COP expectations for 7kW vaillant

Hardware Heatpump
1

Hello,

Had Vaillant AT+ 7kW with buffer installed a couple of months ago.
6.3 kW heatloss in design (actual seems to be around 4.5 kW)
50C design flow temp at -2.9 with SCOP 3.65

All radiators were replaced to oversized (microbore 15mm only to radiators), around 10m run from ASHP to cylinder and buffer installed in garage.

1 zone, no TRV. DHW 250L to 48C once a day. Heating - constant temperature, no setback. Weather compensation. Active room mode. No short cycling. 4 lock shields are half closed in bedrooms, was too hot upstairs otherwise.

Combined COP was terrible at the beginning - lowest 2.2 with outdoor avg -2C, installer attributed this to defrosts but it didn’t improve much when it became warmer outside - stayed around 2.5 - 3.0. This was with average indoor temperature 18-19C instead of design 20-21C.

There was about 6-7C difference in return temperatures measured on the buffer.

Brought it up with installer again, long story short - to reduce mixing in buffer, primary pump speed was reduced 1200->850lph (set to 50% on WizAI and the rest by adjusting physical valve), secondary pump speed was set to highest. Heat curve from 1 to 0.5-0.6.

This resulted in 3-4C return temps difference and COP improved - I saw 4 for the first time when it was +10 outside.

Now when temperatures dropped again it is around 3-3.5. Around 0.3-0.5 COP improvement.

Comparing to the Czech tables for 35 flow temperature / 35 compressor speed (this is what it is mostly running at) I should still be getting around 1 COP more. Pump speeds can’t be adjusted further.

Are there any options left to improve efficiency, or my expectations are unrealistic and 1 COP difference can be explained by not having ideal conditions as presumed in Czech tables? Installer seems to have closed the case by now.

I lurked through various forum posts and there were several user stories for similar heatloss houses and same ASHP and microbore with no UFH that could achieve 5 with +10 outside while having indoor temperature in the 20Cs, above 3 at worst when cold.

Bills while on the best tariff seem to be on par with previous gas bills on an ancient Economy7 tariff, would probably be around 30% higher if I stayed.

Another issue is secondary pump on the highest speed generating a lot of background noise everywhere in the house (like a working bathroom extractor fan in the next room)

Thanks

Screenshot_4
Screenshot_41920×1210 164 KB

* Havenwise is mentioned because it distorted average COP on graphs - was blasting at high temperatures or idle..

2

So how big is the buffer, and what is the overall system volume? is it set up as 3 or 4 port buffer, and why did the installer think it was even needed? Personally I wouldn’t have one in the house, and even manage with no volumiser, but there are rumoured to be occasions when they make sense, and don’t damage performance too much. :slightly_smiling_face:

At zero c outside, 50c flow, and defrost, a COP of 2 to 3 sounds not too bad though. The good news is that is only a few days a year; you want to know how it does when it is average .. 8c round here.

1 Like
3

It is 50L Caleffi, 4 port. I didn’t know much about it to question this before install, I believe it was because of microbore or could be so installer didn’t have to deal with flow errors.

4

Hi @vbob.
If you do a search (the magnifying glass near the top of the page) for “buffer tank”, you’ll see tons of data and opinions in several threads on their use (much of it contradictory, unfortunately), but one thing that most commentators seem to agree on is that if you do have a 4-pipe buffer, it is at its most thermally efficient if the flows in the primary and secondary circuits are equal (or nearly so). The flow noise in your secondary circuit might be because your installer tried to set up these equal flows, which (if the pipework is undersized) would result in high water velocities hence noise.

5

Thanks Sarah.

I did have a look before and, as you say, opinions seem to range from ‘repipe it to volumiser’ to ‘leave it alone’.

This was the goal with flow rate changes, is it realistic to bring return temperature difference close to zero, minimum I could get was 2-3C delta. I don’t think flow rates can be adjusted further - secondary is already on max, and for primary 800lph is required minimum for this unit and it is at 850lph now.

Also it is trial and error without being able to actually measure secondary circuit flow rate, mixing can occur if both secondary pump speed is higher and lower than primary?Wonder if there are other means to match it rather than measuring return temperatures on the buffer.

On noise, it is louder where pipework goes from the back to the front of the house in the garage loft. Different to noise of water running in radiators - that can be heard too, but is subtle. Thinking could be clips on wood resonating through walls or secondary pump noise being amplified by pipes.

Also found in other topics, sensor calibration can be affecting COP shown, but not sure how to measure it with buffer in place while ASHP is idle.

6

Measuring the temperatures at each of the 4 ports of the buffer can help determine if the two pumps are adequately matched. Can use cheap digital thermometers like Mark did here using John’s spreadsheet to calculate how it affects power delivery.

7

I used thermal imaging gun, but it is quite troublesome to do it consistently - it is in outside garage with stiff up and over door.

Thanks for the link, I actually bought exactly the same sensor to test it, then realised it comes without power supply. Probably need to check wireless options with app/history.

8

You are (perhaps unwittingly) asking some good questions which touch on some complex issues. FWIW here’s my perspective, building on @Timbones response.

Forgive me if you already knew this, but it may be helpful to understand energy flows around a buffer tank from the perspective of “heat balance”, i.e. heat in = heat out. That is:

M * (Tin - Tout) = m * (tin – tout)

where M, m are the mass flows (kg/s) primary and secondary circuits respectively, and T, t are the temperatures (ditto).

(Strictly, this assumes that the fluid specific heat in each circuit is the same – perfectly reasonable if T and t don’t differ by more than a few degC. It also neglects heat loss from the buffer tank, but this is reasonable if the tank is well insulated.)

In theory, M is measured at your installed primary circuit flow meter, and Tin and Tout are set and reported by your heat pump controller, e.g. weather compensation settings, DT control etc. (though all of these may be subject to some instrument error). As for the secondary circuit, m depends on your secondary circuit pump performance curve, which depends on the circuit hydraulics, which in turn depends on radiator TRVs positions etc. which may be changing over time.

But once you are at steady state (all temperatures constant with time), you can use the above equation to estimate m, based on M and the four inlet/outlet temperatures, which you can approximate if you get your imaging gun working (or you install thermocouples as @timbones mentions).

Having said that, you will always get some mixing in a buffer tank, even with perfectly matched primary/secondary flows (blame the Second Law of Thermodynamics for this), and the only way to reliably monitor your secondary circuit is to install a MID-certified heat meter in it.

I have done this, in addition to fitting pipe surface thermocouples on the four buffer tank connections on my setup, and I have satisfied myself that the reported heat balance data from my heat pump (a Samsung) are accurate to within a few percent of the heat meter, but that the absolute readings from the buffer tank thermocouples need to be adjusted by up to 0.7degC for true consistency (this despite thermal cement and insulation).

Finally, I would suggest that you don’t get too hung up (for now) on instantaneous estimated CoP. In general, commercial heat pump instruments are not that accurate (though hopefully good enough for stable control!), and any CoP estimates based on them should only be used on a relative (as opposed to absolute) basis. That is to say, they may good enough to check whether or not a change in your settings has improved things, but (without a good heat meter) maybe not good enough to switch to a different electricity tariff.

2 Likes