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Central heating pump
- Thread starter sherylryan
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View the thread, titled "Central heating pump" which is posted in Central Heating Forum on UK Plumbers Forums.
Hi John, interesting reading and out biggest problem is our troubled manifold is running 70% of the time by itself, where are rads are positioned they receive a lot of solar gain. Our troubled manifold is on a thermostat with a 3 degrees difference to turn back in. The biggest problem is thermostat gets up to temp but with 2 hours the sensor has detected the 3. Degrees drop and turns back on. Where does a LLH go on the system, maybe this is our only option. You are very helpful, thank you.
The thermostat has a floor probe, Heatmiser neo stat, on the wall in the kitchen.
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 temp low, then boiler turns off and pump pumping only temp rises
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Very interesting, you can see that the boiler return stays very constant at 39C once system up to temperature which is correct as this is the UFH loop return. What would be nice to see is the UFH manifold (mixed) flow temperature during the same period, if you can only monitor two then run the boiler with the boiler flow temp and the manifold mixed temperature. The boiler flow&return were ~ 24C on start up, after 5 mins 40C/27.4C and 70C/39C at burner off after 32 minutes. When circuits up to temperature, then burner cycle time was ~ 12 mins, 6 on, 6 off, so around 13kw average boiler output?.
There are two different requirements here.
one: when on this UFH only, boiler circ pump should stay running continuously with ufh demand and boiler burner cycling on/off.
two: when no demand from UFH, boiler circ pump should also shut down as well as the zone valve and manifold pump, presume this is what is happening?
Once all tests carried out you may consider reducing the manifold temperature from 48C to say 40C which will give longer run times without the (safety) floor sensor acting, even though 2 hours is a reasonable run time. Once steady demand conditions are established (not bad, just now) then the burner on time as a % of the cycle (on/off) times should accurately reflect the UFH & boiler demand. If the burner run time is say 62% of the cycle time then the boiler output is 26*62%, 16kw.
There are two different requirements here.
one: when on this UFH only, boiler circ pump should stay running continuously with ufh demand and boiler burner cycling on/off.
two: when no demand from UFH, boiler circ pump should also shut down as well as the zone valve and manifold pump, presume this is what is happening?
Once all tests carried out you may consider reducing the manifold temperature from 48C to say 40C which will give longer run times without the (safety) floor sensor acting, even though 2 hours is a reasonable run time. Once steady demand conditions are established (not bad, just now) then the burner on time as a % of the cycle (on/off) times should accurately reflect the UFH & boiler demand. If the burner run time is say 62% of the cycle time then the boiler output is 26*62%, 16kw.
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.
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Yes when room stat is up to temp it shuts everything down.
Here is another boiler graph after they downsized the boiler to 26kw
Also this afternoon I watched the boiler start up from cold, 4.02pm start up 32deg, 4.15pm 45 deg, 4.22pm 49deg, pump set to 1.45 bar, 1840rpm, return flow 43.1deg and that’s where it sat for 1 hour
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No red is the burner firing, blue is the boiler pump circulating only
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
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,.
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,.
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Here is a graph of the radiators calling for heat, BLUE IS BOILER FIRING, Red is pump only circulating
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.
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.
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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.
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.
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.
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)
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.
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.
So the o
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.
The pump set is only 3 weeks old are you suggesting the mixing valve could be faulty? Hence the water entering the manifold at a temperature higher than the set value?
Possibly but unlikely, the boiler circ pump pressure though at 1.4bar or whatever may be preventing it from operating correctly which is why I suggested throttling it somewhere. Most boiler circ pumps will only be running with a head of 4M (0.4bar) or so. I throttling it works, re install your old pump.
Yes I agree re installing old pump, I will try throttling the pump back again and also turn the manifold pump back to 2, it’s currently set on 3. I must admit the floors are heating up better with the new pump set as opposed to the pump set on the good manifold. John thank you very much for all the information you have supplied to me. To balance a ufh manifold is it only done with the flow meters, or can you slow the return water?
I wouldn't reduce the flow rate excessively as the loops are parallel circuits and you want to keep the flows equal, temperature control is the way go INO.
temperature reduction/control is the way to go O
Hi John, well I have throttled back the boiler pump to 1 bar and the manifold pump to speed setting 2 and I achieved 65 degrees flow, and 49 degrees return at the boiler and 45 degree flow through the manifold and 35 return. The boiler is cycling though of 15 mins on 1.15 mins off. I feel we are getting somewhere 🙂
Hi John I thought that as well, yes underfloor only on, once the manifold was up to temp would the excess water return back to boiler, making the boiler return higher? I did notice however that with the boiler thermostat set to 60, the flow did get as high as 70 at one point. I see there are two ports on either side on the top of the boiler ther thermostat probes are in the left, I am picking that has actually been set up correctly.
Maybe this TMV is sending mixed flow temperature water back to the boiler?, I always thought it was supposed to be the UFH cold water return.
The boiler thermostat should control the flow temperature at whatever setpoint its set to.
Did you check that the boiler flow and return pipes are installed correctly?.
The boiler thermostat should control the flow temperature at whatever setpoint its set to.
Did you check that the boiler flow and return pipes are installed correctly?.
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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.
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