@heacol  I am sorry that this provoked such a reaction. Misinformation applies to me as well so I did warn you. I never made any definitive statements. Heatpumpmonitor is the only resource I know of that provides some insight into what is happening out there. I said that “I presume….” about Vaillant and asked where else this variability of flow would come from otherwise. Likewise, why the prevalent low flow dT’s when these are top performing systems? I calculated a notional 5C dT power to see if there was any correlation to explain this e.g. oversizing? I put this out to get comments and a possible explanation of what the trend is here and what could be learnt from it. As there has been a lot of discussion and misunderstanding about fixed flow and variable flow, perhaps an explanation as to how Vaillant systems manage to alter flow with essentially fixed pumps would be very useful or are my calculations a red herring? Are you saying that we should not take these systems as representative? We are all trying to understand what the best configurations are and what the pitfalls are. If you have detailed insight into how different brands work, or don’t, this would be very helpful to all. Case studies are all we have. Bly goed, meneer. 😊

Posted by: @heatgeek

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Beware disinformation on this forum. I took some time out to verify the statement made about all the top performing heat pumps on heatpumpmonitor.org being fixed flow.

The statement was "not a single one of the top performing heat pumps on heatpumpmonitor.org have a constant dT of 8C. At warmer OATs they mostly run with a dT between 2 and 4.", which you then confirm:

Posted by: @heatgeek

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iii) In most cases, the flow dT’s are very low (2-4C)

BTW the site can give you the actual flow rates (by switching on "Show flow rate" in the "Show detail" panel), here's a 7kW Vaillant (one of many Valliants) showing a fixed flow rate for CH and a different fixed flow rate for DHW:

@heatgeek No offense taken or inferred. The problem is, no-one actually knows but many claim to, and are disseminating unverified information as fact. This is the very reason there are so many who have such poor systems. I do not blame the installation base, I blame the manufacturers and leading training organisations for not supplying adequate training and information to the general installer to do a good job. The required MCS training is a 3-day course producing "FULLY QUALIFIED HEAT PUMP ENGINEERS 🤣 🤣 🤣 "  Open zone appears to work well on systems with delta T control, but it appears to make Valiant units work very poorly, the reason, I think, is due to the lack of Delta T control, why I surmise this? A number of leading Heat Geeks have told me that they perform much better, at part load, with a blocked filter, and at full load when undersized. When I asked them to justify this, they all threw their toys out the cot. However, (I am talking from experience and the analysis of a large number of units) they do seam to perform reasonably well with buffer tanks, very high system volume or thermal mass (under floor) and on-off thermostats, or and a very high heat curve, the reason? This matches with what I was told, when they switch on, they run hard, creating a larger Delta T and then off again.

The other thing this industry completely ignores, but is actually incredibly important, is the balancing of the system. There is no way to balance a radiator heating system accurately at low temperature and 5-10 Deg Delta T, temperature gauges/meters are just not accurate enough and with the modulation of heat pumps, the load, hence the Delta T and flow rate is continually changing. It is easier to do it with a boiler, as there is a Delta T of 20 Deg C and a flow temperature of 70 or 80 Deg C. If the radiators are not balanced, there is no chance of getting good performance, I am just as guilty as every one else. If you notice, most of the best performers on OEM are under-floor at very low temperatures. Balancing radiators is about as accurate and reliable as reading the tea leaves.

I, and others have realized these problems for many years and have been working hard to come up with a solution for both. It will not be too long before they are on the market. Hopefully they will make systems more predictable and easier to commission.

@robs  Thanks for this. The statement about 8C was not mine. It came from a Brendon quote. The flow rate shown is the current one. Are you saying that this CH flow rate is maintained at all temperatures with WC? If the flow rate does change, how does it change (what mechanism)? The historic data suggests that it does change with OAT. This is what I have been trying to tease out all along.

@heacol  Thanks, mate.

@heatgeek 👍

@heacol   I agree entirely with you about balancing radiators on low dT. I made the mistake of putting 2 radiators together with my UFH, which is the majority of the house instead of going the whole hog with UFH. I also needed 2 towel rails in the bathrooms to provide primary heating as the UFH was miniscule. I bit the bullet and put all the rads on a separate HP running at 50C together with DHW, with the UFH running continuous low temperature open-loop on a Split HP with PWM pump. Not the most economic, but head soothing.

Posted by: @heatgeek

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@robs  Thanks for this. The statement about 8C was not mine. It came from a Brendon quote. The flow rate shown is the current one. Are you saying that this CH flow rate is maintained at all temperatures with WC? If the flow rate does change, how does it change (what mechanism)? The historic data suggests that it does change with OAT. This is what I have been trying to tease out all along.

@heatgeek Sorry if I miss attributed that. Yes, the same CH flow rate is maintained at all OATs. Here's the same heat pump running at lower OATs:

And a higher OATs:

The CH flow rate stays at approx. 20.7 lpm. As @heacol said "Vaillant Arotherm heat pumps do have a variable speed pump, however, unlike the Daiken, it does not track the flow temperature maintaining a fixed Delta T". 

@robs The flow rate is determined by the capacity of the unit. If you close a valve in the flow, the pump speeds up to try to match the desired fixed flow rate. Why? I have no idea.

Posted by: @heacol

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Open zone appears to work well on systems with delta T control, but it appears to make Valiant units work very poorly, the reason, I think, is due to the lack of Delta T control, why I surmise this? A number of leading Heat Geeks have told me that they perform much better, at part load, with a blocked filter, and at full load when undersized.

Interesting

Have you got some examples/evidence you can point to.  So far my Vaillant 7kW operated 24x7 open zone no buffer tank is working out 13% cheaper than gas would have done over the same period (November 2024-February 2025), so Im happy.  But if you are right Im possibly about to become unhappy (perhaps I should block a filter?)

Posted by: @heacol

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The required MCS training is a 3-day course producing "FULLY QUALIFIED HEAT PUMP ENGINEERS

Really?  What are the prerequisites, presumably just some plumbing experience in which case thats a bit of a joke isn't it?

Posted by: @heacol

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The other thing this industry completely ignores, but is actually incredibly important, is the balancing of the system. There is no way to balance a radiator heating system accurately at low temperature and 5-10 Deg Delta T,

Im with you on that.  I cant see how any installer can ever hope to balance a system in the time they can realistically allow.  For now it seems to me householders have to do this, and also have to adjust the WC curve because that also takes elapsed days not tens of minutes.  There seems, by repute and from my limited experience, almost to be a conspiracy of silence about this, with many (most?) installers instructing householders not to touch the very thigs they have to touch to get the system working properly.  I can understand why (Im not sure I could defend the practice, but equally im not sure I could criticise it strongly), but its dodging a problem surely.

@jamespa you are right about the cospiricy of scilence regarding system balancing. There is no industry consensous, I put a post on linked in at the end of last year about it, there were approximtly 100 replies but not 1 suitable methode.

Not to worry, I have somehing coming out shortly to sort the ballanceing of radiators and I am testing a product that will sort your delta T out. If it works, I will let you know.

@heacol Of course, balancing radiators properly is not a five minute job for the installer at the end of some four or more days of frenetic work installing the whole system. I witnessed my installers tearing around my ten radiators at the end of a very mild February day when the heat pump wanted to switch off and there they were trying to get the radiators all to warm up! Luckily for them, I was happy to carry out this task myself. Regards, Toodles.