Posted by: @ianjrw

Hi, new here and looking to get ASHP installed asap.

What should i be asking potential re on-off thermostats? Seems that those we've spoke to are looking to install Honeywell or equivalent...

Thanks in advance for any insight.

Hi Ian,

Welcome to the forum.

I would advise against having any form of thermostat (On - Off device) with an ASHP. For most efficient operation I would suggest having the heat pump manufacturers control system, which should then be configured in weather compensation mode, ideally with modulation.

@ianjrw As mentioned above, can you ask your installer about using the heatpump's own controller?  What brand of heatpump are they suggesting?

Let me draw an analogy.  Imagine you want to drive at exactly 70mph on the motorway.  You could do one of several things:

1. Set your car's cruise control.  The car will sense that you're going uphill and need a bit more power, or going a bit fast and back off the power.
2. See you're going slowly and stamp on the accelerator.  When you're going too fast, stamp on the brake
3. Like 2, but switch from accelerator to brake every 5 seconds, deciding how much of each you're going to need

#1 is using the heatpump's own controller - a nice smooth, efficient ride.  #2 is like a traditional on-off thermostat, eg a mechanical one.  It can be quite jerky when you switch from going full power to brake.  #3 is like Honeywell TPI: it seems slightly more comfortable (you can't feel the jerks so much) but your engine really doesn't like being pulsed like that and won't run very efficiently.

Now, analogies are never perfect but the idea here is 'brake' is like the heatpump spinning up and down - it has an energy cost that doesn't contribute to heating your home.  So we want that to be as gentle and infrequent as possible.  The heatpump isn't actually as bad as #2 and #3 because it can get some feedback from weather compensation (how cold it is outside) and flow return temperature (how much heat your house managed to take) so it can compensate to some extent even with on/off control, but there is still somebody else doing the driving so it doesn't have free reign to make the best decisions.

That said, using an on/off controller can make sense if used like the ignition key on your car (today I'm not driving, so I don't even start the engine; please make hot water 4-6pm).  But it's better not to use it for speed (temperature) control if you can use the heatpump's own controller instead.

Thanks Alec, Derek and Prunus. Thats helped me get context to address the issue directly.

@prunus sorry an on-off control on a heat pump should never be used, or included in an install.

the controller should always be used to put  a pump into standby so it can run its own “no heat demand” protections.

For large houses where regularly unused rooms exist, and  zones of UFH the heat pump manufacturer should be able to supply mixer stations and sub zonal controls.

just because something appears to work doesn’t mean to say it is right, or won’t have an negative effect somewhere 

@alec-morrow I agree, but there are two different things people use controllers for.

One is temperature control, ie the core control loop of when to call for heat based on room temperature.  As you say, much better to do that via the heatpump controller.

But the other is other system control - ie should the heating be on today or not. That might be controlling when to run the hot water, when to turn off the heatpump because you're going away, that kind of thing. Some heatpumps have relatively primitive controllers in that regard: you can set 7 day programmes but you can't  tell it to run the hot water because electricity is cheap for the next half hour, or to bump the temperature because you've just landed at the airport back from holiday, etc.  As I suggested above, these are more like operating your car ignition key or setting your cruise control and letting the car do the rest.

Multi-zones and TRVs is another area.  My Chofu/Grant has native support for two zones and one mixing valve - that's it. If I had a multi zone UFH setup or smart TRVs I'd want something to control them. It isn't in the heatpump temperature control loop, but it is the equivalent of a human going in and twiddling the TRV - which is something the heatpump is never going to do.

Basically, if I set the temperature to 18C I want the heatpump to get on and do it - I want to be nowhere near that control loop.  But sometimes I might want to decide to change that number based on factors that are beyond the realm of what the heatpump knows about, and an external controller should allow changing that number without me standing in front of the unit pushing buttons.  Not all heatpumps allow that (mine doesn't).

Some of the smart controllers like Nest may be able to do those things (I know Home Assistant has 'climate' controls for talking to a range of heating/cooling appliances and reading/setting temperatures).  But if you're wiring them as a classical on/off thermostat you're doing it wrong, which is why you have to ask the installer carefully how they're going to wire it.

@prunus a decent controller combines both. i suspect all heat pump controllers have a setting deferring to outside temperature to shut it down completely above, by default, an outside temperature of 21c

@derek-m @prunus Hi, I've got my 8KW Midea in and running and have set my weather comp to 45@-2 & 25@17 - seems to be coping admirably with the two frosty nights we've had here and is toasty the rest of the time. It seems that the Heat Loss was a bit aggressive for upstairs as we've got 20C downstairs with rad thermovalves on full, whereas we've got 21C upstairs (specd at 18/19C) with rad thermovalves set just over half. All rads are at least 100% wattage of HL survey spec.

We currently don't have the room stat (Honeywell) plugged in - we're seeing how we get on with just the weather comp.

Q1: Is it OK to use the rad thermovalves to control upstairs temp in this way - does it restrict system flow or anything odd? ANy energy usage implications?

Q2: If only using weather comp (no stat) what happens when temp outside rises to a temp that we dont need heating e.g.+19 degs? Does the ASHP just not kick in for heating?

Thanks for any ideas/help!

Posted by: @ianjrw

@derek-m @prunus Hi, I've got my 8KW Midea in and running and have set my weather comp to 45@-2 & 25@17 - seems to be coping admirably with the two frosty nights we've had here and is toasty the rest of the time. It seems that the Heat Loss was a bit aggressive for upstairs as we've got 20C downstairs with rad thermovalves on full, whereas we've got 21C upstairs (specd at 18/19C) with rad thermovalves set just over half. All rads are at least 100% wattage of HL survey spec.

We currently don't have the room stat (Honeywell) plugged in - we're seeing how we get on with just the weather comp.

Q1: Is it OK to use the rad thermovalves to control upstairs temp in this way - does it restrict system flow or anything odd? ANy energy usage implications?

Q2: If only using weather comp (no stat) what happens when temp outside rises to a temp that we dont need heating e.g.+19 degs? Does the ASHP just not kick in for heating?

Thanks for any ideas/help!

 

Hi Ian,

Since heat rises there is always the possibility that upstairs rooms could be warmer than those downstairs. I think that the accepted norm is to use TRV's to control upstairs temperatures, which in turn should reduce energy consumption to a degree. Because of changing weather conditions, with effects such as solar gain or wind chill, it may be possible to control indoor temperatures with precise accuracy, but correctly adjusted TRV's should help to keep temperatures within acceptable limits.

As the outdoor temperature increases, the heat pump, when running on weather compensation, should modulate the output energy to meet the present demand. There will be a point where the heat pump cannot pull back any further, so that as the indoor temperature increase, the heat pump will start to switch on and off. When no further heating is required, the heat pump will remained powered, but will not run unless there is a demand for hot water.

@derek-m thanks Derek. I've not got to know the Midea controller yet but should there be a way to just 'turn off' the Heating? e.g. when i go away on my 4 week 😉 summer hols?

Posted by: @ianjrw

@derek-m thanks Derek. I've not got to know the Midea controller yet but should there be a way to just 'turn off' the Heating? e.g. when i go away on my 4 week 😉 summer hols?

Hi Ian,

I have attached a copy of the Midea manual, the password for which is midea2020.

There is a holiday mode shown on page 92. The alternative would be to switch the unit off.

@derek-m perfect! ta.