Posted by: @derek-m

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Posted by: @davidalgarve

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Thanks JamesPa - have only just picked this up.

I have the contractor visiting who is primarily a solar installer but he added the DHW system and I have a lot of respect for him, although he has some fixed ideas.

I have started to explain the perceived benefits of taking the buffer tank out of circulation, but it occurred to me that I am uncertain how I will decide on the speed of the built in Ecodan pump.

What method will I use to evaluate the performance at the available speeds? 

I agree with James. I think that I am correct in saying that the water pump speed on the Ecodan can be varied from 1 to 5, so probably start with the pump set at 3 and see what DT you get. You may find that you need the pump speed lower in Summer and higher in Winter.

 

There is a recommended flow rate range for each model (its quite a large range), and I think you might be able to display measured flow rate.  if you can, set it in the middle to start with.

Derek M is correct in that the Ecodan has a 5 speed pump and this is currently set at max of 5. I was just hoping for a meaningful measure of performance, either to optimise the system or to have reason to switch back to a two pump system, possibly with a better (and this time variable speed) secondary pump. 

Posted by: @davidalgarve

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Thanks JamesPa - have only just picked this up.

I have the contractor visiting who is primarily a solar installer but he added the DHW system and I have a lot of respect for him, although he has some fixed ideas.

I have started to explain the perceived benefits of taking the buffer tank out of circulation, but it occurred to me that I am uncertain how I will decide on the speed of the built in Ecodan pump.

What method will I use to evaluate the performance at the available speeds? 

I have just remembered that with Ecodan's the water pump is external to the heat pump, and the water pump speed may not actually be controlled by the controller, it will just be started and stopped. If this is the case then the speed will have to be set on the pump itself, as James suggest with reference to the flowrate reading on the controller.

If you have two cables connected to the water pump, power and control, then it should be possible to vary the speed using the heat pump controller.

James / Derek. Over the last year or more there has been a lot of debate and speculation about the optimum flow rate for the Ecodan range. Some people favouring a high flow rate and some targeting a low flow rate. It would also appear that Mitsubishi have responded to this contradicting debate by issuing a new data book (October 2023) which now quotes a single optimum flow rate for each size heat pump model. 

however a good flow rate is needed.... if the flow rate has been adjusted towards the lower flow range it will never be able to reach the full heat output potential when it’s needed. Likewise if the flow rate is too high the DT May drop too low. 

What I've seen with out Ecodan is that it doesn’t need a variable pump with PWM control.  The system selects one of the 4 part-load outputs then modulates within its range. This makes it very versatile in its output range. And provided with only one fixed flow rate.

the below chart shows the new optimum flow rates.

The new data book has also renamed the 4 compressor outputs see below

I also know the heat pump, when fitted with Mitsubishi cylinder they are usually fitted with plate heat exchanger and an integrated (adjustable) circulation pump. But when they have a 3rd party cylinder the circulation pump usually needs to be adjusted manually.

I am finding that this topic is becoming more complicated than I can handle.

As I understand the Ecodan Hydrobox, it has a single pump with 5 speeds available and I don't see anything which indicates that it compensates in any way, after a speed is selected. The resultant flow must, surely, depend on the pump curve at the chosen speed and the head imposed by the radiator system.

I have no means of measuring flow and I suggest that it is not a meaningful parameter. At the moment, I don't have temperature measuring on the radiator flow and return, but I can arrange for it to be fitted if I go ahead with the modifications involving removal of the secondary pump. 

Am I correct in thinking, therefore, that the optimum pump speed setting must be the based on flow vs return temperatures and that a reasonable aiming point would be 5C.?

You are correct there are 5 speeds available set through the settings menu, under pump speed, these are fixed rates and the ecodan cannot change them.  You can see the flowrate you are getting at each pump speed.  In the same settings menu look for running information then type in code 540 and this shows the flow rate in litres/min.

Thanks Gary. That is a new, and interesting bit of inside information that I am pleased to learn. I think this may be useful in balancing the radiators and seeing how SRV's affect things.

However, I still do not see how knowing the flow in litres/ minute helps me to evaluate the best pump speed setting. Am I correct in saying that it it is Delta T that counts and if so, what figure am I aiming for? 

The quantity of thermal energy being transferred from the heat pump to the heat emitters is dependent upon the flowrate and the DT. If you were to increase the flowrate from say 10 lpm to 20 lpm then you double the quantity of thermal energy, provided that the DT remains constant.

Reducing the DT from say 5C to 2.5C would halve the quantity of thermal energy being supplied.

Neither flowrate or DT are that critical, provided that both are within a range that allows the transfer of sufficient thermal energy under all expected operating conditions.

I would suggest that the water pump speed should be set to provide a DT of approximately 5C,when the heating demand is in the 50% to 75% of overall maximum. Don't be concerned if the DT varies from 5C, provided that your home achieves the desired indoor temperatures.