Dan F

Ours modifies flow temp based on room temp, not return temp!

This the the case with basic weather compensation. Most manufacturers have some kind of "room temperature modulation" option which wil adjust the curve based actual room temperature. This approach helps adjust for solar gain and high wind as well.

Which ASHP is this? What is hysteresis? My understanding is that with a target flow temp of 23C and a 7K hysteresis (as an example) with an open loop system the ASHP is going to run until flow temp reaches 30C and then shut down. Is this how yours behaves, or does it do something different e.g. mark-space ratio. I assume you much have hysteresis much lower than 7K? Otherwise, if flow temp does have to ramp up to 30C before ASHP shuts down, this risks temperature fluctuations and possibiity to overshoot.

It's not marketing (and I haven't seen it mentioned in marketing materials), it's a control strategy that is important to control cycling when heating load is less than minimum output. Without an integral-based control mechanism how would ASHP control cycles? Using simple hystersis would result in more temperature swings impacting efficiency and comfort AFAIU..

I know various people, including a Vaillant-approved installer, who have had to purchase and install their own replacement boards! What kind of "persuasion" was required? Is the new board already installed now? (make sure they move your cooling resistor over, and don't take it away with the old board. if you have one installed)

You don't have to disable anything, this is all automatic. We have UFH on ground-floor and Comfopost on first floor. (not a great idea as boosting MVHR to increase first-floor cooling brings in more warm air on ground floor) If you have UFH and/or fancoils on both floors there is no reason to install Comfopost no. In our case installing UFH on first-floor wasn't justified and given the cacluated cooling load was so low we felt that fancoils weren't justfied either. The comfopost was more of an insurance than anything else. What we realised living here though is that bedroom temperature can be quite important for some people to sleep well, and controlling bedroom temperature would have been very easy and effective with fancoils, but much harder with comfopost where you need to run it for extended periods of time and maintain the whole first-floor at a lower temperature.

2.8kw cooling is actually a fair amount for a very insulated house in the U.K, assuming you have shading/overhangs. Our calculated cooiing load was <1kW from what I remember. The thing is, and I didn't realise this at the time, those numbers are based on: - Water temp of 7C which isn't particulatly efficient. - Air temp of 28C But post-MVHR air temperature is never 28C assuming you keep internal temperature under control. If external temperature is 41C, internal temperature is 21C and MVHR efficency 90% you are looking at closer to 23C. And, if post-MVHR temp is 23C and not 28C cooling power is only 75% of what is quoted. On top of that, you can't really compare cooling power of Comfopost with cooling power of fancoil if you need run mvhr on boost to increase cooling, as it's cooling warmer incoming air, not air at existing room temperature. A simpler approach to stopping first-floor heating up via MVHR on all but the very hottest days is to turn MVHR to minimum during the day on very hot days. Not as effective, but lot simpler..

I have one. They are useful for ensuring MVHR doesn't gradually heat up the house when external termperature is high, but I wouldn't recommend for "active" cooling and if I was doing this again I'd definitly opt for fancoils instead. That said, because we have automatic external blinds and cooling load is very low we aren't in a rush to install fancoils and Comfopost does help keep first floor temperatures more comfortable than if we didn't have it installed. If we didn't have the external blinds though, it would be a different story. You do have to run the comfopost fairly constantly for this work though, and I've gradually adjusted circuit flow temp to ensure this doesn't cause ASHP to cycle like mad. I've put a Loxone automation in place to automatically turn on comfopost circuit when MVHR is delivering warm air to bedrooms, and boost if required.

Yes, any venetian blinds. We got them supplied with windows and control them via Loxone. Our are https://www.roma.de/, but AFAIU they don't sell into the U.K directly. Check with your window supplier as it simplifies things if you get them via then. Also, need to consider recess in construction for blind-box if you want them hidden.

Don't ask them for a price. Tell them it doesn't work as designed and you want it fixed under warranty.

It's the heat pump board that matters, so your unit has the bug and: i) The default -60º min behaves like -30º min ii) It ignores any attempt to change the value, eg. to -100º min, This is bad news for over-sized systems and for systems like ours where adjusting this makes sense. See https://energy-stats.uk/vaillant-arotherm-firmware-351-06-07-problems-energy-integral/\ I had Vaillant switch mine out FoC, but I don't think anyone else has had any sucess.

Exactly. Main reason to keep hysteresis at 7K or to keep the integral start at -60 ºmin is comfort (or any limits you need to consider for the flooring type if open loop). But in a system like ours, this isn't a concern. Using -120º will have a direct impact on the amount of cycling when demand < min power and in turn a small impact COP. Adjusting the hysteresis probably want make any difference in practice, but no harm in increasing. This brings me to another question though, do you have buggy firmware that doesn't allow this to be configured, or newer one?

Yes, thats correct. Yes, hysterisis of 7K will mean compressor comes on at 18C. You ideally want your system designed and installed to never really use the hysterisis though IMO. If your system volume is good, but not very big, it can be a good to increase the hystersis just so that when the compressor comes on (before it dials back to minumum) it doesn't hit the 7K and turn off prematurely. This does however depend on if you have a buffer, are/aren't using mixed circuit and what your emitters are though. In some cases it may not be a good idea to have water 7-15k above target even for a short period of time. In my case I have a buffer + mixed circuits so it only means the buffer gets hotter, the circuits don't. But if you had open-loop with radiators this might not be a good idea.

It targets a flow temperature and adjusts the compressor speed to achieve this, it doesn't target a ΔT. The "Compressor hyster. Heat." is not a target ΔT, but rather the maximum difference between target and actual flow temperature allowed before the compressor shuts down. In a large volume system when heat demand is low (autumn/spring) the primary (or potentially only) mechanism that decides when to turn compressor on/off is the energy integral calculation. This is because the flow temperature never overshoots enough to hit the 7K hysterisis limit. But, with a low-volume system, or one with too many zone controls, the 7k hysteresis will likely be the primary mechanism that controls when compressor turns off.

The aroTHERM works a bit differently. It uses a constant flow rate and varies delta-t between around 2 and 10K.