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ASHP Flow Rates

I have a older 5 bed brick house (Edwardian Terrace) with a new ecodan ASHP heatpump installation. We run it 18+ hours in the day with a flow rate no greater than 45 C. We have c. 10 radiators.

I have reading the voltage from the Sika Vortex (Heat flow sensor for ASHP - #12 by dlongson) and I am getting a flow rate of acboutr 12 l/min. When calcualting the temprature difference and the power output, I get a reasonable COP of 3 ish. This suggests that the flow rate is correct.


How can the flow rate be so slow? I have two circulation pumps comsuming 50W, so there is not an abscence of pressure. One of the pumps is a Grundfos ALPHA3 which shows a head of 6m.

The radiators on the ground floor that are furthers from the 22mm pipes comming down have their lock shield valves fully open and just do not seem to be getting enough flow.

What to do?

Replacing any of the pipework on the ground floor is a major excercise as we have hard floorings everywhere.

Thanks, George


You might do better posting this on the buildhub forum →

There’s a huge amount of knowledge there.

My guess is you’ve been sold something which inherently won’t work. How airtight is your house? How well insulated is it? You need a well insulated reasonably airtight house for an ASHP to be efficient. They work best delivering lower temperatures, UFH which runs at a maximum of 35C is great. To have an ASHP drive existing radiators, it would need to be working at say 60C which is very inefficient and is why it is recommended that much larger radiators are installed, so that the system can run at a lower temperature.

But like I said, drop the question on the buildhub forum and I’m sure there’ll be plenty of people along to help.


Thanks I will do that.

I am not sure ‘inherently won’t work’ is correct. The system works brilliantly, other than this.

I was very aware that when it was installed that a number of the radiators were undersized, and we elected not to replace them but adopt a ‘suck it and see’ approach.

Hi George
I’ve got a fairly large detached 1920s house - with all the problems that entails. We have a 14kW ASHP and a 6kW one (which isn’t on at the moment). Like you, we realised the cast iron rads were wrong for a deltaT30, but we’ve set the pump to flow at 45 degrees, returning at 39-40 degrees, and in the rooms where I’ve added some rads now, it all works really fine. The room temps are around 18 degrees, backed off to 16 at night and with a hysteresis of 0.5 degrees. The trick really is getting the rads right.

As regards the flows - do you mean in the primary circuit to the buffer? On our big pump we’ve got 30l / min according to the Sika, and 17/ / min on the small pump. Looking at the Emon data, I’d say that there are no problems heating the primary circuits quickly - the issue is more losing enough heat in the rads - i.e. if the rads were the right size, I think we’d see the flow of the AHSP struggling to maintain 45 degrees when it’s -3 desgrees outside - but that’s not the case…

I’m just a normal punter – not a heating engineer nor indeed an IT person. So all I can do is describe what I have, rather th n advise.

Both my ASHP are connected in parallel to a buffer (100 litres, IIRC) .Each of those primary circuits has a Grundfos pump running at a fixed (max) rate. The larger ASHP is plumbed in with 28mm, the smaller with 22mm, hence different flow rates. My 45 and 40 degree flow and return temps are measured there.

On the other side of the buffer is connected the flow and return for the CH, from which there is also a feed to the indirect DHW tank - all suitably zone-valved. It is probably relevant to your issue that we have another TWO pumps on that secondary circuit – one on each of flow and return. In addition, much of the pipework is 22mm. As you say, the water in the secondary circuits, and the primary, is actually the same, via the buffer – but I have 4 pumps in total…

I think the setup I have is one of the ones in the Mistubishhi set-up illustrations

I suspect this is out of scope for the OEM forum more generally, but happy for you to PM me if you wish to communicate further. I have known my heating engineer for a long time, and trust him…I think he’s rectified quite a few underspecified installations.

Just an FYI, we have a 14kW Ecodan in a 4-bed 1822 house and it’s trundling along fine. We did get new rads though. Our insulation is lousy.

Here’s the last couple of days which has been unusually intense because it went down to -7 °C (yep I know that’s not cold for lots of people here :slight_smile: )

Notably we didn’t have the flow up to even 40 °C most of the time.

I’ve never seen the pumps run that much. It settled in at about one-third output (5kW) all through the night when it was “cold”.

Here’s the pump running stats from the same time window:

Those images are from my app running in EmonCMS.

Here’s some other stats I collected which shows the proportion of the time the radiator pump was running based on the average temperature for the day. I’ve used three different control algorithms over time. We’re using “gentle” these days and you can see it never ran for 18 hours a day because we like to turn it off and let it settle.

The MMSP system Mitsubishi provide doesn’t have a flow rate but our Grundfoss UPM3 Auto L is doing between 25 and 70 l/min I believe.

Not sure if that helps much.

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Can you please let me know a couple of things about your installation…

  • what kind and number of pumps do you have on your primary circuits?
  • what settings do you have your pump on, and what power are they consuming?
  • if you increase the power on the pumps are you able get an even higher flow through the primary circuits?

I realised yesterday that when just heating the DHW I am only getting a flow of 13l/m though the 22mm copper pipework. Which does not seem to be enough, it is right at the lower end of the flow rates the ecodan unit can use.

If I am only getting 13l/m through a relatively unrestricted DHW circuit, then I am unlikely to get a better flow rate through radiators that are distributed all over the house.

I do appreciate that you are not an expert and will not ask you what you think I should do, but If I can understand what your configuration is, then I can consider why my mine is not correct.


Hi George

I will PM you