J1mbo

@Dan F - the system is built around ebus. Vaillant do provide the vr920 Internet gateway option but there is no published API. But, I would think carefully before going 3rd party controls. The system is designed o work with the vr700 which provides the flow curves based on outside, inside, and set point. Without this control the system will need to always be set to a higher flow temp to achieve an acceptable warm up period and hence the efficiency will be lower. Also bear in mind a significant volume of water is needed in the system circulating to avoid short cycling. hope that helps.

I don’t have PV and cannot get it due to location. The room temp modulation makes the system more responsive as it turns up the flow when there is more deviation (and vice versa, leading I suppose to lower average flow temperatures).

Thanks very much for the replies. So I was hoping there would be something off the shelf but equally I’m happy to make something. @ProDave - I have a Vaillant system with room temp modulation as well as weather compensation. This means system can overshoot unnecessarily when it’s sunny, as it will continue to heat beyond the set point, backing off gradually. So I thought it might be possible to improve it by using the manual cut input to shut it down based on certain combination of sun intensity and inside temperature. And, well basically a little project too :-)

Does there exist a device to provide a switch off signal to an ASHP based on solar intensity? Much like the sunshine sensor fitted to VWs Climatronic system.

You won't get the full warranty without the approved installer and given RHI will pay almost the entire cost, it's seems counter-productive to pass on it to me.

Quick calc suggests required DHW reheat time is likely to be the key sizing requirement for this property.

degree days

R290 heat pump, 35°C flow (would that be enough?) SCOP approaches 5. Electricity 15p so effective cost a bit over 3p. Oil 11 kWhr per litre so break-even is about 30-35p per litre assuming 90% efficiency. Although single phase, cost of oil boiler I was considering was £6k with no RHI obviously. Heat pump £14k with £12k RHI payments. RHI would pay more with UFL due to higher SCOP, but there is a cap. Look at the EPC, it will tell you estimated annual heating demand in kWhr, then you can ascertain the heat pump size from that as a starting point and associated heating energy cost. Supply upgrades: What do you need to upgrade | UK Power Networks IIRC cost for me was about £10k, it wasn't needed in the end. With stone walls it will need a lot of energy. Maybe it will need multiple heat sources, some nice log burners around the place in addition to UFL. Quick calc suggests 150m2 would need 55°C flow to provide 16kW, for example.

You might find that the DHW is not material enough to bother with timed programmes, relative to the convenience of having it always it always hot.

If you go Vaillant then the weather and room temperature modulation will be key to getting the systems to perform - so go with their controls, exclusively. But this only reinforces the need for diverted control on DHW - it will back off the flow temperature as room temperature set point is reached. Right now I’m seeing 31*C flow to the rads for example. That’s just not going to work with DHW.

Certification is required to work on the unvented side (even though the ASHP cannot actually boil the water, though the immersion clearly can) and the system needs glycol. The other issue is that the system only gets 7 years warranty if installed and serviced by an approved installer.

Ask the Vaillant people who designs their systems. It may well be Vaillant themselves, which adds some peace of mind. Ensure the installer you chose has the appropriate Vaillant certifications to get the full 7 year warranty on the system. Although they have their own cylinders, any heat pump cylinder would do - the coil surface area is larger than standard cylinders. Typically 3m2 or more. If you're in the UK and looking at RHI, it's worth playing with the design flow temperatures (even if this means changing some radiators) as lower flow temp = higher COP from the MCS product performance tables = higher payments.

It sounds to me like it's end of life.

If the compressor is fixed speed and the flow rate is at least as fast as yours and the return temperatures are the broadly the same then the compressor hot side should surely heat up at the same rate if the compressor is within spec. Is there any diagnostic menu giving the refrigerant circuit pressures, high pressure in particular?

Obviously the DHW will only ever reach the HP flow temperature so the HP in that config will surely have to work on a fixed flow temperature basis and that will cripple efficiency.

It sounds like an odd solution. Usually the DHW and CH will be controlled with a diverter valve because the required flow temperatures are very different. There will be quite a step across a heat exchanger and buffer but this will vary with load. When continuously heated to a steady state and with the pump speeds all balanced it might 5°C or less.

Probably your Vaillant designs have a small c.45 litre buffer in them. Drawing 5K from it is only 0.25kWhr. In any case, the system will not run the CH pump whilst serving DHW.

Hydraulic separator is a plate heat exchanger. It has the advantage that the radiator/UFL circuit is independent of the ASHP, so any maintenance on it doesn't mean refilling everything with expensive glycol. The disadvantage is that it will create a flow temp step across it, i.e. the ASHP will need to product 3-5°C warmer flow than without it. Buffer tank requirements are set by the ASHP vendor and are there for defrost. Again, they will create a flow temp step across them when plumbed in a 4-pipe configuration. DHW demand will likely impact your choice of output. The heat pump provides no central heating whilst it's warming the cylinder.

If it was working fine, and now isn't and nothing has changed, perhaps the compressor is simply end of life? The DHW would continue to be heated (more slowly) as it's a closed system, when nothing drawing, whilst if the effective output is lower than the heating circuit loss then it will never heat up fully. That said it looks like it is reaching target, I presume the 31°C in brackets? As the energy integral is -57° minutes.

Ventilation losses are probably another 3kW or more for a dwelling of this size. The fabric might not perform to spec, especially if it's dot-and-dab on thermalite and not sealed at the top. DHW may well be using 750 - 1,000 litres per year.

Take your target flow temperature, subtract the room temperature, that gives dT. Then use the formula. e.g. 45°C flow and 21°C room temp, dT = 24. Power output per section = 0.80314 x (24^1.32266) = 53.75 Watts System comprised of 63 sections therefore total output = 63 x 53.75 = 3,385 Watts. Plus the two towel radiators, probably 100 W each. If the flow temp were increased to 50°C then the power output would increase to 4.3kW. At 70°C flow, it's about 9kW. So currently you are heating 9kW x 4 hours = 36 kWhr per day. This is an average load of only 1.5kW. With 3.5kW running 24x7, the house will have a higher average temperature but probably a lower peak temperature everything else being equal, because the total energy added per day would increase from 36 to 84kWhr. Re buffer - the system needs enough capacity to avoid short cycling at part load and enough available circulating water to provide for the defrost cycle. These radiators probably have about 0.6 litres per section so system volume will be only 40 litres plus the volume in the pipes plus the towel radiators. A 9kW heater will heat 200 litres from 40 to 45°C in about 12 minutes (assuming 3kW losses from the radiators in the process) so a buffer of something like 150 litres is probably about right. You could try running electric heaters totalling 3.5kW for a couple of days 24x7 instead of the wood and see how warm the house gets. But the heat load increases as the temperature outside decreases so that needs to be factored in too.

Probably Tropical95 rad elements based on the pic.

For completeness, here's a part-throttle reading: A5.5/W42 - cooling 3.8kW, consumption 1.2kW, Compressor load 27.2%, COP 4.1. Building flow 1980 litres per hour. The HP shows the evaporator cooling and compressor consumption metrics. COP I've calculated as (cooling+consumption)/consumption. Obviously there will be some losses in the compressor so maybe 80 or 90% of the compressor consumption ends up as heat in the water... I don't know.

Heating mode yes

It was 8°C here this morning so I took a couple of readings from the HP interface: A8W55, cooling 8.1kW consumption 5.2kW, COP 2.6 A8W42, cooling 11.6kW consumption 5.3kW, COP 3.2 assuming zero losses from the compressor... so a bit less than that.