Ecodan flow and return pumps

[Benpointer]

We have a fault with our Ecodan 85 FTC2 controller whereby the controller does not switch on the Flow and Return pumps (F&G on the schematic below) when the UFH is demanding.  It does switch those pumps on when HW is on demand.   

 

Despite this, the UFH pump (E - which does power up as it's controlled directly from the UFH circuit, not the FTC2) seems to draw enough water through the ASHP to provide heating to the floors on most days.  It seems to do this in short (5-10 min) cycles with the flow temperature quickly rising to meet the flow target temp, the ASHP cutting out, the flow temp dropping and the ASHP cutting in again.

 

However, on cold days such as those experienced before Christmas, the heating is not reaching the room stat targets.

 

I have a support query lodged with Mitsubishi for this but in case they fail to come back or simply say our system needs replacing, I have the following question for this forum:

 

Is it safe to run the Edodan flow and return pumps even when the ASHP is not operating?  (i.e. have the water circulating through the ASHP when the compressor is not running.

 

If so, I could run these pumps off the same circuit as the UFH pump (E) because we don't use the ASHP to heat our hot water at all.

 

 

 

 

Edited January 11, 2023 by Benpointer
Correction

[akjos]

Has this ever worked or is it something which just happened now?

Overall it seems to me you have too many pumps in your system. Has the installer given you any information why they put a pump on your return pipework? Your flow pump (F) should be able to handle the circuit by itself.

The only issue I see with your UFH pump pulling water through your ASHP is that it might throw a fault code (flow detected when there shouldn’t be). I know Samsung does this but not sure about Mitsubishi.

BUT if your UFH pump really does that, then it means your system can be powered completely by a single pump. So you’ll only have pump F and nothing else. No need for return pump and also no need for UFH pump if that’s the only thing you have (i.e no radiators). Also you wouldn’t need a mixing valve and instead control water temp via weather compensation. Much simpler system.

[PeterW]

33 minutes ago, akjos said:

BUT if your UFH pump really does that, then it means your system can be powered completely by a single pump. So you’ll only have pump F and nothing else. No need for return pump and also no need for UFH pump if that’s the only thing you have (i.e no radiators). Also you wouldn’t need a mixing valve and instead control water temp via weather compensation. Much simpler system.

That doesn’t work. Mixers are there to protect the floor and also balance flow, and the manifold circulator will pull a very small amount of water when needed based on the blending temperature. Remove that lot and the ASHP will short cycle when you have only one open loop toward the end of the heating cycle. 
 

Don’t know why there is a flow and return pump unless the pipe lengths are long, but a single on return would be fine. I would be checking wiring and controls. 

[Benpointer]

Q: Has it ever worked?  Well, it's hard to be sure (because we have never actually checked that pumps F&G were spinning in the past) but the house has always able to maintain a good temperature even through periods of severe cold as in 2010, 2013 and 2018 (the 'beast from the east').  So I suspect it was working ok up until recently.

 

Q: Why is there both a flow and return pump?  No idea, I suspect the original plumber didn't really know what he was doing.  I also suspect it's overkill but... that's not the issue at the moment - neither of the flow or return pumps are switching on when the UFH is on demand.  (Edit:  In fairness to the plumber, the Ecodan installation manual shows both a flow and return pump.)

 

Re checking the wiring and controls: The pumps come on when the HW is on demand, so I think the wiring is ok.  The FTC2 is probably at fault but Mitsubishi want £240 just for someone to come and have a look (!)

Edited January 11, 2023 by Benpointer
change

[ReedRichards]

6 hours ago, Benpointer said:

 we don't use the ASHP to heat our hot water at all.

 

41 minutes ago, Benpointer said:

The pumps come on when the HW is on demand, so I think the wiring is ok. 

 

I found these two statements confusing.  You mean you can heat the hot water with the ASHP but you don't?

[Benpointer]

Yes.  We can heat the HW and the ASHP flow and return pumps F&G switch on when HW is on demand. 

 

But no, we don't heat HW from the ASHP because (sigh) there is a long-standing issue where the controller errors every few days when trying to heat HW.  We had someone look at this a few years ago and the upshot was, quote "It's a known issue with the FTC2, which is out of warranty and Mitsubishi won't fix, so you could either pay for a new controller or, since you have an immersion heater, use that".  

 

Which is what we have done ever since.  It's not so bad as it sounds as we have solar HW too but still we are spending money on the immersion heater. 

[Benpointer]

I'd just like to reiterate (paraphrase) my original question - could put the flow & return pumps on the same circuit as the UFH pump?

 

That would mean every time the UFH is demanding the ASHP flow & return will run.  (It won't work for HW but we don't use that anyway.)

[ReedRichards]

On my system the ASHP, and it's controller, and the third party heating controller, and the internal central heating pump are all on the same (electrical) circuit.  I had assumed this was the norm.  

[akjos]

10 hours ago, PeterW said:

That doesn’t work. Mixers are there to protect the floor and also balance flow, and the manifold circulator will pull a very small amount of water when needed based on the blending temperature. Remove that lot and the ASHP will short cycle when you have only one open loop toward the end of the heating cycle. 

It does work but it depends on how the system is set up. Since we don't know much about this exact design, we can't say for sure.

You are right about the mixing valve but this would apply in systems where you have a boiler running higher flow temps for radiators and you want to mix down the UFH circuit. If the UFH circuit it the only thing you have, no need to mix anything, just setting the flow temp lower on the ASHP would work and be more efficient for the heat pump.

Short-cycling doesn't seem to be taken care of even in the current design. As @Benpointer mentioned, it runs only in 5-10min cycles.
So the ASHP starts up and heats up the flow temp. Then the mixing valve closes shortly after and the ASHP only pushes flow through the bypass valve. This is a short circuit and the return temp quickly rises, leading to the ASHP shutting down. Cycle continues after a while.

 

A longer run-time can be achieved with just one pump if the following is done:
1. Have the UFH loops always open. No zones or actuators.
2. Control the flow temp on the ASHP via weather compensation. So higher temp when it's cold outside and lower when it's warm. This has to be calculated though so that you have the required heat output on your UFH based on the heatloss of your house.

3. Have one pump push everything through the ASHP and the UFH when there is heat requirement. If you have a highly efficient house you could even run the UFH circuit 2-3 degrees above room temp continuously and it will act as a self-regulating surface (i.e heat when required and absorb heat (cool) when your room temp overshoots a little).

 

Anyway this is a change to the whole system potentially.

 

 

8 hours ago, Benpointer said:

I'd just like to reiterate (paraphrase) my original question - could put the flow & return pumps on the same circuit as the UFH pump?

 

That would mean every time the UFH is demanding the ASHP flow & return will run.  (It won't work for HW but we don't use that anyway.)

 

I'd try it just to see if flow and return pumps actually work. I suspect they might have failed though. In your current design, flow and return pumps are on the same hydraulic circuit without any hydraulic separation and they push-pull on each other, leading to preliminary failure. It's possible this has happened, but you can easily verify by doing what you suggested and see if they run.

But I think it's general consensus you don't need one of those pumps. I'd remove the return one.

 

Edited January 12, 2023 by akjos

[PeterW]

1 hour ago, akjos said:

If the UFH circuit it the only thing you have, no need to mix anything, just setting the flow temp lower on the ASHP would work and be more efficient for the heat pump.

But the OP has DHW on this so unless he’s got split temps set on the ASHP and this is set up as W plan using a pair of 2 port valves and can only do DHW or  UFH, then you need the blending and the pump set. 
 

1 hour ago, akjos said:

1. Have the UFH loops always open. No zones or actuators.

That leads to overheating of smaller zones - only works when you have a single large zone. Much better is to use self balancing actuators that reduce flow to balance to the d7°C or similar.

 

1 hour ago, akjos said:

But I think it's general consensus you don't need one of those pumps. I'd remove the return one.

Agree however the best practice is to have the pump on the cool side for longevity and pull through valves so the flow pump would be removed. 

 

1 hour ago, akjos said:

Short-cycling doesn't seem to be taken care of even in the current design.

Yes it’s why best practice is to include either a large volume LLH or buffer in systems needing heat when the min output of the heat source is above the max input needed into the circuit. 

[akjos]

44 minutes ago, PeterW said:

1 hour ago, akjos said:

1. Have the UFH loops always open. No zones or actuators.

That leads to overheating of smaller zones - only works when you have a single large zone. Much better is to use self balancing actuators that reduce flow to balance to the d7°C or similar.

 

👍 yes agree the flow rate needs to be set in each zone according to the heat loss. Self balancing actuators could do that but are pricey. The same can be achieved by simply setting the correct flow rate on each zone using the manual flow rate gauges and screws.

 

For example: Total house 100m2 with heat loss of 5KW (or 50W p/m2). A small UFH zone 5m2 (ex. bath or whatever), would have a heat loss of 250W. Flow for that zone can be calculated using: l/s = KW / (delta T * 4.2)    ==> l/s = 0.25 / (5*4.2)  ==> 0.012 l/s or around 0.7 l/min needed for that zone. Anything more will lead to overheating.

[Benpointer]

Regarding the flow and return pump.  This is from the Ecodan Design, Installation & Servicing Instructions manual:

 

"Each of the Ecodan® models require sufficient primary flow rate for adequate operation. Due to the large resistances caused by the plate heat exchanger in the Ecodan®, two domestic circulating pumps in series to produce the required flow rates. These pumps must be set to the same speed or damage to the system may occur."

 

And this diagram:

 

[Benpointer]

3 hours ago, akjos said:

[SNIP]

 

I'd try it just to see if flow and return pumps actually work. I suspect they might have failed though. In your current design, flow and return pumps are on the same hydraulic circuit without any hydraulic separation and they push-pull on each other, leading to preliminary failure. It's possible this has happened, but you can easily verify by doing what you suggested and see if they run.

But I think it's general consensus you don't need one of those pumps. I'd remove the return one.

 

 

The flow & return pumps definitely do work - they work if HW is being demanded.  

 

It just seems that the controller is not triggering the flow & return pump relay when UFH is requested.  Hence my thought of putting the flow & return pumps on the same electrical circuit as the UFH pump.  (That would mean the system would not work for DHW - but we never use it for that anyway.)

Edited January 12, 2023 by Benpointer

[HughF]

21 hours ago, Benpointer said:

If so, I could run these pumps off the same circuit as the UFH pump (E) because we don't use the ASHP to heat our hot water at all.

 

Not if the ecodan needs PWM control of those pumps to control the deltaT

[PeterW]

5 hours ago, akjos said:

A small UFH zone 5m2 (ex. bath or whatever), would have a heat loss of 250W. Flow for that zone can be calculated using: l/s = KW / (delta T * 4.2)    ==> l/s = 0.25 / (5*4.2)  ==> 0.012 l/s or around 0.7 l/min needed for that zone. Anything more will lead to overheating.

doesn’t that only work with a perfect floor as you need to know the pipe volume / spread in the floor as that has a direct influence on the transfer or are you assuming 100mm spacing ..?

 

 

[Benpointer]

Problem solved!

 

Somewhat embarrassingly, it's turned out to be a failed zone valve head on the UFH circuit - the microswitch not sending back to the FTC2 to trigger the ASHP pumps.  

 

New head fitted today - pumps now working!

 

Thanks all!

[PeterW]

Good to hear all resolved !! 

[akjos]

On 12/01/2023 at 15:01, PeterW said:

On 12/01/2023 at 09:37, akjos said:

A small UFH zone 5m2 (ex. bath or whatever), would have a heat loss of 250W. Flow for that zone can be calculated using: l/s = KW / (delta T * 4.2)    ==> l/s = 0.25 / (5*4.2)  ==> 0.012 l/s or around 0.7 l/min needed for that zone. Anything more will lead to overheating.

doesn’t that only work with a perfect floor as you need to know the pipe volume / spread in the floor as that has a direct influence on the transfer or are you assuming 100mm spacing ..?

 

Good point, it works for any floor as long as the flow temp is set to match the required heat output.

 

For ex.: 

- 200mm centres would require a higher flow temp to give the 50W/m2 vs. 150mm which would require a lower flow temp to give the same 50W/m2.

 

I calculate using a delta T of 5'C as that's what most ASHP target on the heat-exchanger I think and also it gives a comfortable floor (low difference between one and the other side of the room).

 

The key is: Design pipe spacing based on heat loss per /m2, then you can calculate the needed flow rate using the formula above.

 

Weather compensation on the ASHP will take care of the rest (i.e when it's hotter outside you need less heat, so the flow temp will drop and thus the W/m2 output too as per above table -- delta T stays at 5 constantly as does the calculated flow rate in each circuit).