J1mbo

The 1.22kW in 24 hour cylinder will lose less than 3K in 12 hours. The issue with ASHP in my experience is more related to the DHW priority, which can leave extended periods of time with no heating in the mornings when everyone is using the shower. I’ve solved this personally by heating the cylinder via the “legionnaires” programme on E7 overnight - to 70+ degrees - then leaving DHW “off” until lunch when the ASHP is asked to reheat it to 48 degrees. Using the Bulb 5p overnight rate would make this even better but even with normal E7 the reduction in COP works out neutral cost wise and ensures I have heating in the morning.

You need to know the capacity of both.

To add, if you have Hive TRVs on the cooler radiators, take them off. I have seen a couple of electronic TRVs that fail to open valves properly.

So is the system set to a fixed flow temperature of 45°C then? And simple on-off switching?

Does the Hive specifically have a configuration for ASHP? I would suspect this is the main problem. I would suggest standard TRVs upstairs and valves fully open downstairs. Whatever control you have make sure that weather and room temperature modulation are enabled. Any control with a TPI algorithm will mess up the heat pump. To balance the system, close the lock-shield valves of all the radiators that are fully warm half a turn and see how that goes, and repeat.

Take the phone dB meter with a big pinch of salt. It will be noisier when colder as the compressor is going to be working at higher output as well as the fan speed most likely.

In simplistic terms, can plumb the existing to a plate heat exchanger and the AHSP on the other side of it. That way the two are separated meaning there is less glycol needed (only between the ASHP and the heat exchanger) and everything else can stay as it is. When work is needed on the house side, it can also be drained and filled as normal. The downside is that the heat exchanger creates a temperature step so costs a little bit of efficiency.

Due to the varying flow temperature requirements the simple answer is: it depends (weather and room temperature compensation means everything is continually varying). I suppose I need to spend some time measuring.

Yes as it will lock-out if the primary circuit pressure is below a threshold. A buffer tank is a tank without a coil - the only downside will be cost to fill it with all that glycol.

The higher flow rates requirements comes from the smaller dT across the radiators that is sought - gas/oil systems typically specify 20°K across the rads, whereas heat pump wants 5. All else being equal, the flow rate is quadrupled. In round numbers, 1m3 per hour for each 6kW of output. Typically we want the water velocity less than 1m per second in the pipes, so more about 6kW then best to have 28mm flow from the heat pump and branch to 2x 22mm circuits to the radiators (e.g. upstairs/downstairs might be convenient).

Setup: 12kW Arotherm Plus, Plate Exchanger, Buffer Tank, Radiator Circuit with UPS3. Radiator side has pump on fixed speed 2, seems to work OK with ~5°C drop across the circuit when everything is calling. I'm not sure how to set the other two pumps. The ASHP 'auto' simply runs it at 100% providing 2,048 litres per hour through the plate heat exchanger. Whilst rated at 12kW, the heating demand is more like 9kW which would indicate a flow requirement of 1,500 litres per hour. But, when the compressor starts it runs at 45% for three minutes before adjusting to load and this can already result in overshoot mid-season which in turn causes shorter cycling than would be preferred. The heat exchanger pump has three fixed speeds. There's very little resistance (28mm piping to a buffer cylinder next to it) and no way to monitor the flow. I'm wondering if there are efficiency gains to be had by tuning these two pumps and if so how!

What controls are you using with it?

There are three modes - Eco (45% on the 12kW but that may vary with the actual unit fitted) - achieves about 3.8 at 48°C; Normal which simply runs 100% until satisfied (achieves about 2.5 but offers much shorter recovery times); and balance which runs 100% until the flow temp is satisfied (cylinder set point plus 5K) then backs down the compressor to reduce overshoot. The latter extends heating time of a 250 litre cylinder by about 20 mins in my experience and achieves a COP of maybe 3 or so. The system is able to recover surprisingly quickly with enough coil area in the tank. For example when the water is cold the 12kW ASHP will deliver about 17kW into the tank, which reduces as the cylinder temp and hence flow temps increase. HTH

Wall-Mounted 40 Litre Decouple Module | Vaillant UK

Do you know the approx volume of water in the system? The DHW tank dropping could be the diverter valve passing, so that the system is drawing heat from the cylinder. The system pressure dropping is a leak. The expansion vessel looks like maybe 18 litre which should be easily enough for 250 litres at these flow temperatures provided it's pre-charge has been set correctly. If it's too small the system pressure will rise dramatically as the system is heated. The uneven radiator temperatures is insufficient flow. Increase pump speed and ensure both valves of affected radiators are fully open. Note than it's normal for the centre of large panel radiators to feel somewhat cooler. What controls are you using? Anything introducing short-cycling or limiting the volume of water circulating will dramatically impact performance.

I don't know how you found that (I spent ages trying) but I thank you for it!

I do run legionnaires cycle with my R290 ArothermPlus once a week and heat the cylinder to 45/48C otherwise. This is because the DHW cylinder is installed in an open vented configuration with cold water tank in loft. Looking at the consumption data the cycle uses about 1kWhr more on average than I use on all other days. The DHW mode is set to “normal”, which is Vaillant terminology for full power (ie shortest recovery time).

I noticed that the manual for the 12kW ArothermPlus calls for 25A “type c/d fuse”, as well as type A RCD. c25 breakers aren’t very common (MK discontinued theirs some time ago for example) so wondering what others are doing here.

It’s done on the EPC numbers. Play with the design to get lowest design flow temp possible as that will increase payments based on the manufacturer performance number (SCOP).

Glycol is antifreeze, the system should be protected based on local conditions. Eg in the Uk, to -18C. The heat exchanger isolates the water in the radiator circuits from the water in the heat pump and cylinder coil. This means that the internal system can be worked on without having to refill glycol as only regular inhibitor and biocide are needed in the radiator circuit. This advantage is at the expense of a few K drop across the heat exchanger.

Might be worth checking that the serial number on the account matches the one on the meter

Ok thanks, not sure I really follow the correlation.

I don’t know what that means.

Am using Ambisense now. The VRC700 controller is however the only reference point for room temperature modulation so that needs to be cited in the typically coolest location. There looks to be some work going on with Ambisense with home assistant so perhaps some bridge will become possible through that.

@gavztheouch - yes got the 12kW. Bear in mind there is work and overheads involved for the MCS scheme in the installers side. In my case installation took quite a while so whilst the quote was similar, it ended up being good value in the end and especially since the RHI scheme pays back almost all of it. System is quiet definitely. It doesn’t seem to meet it’s quoted numbers - looks like it will be about 20% more in the end. My concern with a 7kW unit would be hot water recovery time, that’s not a lot of power especially if you’re planning to use the (default) eco mode for DHW which is about 45% output.