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ASHP Flow Rates

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#1

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) 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.

OMG.

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

#2

George,

You might do better posting this on the buildhub forum -> https://forum.buildhub.org.uk/

Thereā€™s a huge amount of knowledge there.

My guess is youā€™ve been sold something which inherently wonā€™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 Iā€™m sure thereā€™ll be plenty of people along to help.

Simon

#3

Thanks I will do that.

I am not sure ā€˜inherently wonā€™t workā€™ 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 ā€˜suck it and seeā€™ approach.

#4

Hi George
Iā€™ve got a fairly large detached 1920s house - with all the problems that entails. We have a 14kW ASHP and a 6kW one (which isnā€™t on at the moment). Like you, we realised the cast iron rads were wrong for a deltaT30, but weā€™ve set the pump to flow at 45 degrees, returning at 39-40 degrees, and in the rooms where Iā€™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 weā€™ve got 30l / min according to the Sika, and 17/ / min on the small pump. Looking at the Emon data, Iā€™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 weā€™d see the flow of the AHSP struggling to maintain 45 degrees when itā€™s -3 desgrees outside - but thatā€™s not the caseā€¦

#5

It sounds as if you may have solved many of the issues we have.

Our ASHP is a 8.5KW model, which never runs flat out. So it seems to be correctly sized. The flow temp to be upto 45C works in most rooms, and more than that seems to increase the energy consumption significantly without getting much extra heat out the radiators.

What do you mean buffer?

Our system has been installed without any form of buffer, header or anything. The water used for the radiators is the same water that circulates through the ASHP. There are two grundfos pumps on the same circuit, one before the ASHP and one after.

There are two rooms on the ground floor that struggling, both are at the far ends of the house. Even with the lockshield valves on these radiators open, and all the others closed, they just do not seem to get the flow of water the need.

After further consideration yesterday, I am wondering if a manifold setup would be better. I can then easily (well, sort of) lay new plastic pipes directly to the manifold and bypass and problems in the existing system.

Lifting the floors and gaining access to the labyrinth of pipes in the current system is not an option. It would involve lifting all the GF flooring.

#6

Iā€™m just a normal punter ā€“ 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 ā€“ 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 ā€“ but I have 4 pumps in totalā€¦

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 himā€¦I think heā€™s rectified quite a few underspecified installations.

#7

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

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

Notably we didnā€™t have the flow up to even 40 Ā°C most of the time.

image
image3828Ɨ1695 1000 KB

Iā€™ve never seen the pumps run that much. It settled in at about one-third output (5kW) all through the night when it was ā€œcoldā€.

Hereā€™s the pump running stats from the same time window:

image
image3789Ɨ409 100 KB

Those images are from my app running in EmonCMS.

Hereā€™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. Iā€™ve used three different control algorithms over time. Weā€™re using ā€œgentleā€ 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.

image
image1880Ɨ812 73.7 KB

The MMSP system Mitsubishi provide doesnā€™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.

1 Like
#8

Can you please let me know a couple of things about your installationā€¦

  • 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.

George

#9

Hi George

I will PM you

Damien