Peter269

Sorry to say that I didn't take it any further. The whole question of controls got too complicated, plus I couldn't find anywhere really for a HW cylinder, so I gave up. Still got my old gas boiler and Evohome system. But glad to see that it has provoked some further resonses!

A week after my first post - thanks for all the useful suggestions, and here's an update. I've been researching, reading manuals, and playing with the sensoCOMFORT simulator. I can now answer some of my own questions and I'm posting the answers here in case anyone else has the same questions. I've taken on board what others have said about the pros and cons of buffers, and I'm now thinking about some hydraulic alternaitves - a) buffer, secondary pump, TRVs on all rads but mainly fully open or fully closed. (my original plan) b) no buffer, TRVs on all rads, but with Automatic Bypass Valve and volumiser c) no buffer, TRVs on all rads bar one (probably the hall) with volumiser d) no buffer, no TRVs, no Evohome. All rads open, fed by ASHP pump open loop. They all have advantages and disadvantages, and I will have to give some thought as to which is the least worst. I might do some modelling. And here's my answers to the other questions - To run the Arotherm open loop, I will need at least a 'heat pump interface' maodule (VWZ AI). The outdoor temp sensor, DHW temp sensor, and (optional?) flow temp sensor connect here. This allows me to set most of the settings I need, including selecting a weather compensation curve (0.4 to 4.0). However it doesn't allow me to specify a heating schedule. I could instead use a 'hydraulic station' (VWZ MEH 97) which has all the same functions but includes integral expansion vessel, back-up heater and diverter valve. I don't have space for that though, so I'll be using discrete hydraulic components. To set a heating schedule, I need a 'system controller' as well, either a VRC 700 or the newer VRC 720 sensoCOMFORT. This allows up to 12 periods a day to be defined, each with its own set temperature. The 'set-back' temperature is used for all times outside of the defined periods. So potentially 13 different target temperatures per day, more than enough! Changing the target temperature modifies the WC curve up or down, so determining how warm the house gets even though there is no room temperature measurement. The sensoCOMFORT also gives a wider range of WC curve (0.1 to 4.0) than the VWZ AI. I haven't got to the bottom of exactly how the ASHP pump behaves. I know that the compressor (and fan?) can modulate and cycle based on degree-minutes, and I know from the spec that the pump can modulate between 9 lpm and 20 lpm for the 7kW model. The VWZ AI allows the pump speed to be set from 50% to 100%, or in Auto, but the only reference I could find to the pump actually switching off is a "summer switch-off temp" which can be set from 10 to 90 degrees. Why does this matter to me? If I retain the Evohome, I expect there will be times when the house doesn't need any heat so I will either have to accept the ASHP pump running 24x7x52 or find some way of switching it off from the Evohome when not required. I've not found the answer, but I'm prepared to suck it and see. So I think I now have enough knowledge to go back to the installer and discuss options with him. Thanks all

Down to the nitty-gritty. Good, this has been one of my big headaches. [it's the internal pump I'm talking about, not the house one] So the ASHP pump (a) runs continuously, or (b) definitely doesn't. Can anyone give a definitive answer, specifically for the Arotherm Plus? Thanks

Perhaps not surprisingly, this thread has focused on the merits or otherwise of buffers. I'd like to bring it back if I may to one of my original questions which was a "how" rather than a "why". Forget about the buffer. Imagine a house with weather compensation but no room compensation. On the Vaillant controller, 'Room temp. mod.' is set to 'None'. Under low load conditions - say the outside temperature is in the teens, perhaps the HP is a bit oversized or there is some extra heating on somewhere - the HP will modulate down to its minimum and then start cycling on and off. What determines the start of each off cycle and on cycle? I guess the off cycle starts when the HP sees the return flow too high (delta-t too low) so it switches the internal pump off. What then causes it to switch the pump back on? Does anyone know please?

Thanks for all your comments, and your patience. Sorry, yes a brain dump, I know. Been on my mind for a while! Not really, I understand the difference, and the Evohome already gives me fairly low radiator temperatures, with a slow warm up time. Very different to my old on/off thermostat. I think the shoulder months might be tricky with just weather comp. For instance, it's turned colder today and outside temps have been 13 rising to 18. My Evohome gave me just a bit of heat in my lounge with a radiator surface temp of around 28-30 (old rads, not yet upgraded). All the other rooms are at or above target temp so all other rads are off. If I had a heat pump with WC and no TRVs, the flow temperature would be about 28 flowing through all the other 8 rads even though those rooms are up to temperature. It might give a good COP but I can't see that it's an economical way of running. I think that's about where I would like to end up. But I couldn't do without a TRV in the lounge. The sun has come out now, the Evohome has shut down, and the room has got up to 24. Might have to open the patio door! With pure open loop, the radiator would still be trying to heat the room. Nice to hear from an Evohome user! Do you intend to use a buffer? I need to think how I would do the balancing, but I guess that would be difficult even without a buffer. You can't just open all the rads, as they will have been sized for the next available increment above the design heat loss and the margin will differ from rad to rad. At least it should be easier with a constant pressure pump. Thanks for the advice on the controls. I haven't yet got my head round all the modules and how they interact. I'll look again at the Sensocomfort. Thanks, that's exactly how I plan to work it.

(new member, first post) I’m thinking of having a 7kW Vaillant Arotherm Plus heat pump installed and have had a heat loss report and quote. I’ve seen that there are several experts on the Arotherm on this site – I would appreciate it if you could have a look over this proposal and tell me what you think Some years ago, I fitted an Evohome controller giving me individual time and temperature control over every room, and I estimate this reduced my gas bill by about 15%. The installer wants to get rid of the Evohome but I would like to keep the flexibility it gives, mainly because (a) there are some rooms in the house we don’t use except for when we have guests, and (b) the lounge has large SW-facing patio doors giving a lot of solar gain. I know I might get a small reduction in COP by turning unused rooms off, but (hopefully) it will be more than compensated by a lower total house heat loss – I’m more interested in running cost than achieving best efficiency. The way I would like to set this up is to feed the heat pump flow into a 40-litre buffer (Vaillant “decoupler”), and set up the Arotherm to run in weather compensation mode only, with no room feedback (ie. “open loop”). A separate recirculating pump will feed the radiators, with the pump and TRVs being controlled by the Evohome, as at present. The CH pump will run in constant pressure mode, so that each radiator should operate at design flow without being affected if other rads in the house open or close. If I didn’t have a buffer, I would be reliant on the internal pump, which I guess modulates to maintain delta-t rather than constant pressure, is that right? In any case, I couldn’t guarantee minimum water volume if some or all the TRVs close down. To set it up, I plan to start with a weather comp curve of say 0.8 and a target temperature of 21’, giving a 45’ flow at -3’ which is what the radiator sizes will be based on. I will use the Evohome to fully open all the TRVs, then balance each radiator to get a 5’ delta-t. This assumes that all the radiators are correctly sized for the individual rooms – in practice some may need turning down a bit to achieve the design room temps, but at least one should be fully open on the check valve. If I set the Arotherm for a 5’ delta-t as well, I think the flows in and out of the buffer should match. Once that is set up, I can use the Evohome to turn individual rooms on or off, and to stop rooms overheating when the sun shines. I might also turn some rooms down during the Octopus Cosy peak time of 16:00 – 19:00. And if we go away in the winter, I can use the Evohome to turn all the rooms down low and then turn them back up a day or two before we return. The Arotherm should just keep the water in the buffer at the correct temperature in all circumstances. I guess I will have to experiment to find the lowest heat curve I can get away with, and it might be different between the autumn when the house and ground has some retained heat, and the spring. So – some questions : • To run the Arotherm open loop, do I just need a ‘heat pump control module’ VWZ AI with an outdoor sensor, or do I need a Sensocomfort or VRC 700 as well? • In this configuration, can I still modify the heat curve up and down to a daytime target temperature with night setback, without room feedback? What settings do I need to do that, and is the night set-back just a single period, or can I set a more detailed set-back schedule to line up with the Cosy tariff? • If I shut off some of the rooms, initially some of the flow from the heat pump will flow through the buffer directly into the return, reducing the delta-t that the Arotherm sees. I assume that the internal pump will then modulate down to maintain 5’ delta-t, so the flows in and out of the buffer should equalise again, and there is no loss of efficiency. Is that true? • If there is no demand from the house – for instance if we go away – the heat pump won’t know about it, but I assume it will modulate down and eventually turn itself off. If the house then starts demanding heat from the buffer, how will the heat pump know to turn itself back on if the internal pump isn’t running? Will I need a sensor in the buffer, and how will this connect? Sorry – lots of detail and lots of questions. Am I on the right track? Any snags? Any advice will be appreciated. Thanks for reading

Hi Just a brief intro - I'm hoping to have an ASHP installed, and I'm not happy with all the details of the quote I've had so I've been working on my own design. So not strictly a self-builder, but a self-designer maybe! Once I'm happy with the design I'll go back for a re-quote with a bit more confidence that I know what I'm talking about. Background is System Engineering, so I tend to (over-)analyse systems and look for ways to improve them! I'll put my first post in the ASHP section Peter