Mitsubishi Ecodan ASHP - DHW & Heating questions

Matchermike

Hi all,

I live in a 2019 new build that has solar, and a Mitsubishi ECODAN PAC-IF063B-E ASHP. Upstairs has radiators, and downstairs has wet UFH, controlled in all rooms by Heatmiser room stats. Upstairs has a single stat on the landing, with TRV's on the rads.

I have had several issues with the system since moving in, that the builder has tried to rectify several times, and that plumbers have given conflicting advice on, so would appreciate some guidance from here if possible please :-)

Firstly the system was left setup to heat the house based on the temp in the room on the FTC5 controller. This controller is in the garage where there are no rads, or UFH so the heating was on permanently! Big bills! A heating engineer visited, and set it to compensation curve, and advised to turn all UFH stats right up, and to lower the compensation curve offset until the rooms were comfortable. This lowers the water temp, and then we can trim at night using the room stats. We tried this and it seemed to work ok.

This though, brought issues to the upstairs rads. They were barely getting warm. Some not even luke warm. Another engineer visited, and said the water temp needs to be higher, turning comp curve off, and setting it to 55 water temp. This now has nice hot rads, but i'm concerned the temp is too high to be efficient, and that there is no compensation curve. He advised that there is a 2 way valve to UFH and 2 way valve to rads, and they both use the same water circuit, so the temp is the same for both. This seems contrary to what the ecodan manual suggests should be installed. Additionally there is no mixer on the UFH manifold.

Should any of the above be concerning? Or should i just stick with the temps at 55, and manage this? Would 2 zone water control be more efficient?

My second concern is the DHW heating. I have this set to go to 50 between 11 and 12 in the daytime. My melcloud app shows the water gets to 50 just after 11, but by 1pm has dropped back to 24 degrees. Even if no one is home. Is this normal? Why would this drop so low so quick?

I'm going to try and draw out what makes up my heating system, and the part codes and piece together how it all works, but appreciate any advice in the meantime please. It's very confusing!

Thanks
Mike

Astria

Turning the comp curve off seems to be a common installer/engineer thing to keep customers happy, although not surprisingly it does increase bills.

You should have sufficient and large enough radiators to support a ASHP, and typically go for a low temperature such as 40c and run it for much longer periods to maintain temperature rather than to "top up" like a conventional gas boiler. Radiators shouldn't get hot with a ASHP system, but rather supply sufficient heat to combat the heat loss of the property. Considering you have a new build, that heat loss should be minimal.

Remember also that the colder it is outside, the worse the efficiency of the system, so try and do any big loads such as bringing the temperature of the house higher to in the day time rather than at night.

Ceol

Matchermike said:

This though, brought issues to the upstairs rads. They were barely getting warm. Some not even luke warm. Another engineer visited, and said the water temp needs to be higher, turning comp curve off, and setting it to 55 water temp. This now has nice hot rads, but i'm concerned the temp is too high to be efficient, and that there is no compensation curve.

The temperature of the radiators is of no importance unless the rooms are cold. Were the rooms too cold? If they were, then you might need to look at balancing your radiators, but if not, then the temperature the radiators are at is fine. The nature of weather compensation curves means that radiators are often cooler than the skin, which could be around 33C. The radiators could feasibly go down to as low as 25C in the autumn or spring and still provide sufficient heat. Since they are metal, they conduct heat away from your skin very well, and so you might think that they are cooler than a different non-metal surface that you touch, but in fact they are not.

I suggest that if the temperature of the radiators is concerning you that you buy one of those cheap infrared thermometers and take the temperature of the radiators yourself. You can even buy little clip on thermometers to attach to the pipes of the radiator. You can then compare those readings to the flow temperature of the heatpump which probably appears on your controller, possibly under the name "Leaving water temperature" or LWT.

Contrary to what you might think, leaving your heat pump heating all the time on a low flow temperature is significantly cheaper to run than having bursts of 55C. This is because it is much harder for the compressor to extract 55C out of the outside air vs a lower temperature like 30C. As you have noticed, this does leave radiators feeling "cool", but 30C is also a lot warmer than your rooms which are probably 20C, so as long as you're comfortable, you shouldn't worry about that.

On the subject of whether you need different zones, this would often be the case, but if this house was designed with an ASHP system in mind, then it's likely OK since the rads will have been designed to work at the same flow temp as the UFH.

On the hot water front, I suspect there's an error with the thermometer, perhaps with its positioning. I would get that looked at if I were you. Does the water temperature actually decrease? Can you have a shower in the evening for example? If you can have a shower, then that suggests that the hot water is at least 40-45C and there's something else at play. Again, if you buy a cheap infrared thermometer, you could actually measure the water coming out the taps.

Mstty

This may help but we have the heating on 24/7 low and slow we call it keep the house at between 18-21oC depending on the solar gain of certain rooms with the Ecodan set on weather compensation and adjusted downwards due to us having it on 24/7.

The upstairs single thermostat is in the hall and set to 19oC. The radiators have not come on up there either as it has not dipped below that.

If you want to check your radiators upstairs whack the thermostat up there to 25oC