I’m hoping to get some thought on which direction should we go.
The background - we have a 7kW Arotherm plus for just over two years and while it’s proving heat, hot water and besides a “snail” shorting the power supply and the initial glycol mixing issue that seems have been sorted now (by running the unit) we are sort of happy with the unit. However. Very unhappy with the performance, not using any external monitor and solely relying on the Vaillant built-in readings it would appear that we only can archive scop around 3.1 - not terrible but not great either.
We have underfloor heating downstairs - about ~35m² and 4 radiators upstairs, a low loss header (or buffer tank) and no design documents or anything.
Recently I read up a bit on buffers to realise it might be an unnecessary thing, however not qualified to determine if we really need it, however came across a piece of information suggesting that the secondary pump should pump out water faster than the primary pump, now only can rely on the information by Vaillant and the dab pumps readings, but would appear that the evosta pump is reporting 1m³/h flow rate and the pump in the ASHP 1-1.2m³/h. Which according to my basic understanding is not ideal as the evosta side should be pumping 20-30% faster than the ASHP side. Tried the obvious and limited the building circuit pump to 50%, which according to the data provided by Vaillant should bring down the AsHP side to ~600l/H (as 1200l/h is apparently the maximum) but still the ASHP flow rates remain 1080l/h with some occasional drop to 750l/h which is 25 - 80% higher than it should be.
Obviously reached out to the installer but doesn’t seem to be overly keen to come and access the issue or deal with our complaints on the performance of the unit. Ultimately I think we’ll need to find someone else to investigate the issue but was wondering if could get some thoughts on what the culprit with the ASHP-side flow rate could be and if that discrepancy in the flow rates would be the reason for the poor scop or just the buffer tank itself?
The big improvement would be upgrading the radiators (maybe to fan convectors or adding DIY fans) so they can run on same flow temperature as the UFH.
Then low loss header, UFH mixer (if fiited) and seperate pumped zones can be removed with the complete system running as a simple setup at the best temperature for UFH.
Everything runs on the same temperature , there is no ufh mixer and there is only one zone. From the pumps perspective there is a pump in the ASHP, that’s goes into the buffer, and from there a pump is pushing water to the UFH and the radiators that are just split off the same pipe.
I would try converting the buffer into a volumizer with removing 2nd pump. I expect the pump in Arotherm will be powerfull enough on its own unless your pipework have high resistance.
Not having the buffer should allow a lower flow temperature to be set on heatpump. Buffers are put in to protect against too much of UFH being closed with activators and per room thermostats.
I don’t know Arotherm, but my Daikin varies its primary pump between about 6 and 30 litres per minute.
I would imagine the Arotherm will modulate the pump according to how it is doing with meeting the target leaving water temp and its flow-return temp difference.
I would seriously avoid replumbing anything without gathering more information first.
If I were you, I’d make/get some kit to understand the temperatiure differences across:
the 4 ports of the buffer. This should provide evidence for whether the primary [heat pump] and secondary pumps [house side of buffer] are imbalanced.
your UFH and radiators. Look out for signs your emitters may need balancing.
In both cases, see whether the flow-return temperature difference is far from the recommended dT (I would think 5C is a reasonable target)
Try to find out whether the secondary pump has different speed options, or can be controlled. (the pump supplied with my HP was very over powered until I reduced its speed to half using its PWM input).
Hi Gmajoros, Apologies for my late comment. I have a small house (98m2) , newly renovated with UFH upstairs and downstairs (1 zone). The initial design proposed by the installer was radiators upstairs, UFH downstairs with a buffer tank, a separation header, UFH mixer valve and 2xpumps for upstairs and downstairs plus 1xpump between the buffer and separation header. I was unhappy with the “system” design and the amount of hardware involved.
I opted for no buffer, no separation header and no extra water circulation pumps. My calculations showed the 4kW ASHP circulator pump was more than adequate to deliver the flowrate with minimal pressure loss. You would have to perform your own simple calculations to confirm. I operate with low water temperatures and one house temperature sensor.
What is your “target” water temperature?
I guess it is set by the upstairs radiators and possibly 40C-45C. If you can use a lower target flow temperature , then your heating COP should improve.
In my opinion, the buffer tank will probably be pulling down your COP due to water mixing within the tank. Guessing 3C-5C, but other folk may have studied/monitored it more.
The additional water pumps will, at best, not adversely influence the water flow and dT, but you should verify with simple digital thermometers (£8 for 5 from Amazon) or an equivalent approach. In my opinion, the best possible set up of your additional water pumps will still draw more current to operate a balanced “system” compared with the ASHP water circulation pump alone.