Midea Heat Pump expensive to run

matt_drummer

Here's how it works in the simplest of terms.

The heat pump is just a source of hot water, it does not need to know the temperature inside the house.

When running the heat pump will heat water to a set temperature and pump it around the radiators.

It will continue to do this until something tells it to stop.

As long as the water is coming back to the heat pump cooler than it left it the heat pump will continue to run.

The heat delivered to the house is a factor of the volume of water and how much it has cooled down by when going around your house.

If the water comes back too close to what it left the heat pump at then the heat pump will stop as it can no longer add heat.

The temperature of the water, how fast it flows and how much it cools down determines how much heat is delivered and consequently how hot the house gets.

If the flow temperature is too low the house may not get as hot as you want and/or the radiators may not be able to get rid of enough heat.

To get warmer you need to increase the flow temperature.

To get cooler you reduce the flow temperature.

Heat pumps can automate this by having different flow temperatures for different heat losses, that is different outside temperatures.

This is generally known as a weather dependent curve, basically an automation of the flow temperature that you find by experimentation.

Your Hive thermostat is working as a temperature limiter by telling the heat pump to stop working when a set temperature is reached.

It also tells the heat pump to start heating if the thermostat is below the set temperature although there may also be timings set within the heat pumps own controller that only allow heating at certain times of the day.

Different heat pump manufacturers use different names for the weather dependent curve, read the Midea manual.

Different heat pump manufacturers also use various methods of operation varying heat output based on either flow temperature or flow rate.

Ideally you want all the radiators working with no trv's in operation as you need the biggest radiator volume to allow the lowest flow temperatures and therefore cost.

But unless all the radiators are the correct size for the room you may find that not all rooms are at your desired temperature, some may be too hot whilst others may be too cold.

In order to run at the lowest flow temperatures you will need considerably bigger radiators than you need with gas or oil.

A radiator that gives out 1kW of heat at a flow temperature of 70c and a room temperature of 20c will only produce about 100W of heat at a 30c flow temperature with the same 20c room temperature.

Running the heat pump at too low a flow temperature will mean that it cannot get rid of the heat produced and it will stop.

Running at too high a flow temperature will cost more to run than you need to.

I suspect the previous owner of your house was running the heat pump like a gas or oil boiler, running at a higher than necessary flow temperature and using the Hive thermostat to control room temperature and cost.

This is quite common as new owners often see the instantaneous electricity cost of running at too high a flow temperature, panic and then limit the run time and turn down the room temperature in fear of huge running costs.

My heat pump never goes above 32c flow temperature and runs 24 hours a day.

The most electricity I have ever used in one day is 24kWh and that includes heating DHW in a tank.

My house if four bedroom detached and is pretty much always at least 22c downstairs and 20c upstairs

Painter23

Thanks a lot, Matt. I've learned more from your post than I've garnered in the last few weeks.

So it sounds like in order to keep the system running for longer at a 'comfortable' temperature I need to reduce the flow rate from the usual 40 or 55 (weather compensation) and then let the Hive thermostat detect when the set point is reached? If I'm finding 17 degrees too warm I guess I just reduce the flow rate and/or the Hive thermostat.

Is it likely I am using more energy by having it come on and off even if only a few hours a day because it's working harder to get up to the right temp? It sounds illogical that keeping it running all the time might actually be saving money but I would need to reduce the flow rate to do this I think or I'll boil !

The previous owner was definitely using it as she would a boiler - on for a couple of hours in the morning and evening.

What I'm unclear about is why the installation company are saying I should have a seperate wireless thermostat that would have been installed with the Midea unit rather than Hive (even though the previous owner says one of their engineers installed Hive for her!). I'm desperately trying to get them out to see it and give it a service.

benson1980

I wouldn't be desperate to get airsourceuk out to see it and give it a service if I were in your position. The fact that you have a hive controlling it, is concerning, and they sound like they generally don't really know what they are doing anyway from what you are saying.

There is no other wireless thermostat that comes with the midea (assuming you don't have a rebadged midea- lots of other makes rebadge them such as baxi, bosch, clivet) and thus essentially you have three options:

- control as you are doing now with a basic on/off third party thermostat (not preferable)
- pure weather compensation whereby heat pump will add heat to your house to match the heat loss based on outdoor temp. No thermostatic control. Can be very effective.
- third party control option designed for heat pumps- these will provide some room influence but won't operate in an on/off fashion. The only two I am aware of that are compatible with midea are homely and passiv. Homely is well proven; passiv less so. Homely comes with some downsides but some people really like it.
- edit. I should have added a 4th option which is to connect via the modbus, and use home assistant or similar to monitor and control. I have custom curve settings that will work perfectly 95% of the time, but in certain conditions I find I just need to bump the curve up a degree. You can basically set something up to do this via automatations. This is what something like homely might do for you, using its algorithms. I use a standalone tapo indoor temp sensor (around £20)

Generally it isn't really sustainable to operate on the basis that you won't touch any of the serviceman settings on your own heat pump as the new homeowner- not least as a lot of heating engineers won't have a clue either. It is very simple to set up a curve and there's a decent youtube video on how to do it. The midea has 8 'default' curve settings but the one you will want is the custom one which is 9. This is where you'd have to try and see what works for your house, and heat loss and tweak it over time, but we can certainly give you a starter for ten on what values to begin with.

One important thing with the midea. When you go into serviceman settings and come back out again it turns everything off. Thus you just need to turn something back on again once you come out.

matt_drummer

Terminology

Flow rate is how much water, litres per minute or per hour usually.

Flow temperature is how hot the water is leaving the heat pump, sometimes called leaving water temperature.

Return temperature is the temperature of the water going back to the heat pump.

You will either have glycol in your water or anti frost valves for protection against freezing.

Anti frost valves will be close to the exit point of the two big pipes from the heat pump.

The heat pump should also have a frost protection setting where it comes on when it needs to, mine is set at 5c outside temperature.

You need to make sure it is all working, you don't want it freezing.

You can take the hive out of the equation by either disconnecting it or by setting it to something like 30c.

You don't need it right now.

You could just set a fixed flow temperature of 30c and see what happens, you are not living there.

Do this either by flattening the wdc or changing the mode to fixed lwt

wdc = weather dependent curve
lwt = leaving water temperature.

If you do this you will get some heat, maybe not enough to live in but enough that it doesn't freeze and you won't spend too much on electricity.

Read the manual.

When you start living there you need to establish what sort of lwt you need to keep the house warm enough.

You will probably have other issues like rooms too hot or too cold but they aren't so difficult to deal with.

But first you need to find out what flow temperature you need and then fine tune later with setbacks and  schedules.

I don't use a room thermostat at all but I do have one that is quite smart, I just don't like one aspect of it.

I have a Daikin heat pump and my thermostat can control the heating by modulating the lwt from the heat pump to keep the temperature in a narrow range. It's idea is not to turn the heat pump on and off but to control the heat output.

Your Hive thermostat cannot do that, maybe a Midea one can?

benson1980

matt_drummer said:

Your Hive thermostat cannot do that, maybe a Midea one can?

I'm assuming the midea control panel in the OPs photo is in the airing cupboard or similar. The Midea doesn't have anything like the Vaillant sensocomfort or madoka.

The benefit of the midea is that it has a modbus connection in this wired controller thus opening up some decent options which can be very easy to set up.

The photo of the controls is obviously showing an outlet flow temp of 40C when 14C outside. My educated guess- there is a curve set up by the installers which is too high, and they limit overshooting with the hive. I'd expect to see an outlet flow temp of low to mid thirties at this sort of OAT, for 90's house with average insulation perhaps. A pretty lazy commissioning by the looks of things, hence why I wouldn't personally be rushing to get them back to tweak any settings.

Netexporter

Just had a skim read back through the thread and I can't see any reference to the capacity of the heat pump. Perhaps the OP could let us know what size unit it is, as if it is grossly oversized it will never run efficiently.

Painter23

Hi guys. Thanks again.

The heat pump is:
Midea R32 Monobloc Heat Pump
MHC-V12W/D2N8-B
Rated input 5500w
Cooling capacity 12.00kw / 3.95
Heating capacity 12.10kwh/4.95

My only reason for getting the installer back to look at it is partly as there is a live warranty with them and Midea (the pump was only fitted in Oct 2024) so I wanted to take advantage of the engineer who fitted it still being around and able to articulate how and why.

House size is about 1860sqft.

benson1980

Painter23 said:

Hi guys. Thanks again.

The heat pump is:
Midea R32 Monobloc Heat Pump
MHC-V12W/D2N8-B
Rated input 5500w
Cooling capacity 12.00kw / 3.95
Heating capacity 12.10kwh/4.95

My only reason for getting AirSourceUK back to look at it is partly as there is a live warranty with them and Midea (the pump was only fitted in Oct 2024) so I wanted to take advantage of the engineer who fitted it still being around and able to articulate how and why.

House size is about 1860sqft.

Ah fair enough. Sounds reasonable. And ASHP size sounds reasonable as well for house age/floor space. Similar to ours and we have a 12kw, with a heat loss of around 8kW at -2.

QrizB

Painter23 said:

Rated input 5500w

So, if running for seven hours a day, that's a maximum of 38.5 kWh.

Yet you're being charged for closer to 45kWh a day.

Have you had a chance to check your bill yet?

Is eg. one of your tradies using your house electricity supply to charge their electric van?

matt_drummer

QrizB said:

Painter23 said:

Rated input 5500w

So, if running for seven hours a day, that's a maximum of 38.5 kWh.

Yet you're being charged for closer to 45kWh a day.

Have you had a chance to check your bill yet?

Is eg. one of your tradies using your house electricity supply to charge their electric van?

It has been cold, especially at night.

The heat pump will probably run on its frost protection setting.

Mine will run at 5c or less outside even if I have turned the heating off.

I would probably start there rather than accusing trades people.