Central heating pump

[sherylryan]

Wondering if someone can answer my queries. We have a firebird C35, downsized to 26kw condensing boiler. 2 Ufh manifolds and rads. When the boiler runs the rads and manifold 50m from boiler the boiler runs perfectly. When the boiler runs the closest manifold 15 m away from boiler heating approx 150sqm floor area, boiler never gets up to temperature, a new pump was fitted running on constant pressure outside of boiler. Pump runs extremely fast on closest manifold, but boiler never gets past 50 degrees. When other manifold and rads are also run with this manifold system runs poorly. Anyone have some ideas for me please.

 

[sherylryan]

Now that I'm fully awake and looking at the trends below, it would seem that the red might be the burner off times and the blue the burner on times, the burner seems to cut out at ~ 72C/75C and cut in at 61C/63C. One way or the other anyway the boiler is maintaining its stat settings so don,t see any big overall problem as long as the UFH flow/return temps are constant and the calculation (LPMx60*dT/860) corresponds to the boiler % cycling time.

 

[sherylryan]

Now that I'm fully awake and looking at the trends below, it would seem that the red might be the burner off times and the blue the burner on times, the burner seems to cut out at ~ 72C/75C and cut in at 61C/63C. One way or the other anyway the boiler is maintaining its stat settings so don,t see any big overall problem as long as the UFH flow/return temps are constant and the calculation (LPMx60*dT/860) corresponds to the boiler % cycling time.

No red is the burner firing, blue is the boiler pump circulating only

 

[sherylryan]

The zone valve, pump and boiler all shut down when thermostat reaches temp and tells everything to turn off. I have attached a graph of the flow and return at the boiler, interesting when the boiler is firing

No red is the burner firing, blue is the boiler pump circulating only

temp low, then boiler turns off and pump pumping only temp rises

Yes that’s correct. That’s what I couldn’t understand when carrying out the experiment, why would the temp rise when pump only pumping as opposed to boiler firing and the temp is low

 

[John.g]

OK, confusing signals allright, what UFH output are you getting by doing the calc, above, and what is the time scale on the trends bottom. the boiler would appear to be firing ~ 86.6% of the time, ~ 8.35M on & 1.29M off, this gives a average output of 22.5kw based on a 26kw boiler, how does this compare with the calculated numbers?.
Edit: These tests were carried out last november and the return temp was 50C which means a manifold flow temp of ~ 60C.

Sometime, you might carry out those tests on the healthy system,.

 

[sherylryan]

OK, confusing signals allright, what UFH output are you getting by doing the calc, above, and what is the time scale on the trends bottom. the boiler would appear to be firing ~ 86.6% of the time, ~ 8.35M on & 1.29M off, this gives a average output of 22.5kw based on a 26kw boiler, how does this compare with the calculated numbers?.
Edit: These tests were carried out last november and the return temp was 50C which means a manifold flow temp of ~ 60C.

Sometime, you might carry out those tests on the healthy system,.

Here is a graph of the radiators calling for heat, BLUE IS BOILER FIRING, Red is pump only circulating

 

[John.g]

The above makes no real sense either as the temperature keeps rising when the burner is apparently off.

What signal/indication are you using to tell you that the burner is firing or not?.

I would suggest monitoring the burner firing time (boiler on/off) by watching the flame actually coming on/off in the viewing window and monitoring the temperatures.
I would also suggest that you do the above on the UFH(s) and monitor the manifold flow temp (which I don't think you have been) and note the total flow as per the flow tubes, the UFH output can then be cross checked reasonably well by using the % boiler cycle firing time. The manifold temperature gauges should also give reasonably accurate temperatures.

 

[John.g]

Is this manifold plumbed correctly?.

 

[sherylryan]

The above makes no real sense either as the temperature keeps rising when the burner is apparently off.

What signal/indication are you using to tell you that the burner is firing or not?.

I would suggest monitoring the burner firing time (boiler on/off) by watching the flame actually coming on/off in the viewing window and monitoring the temperatures.
I would also suggest that you do the above on the UFH(s) and monitor the manifold flow temp (which I don't think you have been) and note the total flow as per the flow tubes, the UFH output can then be cross checked reasonably well by using the % boiler cycle firing time. The manifold temperature gauges should also give reasonably accurate temperatures.

I monitored the boiler exactly how you suggest. I will do the manifold today, it’s very confusing as to why the temp rises when the flame is off.

 

[John.g]

It sure is, and the other puzzle is that Laundry manifold (the good one?) above which seems to be sending mixed water temperature back to the boiler, also the pump is connected between the UFH cold return and the manifold flow, the troubled manifold is plumbed the normal way like below.

 

[sherylryan]

It sure is, and the other puzzle is that Laundry manifold (the good one?) above which seems to be sending mixed water temperature back to the boiler, also the pump is connected between the UFH cold return and the manifold flow, the troubled manifold is plumbed the normal way like below.
View attachment 76356

It sure is, and the other puzzle is that Laundry manifold (the good one?) above which seems to be sending mixed water temperature back to the boiler, also the pump is connected between the UFH cold return and the manifold flow, the troubled manifold is plumbed the normal way like below.
View attachment 76356

The troubled manifold was plumbed the same as the laundry manifold until 4 weeks ago, we changed the pump set. We are going to replace the laundry pump set the the same as above as well.

 

[John.g]

Just check also that the flow through the boiler is in tht right direction.

 

[sherylryan]

Just check also that the flow through the boiler is in tht right direction.

Ok I’ll have a look, thank you

 

[sherylryan]

Ok I’ll have a look, thank you

Well I have watched the flow and return on the manifold after the mixing valve which was set to 42 degrees, I found the flow going into the manifold reached 52 degrees. The boiler fired continuously for the entire 2 hours and 25 minutes. As far as the flow rates through the circuits were 5 @ 3lpm, 3 @ 4lpm, 1@ 2.5lpm, a total of 29.5lpm. As per attached photos the flow meters were turned down low but still showing extreme flow through them. (Sorry about the sideway photos)

 

[John.g]

Very good, all adds up thermodynamically any way, S.sheet shows boiler output of 24.7kw, almost flat out, hence constant firing.
Suggest throttling hot water before the manifold to see if mixing temp drops, this would then point to excesss boiler feed pump pressure and/or reduce manifold pump to speed 2, at least that will point you, hopefully, in the right direction.

 

[sherylryan]

So the o

Very good, all adds up thermodynamically any way, S.sheet shows boiler output of 24.7kw, almost flat out, hence constant firing.
Suggest throttling hot water before the manifold to see if mixing temp drops, this would then point to excesss boiler feed pump pressure and/or reduce manifold pump to speed 2, at least that will point you, hopefully, in the right direction.

View attachment 76367

Very good, all adds up thermodynamically any way, S.sheet shows boiler output of 24.7kw, almost flat out, hence constant firing.
Suggest throttling hot water before the manifold to see if mixing temp drops, this would then point to excesss boiler feed pump pressure and/or reduce manifold pump to speed 2, at least that will point you, hopefully, in the right direction.

View attachment 76367

Thanks John you are very helpful. So our boiler originally 35kw was fine and didn’t need to be downsized at all. If it’s firing ar 24.7 kw with the Troubled manifold it clearly isn’t going to run the 2 x UFH manifold/circuits, 15 radiators and 28m of trench, in fact I wonder when all of them are firing 35 kw was also to small as well.