Cheers, Wayne
 
I had the same issue with my LG minisplit. They sold add an on board with a contact closure for this purpose. It was about $125 or so and I needed a 240 volt transformer to power the thermostat. Now I can control one of the units from a Nest thermostat.

I really like the idea of a relay controlling a resistor, good thinking!

I'm with reg though, it's a really basic feature to control on/off with an external thermostat.
 
In case anyone is looking to do something similar, here is what I did.

I could not get my Rube Goldberg relay/heat pad to work - the temp sensor was buried too deep in the unit.

But I did find the ambient air sensor that Wayne saw in the parts diagram. It had a simple two wire connector to the circuit board. I clipped one wire and the unit stopped heating and showed fault E2 - "Faulty ambient temp sensor". I connected the wires together and it came back on. This worked fine for several cycles, so I just connected that wire to a Honeywell wifi thernostat. The thermostat acts as an inline switch, turning the temp sensor on when It calls for heat.

It's been running fine now like this for a day, so I think it will work.

Thanks again.
 
Some more advanced split type heat pumps have "vacation mode". That is usually 5C or 8C or 14C and can not be adjusted.
It is very hard to randomly switch it on (even if it is available). Read the manual thoroughly, it might be somewhere.

Heat pumps don't like to be switched on/off very often. This is why inverter type pumps last multiple times longer. They never
switch between off and 100% on. It's like switching on car engine and revving it to the max at the moment you start it.
They* are efficient only if they work at optimal load (partial). Some have ECO button. That usually means they don't really heat
up enough according to temperature that has been set :lol:

*inverter

Interesting way to trick the heat pump, mess with thermostat :lol:

I would add a specific resistor to fool it to think it is warmer inside.
You need to measure some resistances at different temperatures and estimate.

Also read the manual, maybe you have special mode.
 
Thanks, Arnis. You mean add a resistor instead of just breaking the circuit to the thermostat? Any idea what size resistor I might try?

I don't think my unit has a vcation mode; it is a pretty low end model. In the US, none of the really good ones seem to be set up for do it yourself (pre charged lines). But I'm satsified with it. So far it has been working fine, and now that I can set the temp much lower, and contol it from a remote location, it's doing everything I wanted.
 
You need to remove the thermistor (temperature sensor) and make a graph about resistance it has at different temperatures.
Using water and normal thermometer :lol:
Then you use Ohm law to manipulate the resistance. If you add in series you add resistance, if you add resistor parallel you will
lessen the resistance.
Graph looks something like this.
log-graph.png

Every thermistor has its own curve. If you know the part number it would be possible to find it on internet.
But it is faster just to measure using water or something :)

Also be sure to manipulate with "air in" sensor not the one that measures the air temperature that gets out of the pump (most don't have that but be sure).

It would be ideal to have optimal resistor that you could set it to appropriate temperature when you are away and also when you are there. Temperature setting on remote would be really high but that is just the trick.

Don't forget to clean the filters. And don't set fan speed to forced minimum, it will make efficiency slightly worse. Set it to auto.
And yes, just add resistor and no need for additional gadgets. Will work like OEM, except temperature settings will be incorrect :)



example. if you add 50kOhm resistor according to this graph (not yours) this will fool the controller that temperature is 3-8C colder. It is not linear unfortunately, but for ambient air you would not even notice the shift.
 
Thanks again Arnis. I may try that, though I am hesitant to fool around with something that is working.

I know these split systems are much more common in Europe and Asia. Are many ofthem sold with charged lines so you can do the installation yourself? Here in the US that is very hard to find.
 
All splits are precharged for up to 5m piping.
Also in cold countries (winter below water freezing temperature) heat pumps are not ON/OFF but
inverter (can speed up, slow down, like Nissan Leaf). Mostly additional heating cable is used so
outside part will have massive ice buildup. I recommend to use it only if necessary (manual switch).

Premium brands are Mitsubishi, Mitsubishi Heavy, Fujitsu, Panasonic, Toshiba, Samsung.
You can contact manufacturer and ask for resellers in your region.
Recommended power per each unit is something like 3-6kW of heat.
Any bigger and usually too much heat in one room. Any smaller doesn't have enough capacity.


ON/OFF pumps don't last long as wear/tear is too great while compressor oil is very cold.
Every time it switches on (and have not worked for 1 hour) there is a lot of wear.

Good pumps have COP (heating efficiency) at least 4.5 (I would say 4.0 is pretty much very good).
That means heat pump draws 1kW energy and gives 4kW of heat. 400%.
Best pumps work down to -25*C but no more. Usually cheaper models die below -15*C.
Cheap models might have COP around 2.5 at -5C, best stuff can get 250% at -15C.
All heat pumps work very well above 0*C outside :)

Price for splits in Europe starts at 500€ (cheap models, no installation) and premiums are up to 1000-1500€.
There are more powerful pumps but I would not recommend those. Better to buy air-water pumps and
connect water to central heating system. Or have two different pumps in different sides of the building.
With big buildings (many families) it is best to use ground-source heat pump. Whatever the temperature
outside, always the same COP. Even if it is 0F outside, earth is always above freezing temperatures.
 
Mine is not a big name brand, but it was the only one I could find set up for do it yourself installation; most here require that the lines be evacuated and then charged. It was a fair price (about $1,400) . It is also pretty efficient, but we measure them by SEER, whatever that is; this unit is a 21 which is fairly high. I don't know how that compares to your COP. It works fine, and is much less expensive to run than resistance heating. It also air conditions, if needed.

I have seen some MItsubishi units that are practically silent. Mine is quiet, but some of them are amazing.
 
It is better to vacuum the rest of the line before releasing the pressure into the system.
Humidity does no good in the system. At least do the piping during hot summer weather :)
Installation does cost but if you do everything yourself and only bring a technician to fill
it... I wouldn't pay more than 80€ for that. It is 10 minute job.
 
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