akjos

I have a similar setup, two UFH manifolds, on on ground and one on first, same piping as you but without any mixing valves and pumps on the manifolds. The ASHP flow and return are directly piped into the manifold as the internal ASHP pump has enough power in my case to push flow through everything. If you remove the mixing valves and still don't have flow in the ground floor manifold, try closing of the first floor one and check again. Water will take the flow of least resistance, so it might be the case that your first floor manifold just passes the whole flow through, so you'll need to add resistance so water reaches the second one. This should have been calculated as design though, and if all flow rates for each loop are correct this shouldn't be a problem. Worth checking though.

Your heatpump needs around 2-2.5x the capacity in flow rate in l/min. So 9kw unit you’ll need 18-22l/min for the unit to work. Otherwise you’ll get flow rate errors. Not the individual rooms or UFH loops you can calculate by the heat output formulas gives above. But you heatpump overall needs the high flow rate to function.

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.