Central heating pump

[ShaunCorbs]

interesting you say that because yesterday the laundry manifold had a flow of 60 at the boiler and returning 59, and that water has to travel 55m there and back.
I checked the flow and return out of the boiler, and the seem to be ok.
Another thought I’ve just had, I think this boiler is way undersized even at the original 35kw. This diesel boiler replaced a 26kw gas boiler which ran all the rads and trench heating and replaced a 16 kw heat pump that was to small to run the underfloor heating, hence why we decided to change. Is there a formula to work out what size boiler one would need.

We already know that when the troubled manifold is on it needs 24.7kw.

What’s the flow rate of all the zones on this manifold ?

 

[sherylryan]

What’s the flow rate of all the zones on this manifold ?

Out of the 7 zones, only 4 are operating as the other zones are turned off. 2 x 2.4LPM, 1 x 2.0LPM and 1x 1.5LPM (small laundry circuit) so approx 8.3 lpm in total. This manifold is 55 m away from the boiler.

 

[ShaunCorbs]

Which is 4kw required so very strange why it needs 5 x that

 

[John.g]

interesting you say that because yesterday the laundry manifold had a flow of 60 at the boiler and returning 59, and that water has to travel 55m there and back.
I checked the flow and return out of the boiler, and the seem to be ok.
Another thought I’ve just had, I think this boiler is way undersized even at the original 35kw. This diesel boiler replaced a 26kw gas boiler which ran all the rads and trench heating and replaced a 16 kw heat pump that was to small to run the underfloor heating, hence why we decided to change. Is there a formula to work out what size boiler one would need.

We already know that when the troubled manifold is on it needs 24.7kw.

Something definitely very strange, I get a UFH heat output of 5.8kw at your flowrate of 8.3LPM and dT of 10C which should only result in a boiler flowrate of 2.77LPM at a boiler return temp of 35C., the measured return is 49C which means that 2.42LPM at 65C is either bypassing through the TMV or through a external bypass to give a boiler return flow of 5.2LPM. But the boiler is outputting ~ 24kw based on your burner cycling times, you may be losing up to 5kw in all that pipework if uninsulated but still wouldn't remotely account for that boiler output, the only logical explanation is that there is another zone (maybe rads) on as well, taking the few temperatures I suggested will show if the TMV is by passing but will not explain the very high boiler output.

 

[sherylryan]

Which is 4kw required so very strange why it needs 5 x that

Something definitely very strange, I get a UFH heat output of 5.8kw at your flowrate of 8.3LPM and dT of 10C which should only result in a boiler flowrate of 2.77LPM at a boiler return temp of 35C., the measured return is 49C which means that 2.42LPM at 65C is either bypassing through the TMV or through a external bypass to give a boiler return flow of 5.2LPM. But the boiler is outputting ~ 24kw based on your burner cycling times, you may be losing up to 5kw in all that pipework if uninsulated but still wouldn't remotely account for that boiler output, the only logical explanation is that there is another zone (maybe rads) on as well, taking the few temperatures I suggested will show if the TMV is by passing but will not explain the very high boiler output.

I’m with you John it is very strange. All pipe work is insulated, we have doubled checked the flow and return today and all seems ok. There is an external bypass on the boiler but nothing seems to be flowing through it, as the pipes are cold. I wonder if it because of the stupid pump setup on the laundry manifold it’s bypassing and going back to the boiler. I’m about ready to rip the whole system out 😂😂😂 I really can’t find an answer.

 

[John.g]

Bypass on its own shouldn't increase boiler demand,
Can you just run on DHW demand only and see what kind of cycling and flow/return temps you are getting.
Also ensure you check those manifold temperatures sometime.

 

[sherylryan]

Bypass on its own shouldn't increase boiler demand,
Can you just run on DHW demand only and see what kind of cycling and flow/return temps you are getting.
Also ensure you check those manifold temperatures sometime.

Ok, I’ll go do that now. I have turned the manifold temperatures down like you suggested yesterday. (Is that what you mean)

 

[John.g]

No, the next time the troubled manifold is on check the temperatures where highlighted with the heat gun as suggested in post 69, we can then see if that TMV is bypassing.

 

[sherylryan]

No, the next time the troubled manifold is on check the temperatures where highlighted with the heat gun as suggested in post 69, we can then see if that TMV is bypassing.

Ok, yes i wil do that. Well I ran the DHW system and this was the result

View attachment 76410
But also while I was standing there, I think I've discovered the problem. We have two pipes running from the boiler to the gas boiler as the radiators are piped to work on gas if something happens to the boiler. On further inspections these flow and return pipes are exceptionally hot, and the DHW is in the complete opposite direction. As you can see from the photos there are 2 valves but these are only shut if the rads are operating by the gas boiler. So it would seem we have flow going along these pipes circulating into the gas boiler, and heading back to the other boiler. This is a 28m loop
( I do hope that makes sense ) This would also explain why the boiler return temperature is higher than the manifold return temp.

 

[John.g]

Can you shut those valves temporarily and put a note on the gas boiler to that effect and see how the UFH operates.
Can't see attachment 76410 re DHW.

 

[sherylryan]

Can you shut those valves temporarily and put a note on the gas boiler to that effect and see how the UFH operates.
Can't see attachment 76410 re DHW.

Yes I can shut those valves temporarily, if the rads call for heat off this troubled boiler they use those pipes for flow and return, both boilers can not operate at the same time. The gas is purely for backup.
Attached DHW graph

 

[John.g]

If you mean that the boiler ran continuously for 45.37 mins then its output was ~ 19.7kwh and if all that energy was inputted to the hot water cylinder then the cylinder volume would need to be 846 liters to only rise by 20C, not likely, if the HW cylinder is say a more realistic 250 liters then it would only take 5.81kwh or 13.5 min to rise it by 20C. still not clear as to what's going on but with these valves now closed then it may be become clearer.

 

[sherylryan]

If you mean that the boiler ran continuously for 45.37 mins then its output was ~ 19.7kwh and if all that energy was inputted to the hot water cylinder then the cylinder volume would need to be 846 liters to only rise by 20C, not likely, if the HW cylinder is say a more realistic 250 liters then it would only take 5.81kwh or 13.5 min to rise it by 20C. still not clear as to what's going on but with these valves now closed then it may be become clearer.

No boiler didn’t run continuously, the blue on the graph is where the flame stops but the pump still runs to circulate the water, the boiler only fired for 29 mins 38 seconds out of the 44.37 mins (sorry was not clear on my outcome)

 

[ShaunCorbs]

There’s a set of valves on the boiler so worth a test

 

[sherylryan]

No boiler didn’t run continuously, the blue on the graph is where the flame stops but the pump still runs to circulate the water, the boiler only fired for 29 mins 38 seconds out of the 44.37 mins (sorry was not clear on my outcome)

Hi John
I run the troubled manifold with the rad valves turned off and the boiler return temp was exactly the same as the ufh manifold return temp. Although the boiler ran continuously for 42.07 mins and only got to 59 degrees. The manifold points you told me to measure were spot on, so the TMV is doing it’s job. I now believe the pump set to constant pressure and being such a big pump is another issue, as water was also bypassing through the external bypass.

As you can see the Rads literally T into the flow and return so no valves to stop the flow of water, with valves turned off water was still flow the this point in these pipes.

 

[John.g]

So install zone valve on rads and problem solved?.

 

[sherylryan]

So install zone valve on rads and problem solved?.

There are zones valves already on the rads but because of the new piping the have stuffed it up, so new pipe work and zones valves I’m picking