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Flow problems with ASHP


gck303

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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 read the voltage from the Sika Vortex (flow sensor for ASHP) and I am getting a flow rate of about 12 l/min. When calculating the temperature difference and the power output, I get a reasonable COP of 3 ish. This suggests that the flow rate is correct at 12 l/min.

 

The radiators on the ground floor that are furthest from the main 22mm circulation pipes have their lock shield valves fully open and just do not seem to be getting enough flow. Most of the radiators are nearly closed, the problematic ones are fully open. Still these two do not get warm enough. 

 

OMG.

 

How can the flow rate be so slow? I have two circulation pumps consuming 50W, so there is not an absence of pressure. One of the pumps is a Grundfos ALPHA3 which shows a head of 6m and also shows a similar flow, of 0.6m/h.

 

What to do?

 

Replacing any of the pipework on the ground floor is a major exercise as we have hard floorings everywhere. I am wondering if installing a radiator manifold and putting new Speedfit  pipework in for these radiators direct to the manifold is a sensible way to proceed?

 

https://emmeti.co.uk/products/heating-manifolds/

 

Thanks, George

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Hi George

 

Welcome to BuildHub.

 

I'm not an expert by any means, but a few things jumped to mind.

 

How old is the pipework? If it's old, was it properly cleaned and flushed when you had the ASHP installed? If it's 22mm everywhere, I can't imagine you having much of a restriction based on furring up, but who knows what crud might have gotten stuck in it from old radiators over the years.

 

Are the rads ASHP-compatible (i.e., designed to run at lower temps)? If they're old, were they properly cleaned when the ASHP was installed?

 

Perhaps it isn't the flow, but that you're losing so much heat on the trip to the distant radiators that they just aren't capable of putting out much energy. 45°C isn't a lot of temperature to work with even with good ASHP-compatible radiators, so losing even a few degrees on a long, uninsulated copper run could make a difference. A bigger pipe is actually worse in this situation, because there's a larger radiant surface area, and the water is moving more slowly for a given flow rate, giving it more time to lose heat.

 

I'm sure someone who knows a bit more about these things will be along soon.

 

@Nickfromwales @PeterW @ProDave

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26 minutes ago, gck303 said:

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

Tis may be a clue.

Are you loosing temperature to the floor where the pipes run?

Get a cheap IR thermometer and see what the temperatures are in the areas around the pipes.

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Thanks.

 

All of the suggestions have been considered and have either been tackled, or cannot be tackled:

- the radiators have been balanced, the problem remains. 

- the system was flushed when the ASHP was installed.  But, it is an old system. Crud may remain. 

- the pipework is a mix of 22mm and 15mm pipework, but it missing much insulation under the suspended wooden floor. Getting to it is impossible as it would require lifting the Amtico, hence my thought to lay new pipes running direct from a manifold. 

- I have a decent IR thermometer and am monitoring the flow/return at the ASHP. The issue is the overall flow. 

 

I am wondering if the biggest issue seem is the poor overall flow. I would expect that a heating system would have a flow of 20+ l/m, the pumps are running consuming 54W and there is 6m of head, but only shifting 10-12 l/m. Surely this is a problem?

 

With an ASHP the flow is critical, I ideally want to get 2 l/m though every one of the 12 radiators in the house. Eg with a 42C flow and 38C return. You need to have a high flow rate, so that the difference between the flow and return can be minimised, allowing to to reduce the flow temperature and increase the COP.  

 

 

 

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1 hour ago, gck303 said:

How can the flow rate be so slow? I have two circulation pumps consuming 50W, so there is not an absence of pressure. One of the pumps is a Grundfos ALPHA3 which shows a head of 6m and also shows a similar flow, of 0.6m/h.

 

Is that 0.6m^3/h a measured flow rate or from the data sheet? If its measured its about 10L/s so there is flow going around the rads.

If most rads are shut off and the open ones are cold it must be going somewhere else...

Is there a bypass that is stuck open? 

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8.5kw. Which is adequate, as it is delivering that heat continually. Selecting an ASHP is a fine balance, as you want to neither over or under spec it. 

 

It is a 8.5kw unit and capable of deliverying a greater output, but the flow is not great enough to take the heat away from the ASHP.  

 

How can I increase the flow?

 

Is 20 l/m an reasonable flow for 12 radiators?

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47 minutes ago, SteamyTea said:

4.2 [SHC of water kJ.kg-1] x 0.33 [flow rate kg.s-1] x 4 [delta T] = 5.6 kW delivered to the house.

 

This seems very low, what size is the ASHP.

 

12l/min is only 0.2 kg.s-1 or have I misunderstood  your calc?

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The 10 l/m is measured and correct. I need to increase this flow to 20 l/m.

 

The radiators are warm. But at the two that are at the far ends of the house are not warm enough. They are a few degrees cooler than the others. At 12 l/m there is not enough flow for all the rads. Some are warm enough, others are not warm enough. 

 

The warm ones have their lock shields almost closed, the cool ones are fully open. 

 

It is not going via a bypass rad. If it was, then the temperature drop would not be low enough. It is correct as having a 5C temp drop. 

 

 

 

 

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35 minutes ago, Temp said:

0.6m^3/h appears to be about right for the Grundfos Alpha data i just looked at. Which model do you have and which mode is it in? 

 

 

It is a 15-50/60. There are two pumps of identical size, one saying it is running at 18W and the other at 34W. I think it is in radiator & 'max' mode. 

 

I have cranked both pumps upto III. Then are both showing 34W on the display. The flow is up to 12.5 l/m. 

 

 

 

 

 

Edited by gck303
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58 minutes ago, Temp said:

12l/min is only 0.2 kg.s-1 or have I misunderstood  your calc?

20 lt/min = 0.33 kg/s

 

I got that from here:

1 hour ago, gck303 said:

I am wondering if the biggest issue seem is the poor overall flow. I would expect that a heating system would have a flow of 20+ l/m, the pumps are running consuming 54W and there is 6m of head, but only shifting 10-12 l/m.

 

This is to get an idea of the power that the ASHP is capable of delivering.

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So it currently looks like you have high/max pump power levels, high pressure and not huge flow. Do you still have a lot of the rad lock valves closed?

Edited by Temp
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21 minutes ago, Temp said:

So it currently looks like you have high/max pump power levels, high pressure and not huge flow. Do you still have a lot of the rad lock valves closed?

 

Yes, this is almost correct. 

 

- There are a number (2) of radiators that are fully open (TRV and lockshield) which are not getting flow. 

- The other radiators (10) are almost closed and getting enough flow. If they are closed any more, then they shut of entirely

 

 

 

 

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6 minutes ago, jack said:

Can you run something temporary (and above-floor) to the distant radiator to see whether that improves things? If flow improved, it would at least suggest an issue with the pipework.

 

You mean unplumb the dodgy radiator, put in some temporary pipe work, and see if that helps?

 

Yes. I could do. I would need to connect it to the existing system somewhere. I was thinking that a radiator manifold near to the main pumps would allow me to do exactly this and give me total control over the problematic radiators. 

 

Does that sound sensible?

 

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15 minutes ago, gck303 said:

You mean unplumb the dodgy radiator, put in some temporary pipe work, and see if that helps?

 

Yes, exactly.

 

Not sure whether it's worth going to the trouble of installing a full manifold at this stage - is there any way to just connect just the radiators of interest with new temporary pipework, ideally in a way that let's you continue to monitor flow rates?

 

Also, I notice you have an IR thermometer. What's the temperature at the inlet to the problem radiators?

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36 minutes ago, TonyT said:

No I meant an air lock in the pipework supplying the radiator from when the boiler was changed out to the ASHP.

 

I will check this. Thanks. I have never done this before, but it is quick and easy. And TBH something and on reading up something I should have done before I wrote this message. 

 

42 minutes ago, jack said:

 

Yes, exactly.

 

Not sure whether it's worth going to the trouble of installing a full manifold at this stage - is there any way to just connect just the radiators of interest with new temporary pipework, ideally in a way that let's you continue to monitor flow rates?

 

Also, I notice you have an IR thermometer. What's the temperature at the inlet to the problem radiators?

 

Once I have drained down the system, and put in the pipes and then repeated it to remove them all it might not actually be that much extra effort. 

 

The temperatures are low, a good few degrees beneath what they should be. High thirties, rather than the 44 coming of the ASHP. And those few degrees do make a difference. 

 

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47 minutes ago, TonyT said:

Best to connect up the cold mains feed to the manifold and push and airlock out with mains pressure water rather than altering the system with temp pipework

 

this would be the first elimination step

 

Which manifold? I do not have a manifold in the system, yet. 

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A further thing I have just noticed....

 

When it is heating the DHW the flow rate is still only 14l/m. This is all in 22mm copper pipe to/from the ASHP, so should I expect that the flow to be greater? There are two -15-50/60 pumps set on 'III' so it should be going mcuh quicker!

 

If with a relatively unrestricted flow into teh DHW tank, then it would not be sensible to get a greater flow into the rads. 

 

Does this suggest that there is a blockage somewhere?

 

 

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