akjos

As JohnMo said, many variables to take into account, which means nobody will be able to tell you what to set your heat curve to without knowing the details. The simple answer is: Don’t fix it if it ain’t broken. The more complex approach, if it isn’t working properly right now or you want a better performing system: 1. Figure out your heat loss. Preferably room by room at certain outside design temp. (Example: -3C). 2. Get the data sheets for your radiators and calculate what temperature needs to flow through them with a delta T of 5C to achieve the heat loss from 1. 3. Look at the Grant manual and see the heat curve lines. Find the one which gives you the needed flow temp at the design temp. 4. That’s the curve which will work best for you.

What you're describing is exactly what is being used in commercial settings for many years: VRF Systems, i.e variable refrigerant systems. You'd have experience them most probably in hotels and similar. Essentially there are big condenser units and then there is a main trunk with both hot and cold refrigeration lines branching to each unit. The cool thing is they can even balance themselves out, i.e if one unit needs cooling, and another heating, they can work between each other as condenser and evaporator and not use the big outside unit. Really cool stuff, but overkill for residential for sure.

When you re-pressurised it, did you bleed all air out? Make sure all air valves are open as trapped air can also cause flow rate issues.

From a theory and calculation perspective I think your conclusions make sense. But in reality this setup would be unpractical. If I understand correctly, based on your following statement: you have modelled the system to receive 45'C flow on both Rads and UFH. Your UFH return is 20'C because the flow rate needs to be very low to only output 2.5kW. UFH has a huge surface area, and feeding it 45'C it makes sense to only require a tiny portion of the overall flow rate. But in reality, you don't want to feed 45'C to UFH and have a 20'C return. There will be big fluctuations across the floor, cold spots, hot spots and flooring most probably wouldn't support it. UFH should be at 5-7'C delta T to be comfortable, that's why the 'more conventional' setup has a separate pump and mixes the flow. Ideally we'd run the whole system on a lower temp, oversize rads and have everything running on a lower flow temp with both rads and UFH giving an acceptable delta T.

From what I’ve read online, Hitachi are one of the best. I was considering them but found a really good price on a Midea unit and got that one instead. If price is good, I’d definitely go for the Hitachi, they’ve been around forever..

@NightMail are you still seeing high usage of the immersion heater even with warmer outdoor temperatures? It's absolutely normal for your COP to fall in the colder months.

There is usually a setting in the ASHP unit where you can specify under which temperature conditions the booster heater should be used for DHW. This is usually meant for very low outside temperatures, where you don’t want the ASHP stopping your central heating in order to heat DHW. So the unit can use the booster heater in those situations and not have any down-time in your central heating. My Midea unit has these setting in the controller, though I’m not sure how the Mitsubishi is configured, or if it is configurable at all.

From what I’ve seen they’re really good units for a good price. Pretty much all in one nice compact package on the monobloc artic series. Contains also an expansion and pump inside the unit. I’ll be getting an 8kw mono for my house deep retrofit soon. Check this out as well: Also here are some technical specs: https://www.microwell.sk/assets/uploads/matrix/files/downloader/TM_Midea_A_series_Mono_M-Thermal_Heat_Pump_R32_2020513_V1_7.pdf

How are you going to distribute the heat? UFH, rads something else? What flow temperature was it designed at? also how big is your property? If your heat loss is 3.6kw, a 11kw unit seems unnecessarily big. Anyway it’s worth knowing your whole design.

Water expands as it heats up, so you definitely need an expansion vessel both on your heating circuit and your DHW. Just search for Robokit online.

For the heat pump itself it’s no issue at all to switch between heating and cooling as much as you like. That’s exactly what it does during defrost in winter constantly. It switches to cooling and runs hot gas outside to melt frost on the coils, then back to heating. As @jack mentioned, you have to pay attention inside where you might have condensation. But if you use fan coil units for cooling, those usually have their own condensate drain.

If I remember correctly, Samsungs don't have an internal pump. So you need one pump external to the unit which will run your system. I'm not sure about the latest generations though. You can easily check it by popping the lid and seeing if there's a pump in there by the water heat exchanger. Flow rate is the most important thing of a heat pump and each needs about 2x-2.5x the capacity in l/min. So in your case (8kw), you need around 16-20l/min flow rate running always when your heat pump is on. In a simple system (no buffer, no LLH etc), you need to make sure all your UFH loops are open and the total flow rate across them matches your required 16-20l/min. If you have zones that close, you will need either a bypass valve to make sure you have the required flow when zones close, or do hydraulic separation with a LowLossHeader. But with the hydraulic separation you will need a second pump as you'll have primary and secondary circuits then. Also, check out Graham's playlists on Samsung fault codes and how to resolve them: https://www.youtube.com/@myheatpumpchannel7577/playlists There are a few on flow rate as well.

Makes sense. Microbore has a high pressure drop due to the small pipe diameter (meaning your pump needs to work harder to get the required flow rate through) so you need the additional pump. Your installer seems to have done the correct thing to hydraulically separate your circuits with a LLH, as otherwise the two pumps would work against each other and could lead to premature failure.

Some monobloc heatpumps have a pump inside the unit (ex. Midea), but others don't and need an external pump (ex. older Samsung, not sure about the newest gen). Pressure drop across your whole heating system needs to be calculated, but if it falls within the capacity of the pump head, you can run the whole heating from the one pump (be it inside the unit or outside). BUT that's only the case if you don't have any 4-port buffer, LLH or CCT which create hydraulic separation. If you do have any of 4-port buffer, LLH or CCT, you essentially have a primary and secondary circuit, then your circuits are separated and each needs its own pump. Primary circuit is from heatpump to 4-port buffer, LLH and back, and secondary circuit is from the separation to your heating emitters and back.

It sounds like your calculation as to heat demand match those on the MCS as well as the Heat pump estimates. So there shouldn't be any issues there... As for the electrical consumption and COP, that's a whole different story and depends fully on how your system is set up. As @JohnMo already mentioned: What system components do you have, how is it plumped, do you have enough system volume, defrosts, run times, flow temps, immersion etc etc etc. You could set up OpenEnergyMonitor to track a lot of this and then calculate the COP. As far as the advertised COP from the manufacturers, I think that's a momentarily value. Meaning your heat pump has started up and is running a constant flow temp of 35'C at 7'C ambient for example. If you checked your COP at that specific time, it would probably match. But if you take it as a whole day it won't due to all the other factors mentioned above. Except maybe if you have a system which is set up to perfectly match your heat demand and run 24/7 with all being one zone, no buffers, no HW. More realistically you need to compare to the SCOP (Seasonal COP) over a longer time period.