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.
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View the thread, titled "Central heating pump" which is posted in Central Heating Forum on UK Plumbers Forums.
Here is a pic of the boiler and the external pump that was fitted, (they run pump at 1.8 bar, constant pressure, variable speed) through to the maze of pipe work. Boiler heats 2 manifolds, manifold one has 10 circuits, 9 on one thermostat heating an area of 160sqm approx, manifold 2 has 8 circuits with 3 thermostats, also runs 15 Radiators and DHW.
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You say that the "Pump runs extremely fast on closest manifold, but boiler never gets past 50 degrees."
Presume the boiler stat is set at ~ 70C, if so, or if set to anything > 50C, is the burner firing continuously or cycling on/off, if firing continuously then would point to a heating demand > the boiler output of 26kw (unlikely?). Have you got 4 pumps in all, the boiler circ pump + the Grundfos pump shown in photo+ one circ pump each on the UFH manifolds?.
Correct the boiler does not get up to temperature on the closest manifold circuit but does on the other manifold and rads. The boiler is set at 70 degrees, the boiler runs continuously for an hour then cycles on and off. We only have 3 pumps, they removed the boiler circ Pimp and replaced it with the grundfos in the photo, plus one on each UFH manifold. The pump in the pic, runs at 1.85 bar, when the closest manifold is on it Ramps up to 2500 rpm, when the manifold 55m away comes on pump runs at 1500rpm same when the rads are calling for heat.
Is it on the closest manifold that it runs continuously for an hour and then cycles on/off? and is the boiler temperature still only 50C?
Can you post a photo of this 1.85bar pump, this head (18.5M) seems extraordinarily high for a circulation pump, even a 8M pump would be considered high, also please post the exact model number and any other details from the pump label.
Can you post a photo of this 1.85bar pump, this head (18.5M) seems extraordinarily high for a circulation pump, even a 8M pump would be considered high, also please post the exact model number and any other details from the pump label.
Hi John
Yes it is the closest manifold (kitchen), yes after this time the boiler is still sitting anywhere between 45-50 degrees it never reaches boiler set thermostat.
Below is pics of the pump, you will notice in the images of rads and second manifold the rpm’s are quite low. But the kitchen manifold is nearly double the rpms.
The pump is a grundfos CRIE 3-4, pump is set to constant pressure
Yes it is the closest manifold (kitchen), yes after this time the boiler is still sitting anywhere between 45-50 degrees it never reaches boiler set thermostat.
Below is pics of the pump, you will notice in the images of rads and second manifold the rpm’s are quite low. But the kitchen manifold is nearly double the rpms.
The pump is a grundfos CRIE 3-4, pump is set to constant pressure
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Can't understand the need for a 1.8bar pump, this pump (if it had a bronze casing) would be suitable to pump cold water from a header tank through the secondary heat exchanger of a combi boiler to provide hot water in the event of too low mains pressure.
Used as a circ pump to supply the hot water to a UFH manifold at this head will probably upset the TMV and the ability of the manifold pumps to recirculate water to give the desired mixed water temp as these pumps are generally only 6M pumps like a grundfos UPS3 or the like. It still doesn't explain the boiler running at 26kw, where is the heat going?
Maybe suggest as a temporary measure, reduce the setpoint to something like 0.7bar and see if any inprovement, also have a look at the manifold pumps and see make/model.
Used as a circ pump to supply the hot water to a UFH manifold at this head will probably upset the TMV and the ability of the manifold pumps to recirculate water to give the desired mixed water temp as these pumps are generally only 6M pumps like a grundfos UPS3 or the like. It still doesn't explain the boiler running at 26kw, where is the heat going?
Maybe suggest as a temporary measure, reduce the setpoint to something like 0.7bar and see if any inprovement, also have a look at the manifold pumps and see make/model.
We can’t understand the big pump either. I have attached a photo of the manifold pump, we have just replaced this as we thought it could be the problem. I just attempted to turn pump down and it does nothing but squeal, but the flow water temperature did increase.
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Definitely a big mismatch there, also the ratio of UFH manifold recirc rate to boiler circ rate will be in the order of ~ up to between 3&5 to 1, for example if you have only one UFH on emitting say 10kw with a mixed flow temp of 45C and return of 38C, the boiler circ pump will only need to supply 4.5LPM at 70C (return 38C) and the manifold pump must recirculate 16.0LPM at 38C, a ratio of 3.6 to 1.
What supplies the DHW demand?.
What supplies the DHW demand?.
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I completely understand, it’s so confusing. I wonder if the whole pump set up is wrong. Maybe we will just take this pump out and put original one back in. The boiler can do the DHW but we have it on electric as the boiler is a nightmare.
This big pump was put in because the boiler condensate pipe melted 6 times, so they assumed the boiler circ pump was not big enough. The baffles inside the boiler were disintegrated due to the heat (pic attached) the pump I tuned it down and flow temp increased, flow return dropped dramatically, turn the pump up and the flow temp drops and return increases .
Pic of pump removed from boiler and damaged baffles
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The plastic melting condensate system was cured by installing a Firebird supplied SS one.
The reason that the 50M run supplied UFH+rads work OK may be because of that long run as, depending on pipe diameter and circ flow probably results in a fair bit of head loss so the head at the manifold is more "normal" then.
I don't really think that the burner is/was actually firing for a hour without any rise in temperature, even if the boiler control stat is faulty, the hi limit stat, would trip/lockout the boiler at ~ 110C.
What diameter is this 50M piping and what are the UFH flow&return temps?
The reason that the 50M run supplied UFH+rads work OK may be because of that long run as, depending on pipe diameter and circ flow probably results in a fair bit of head loss so the head at the manifold is more "normal" then.
I don't really think that the burner is/was actually firing for a hour without any rise in temperature, even if the boiler control stat is faulty, the hi limit stat, would trip/lockout the boiler at ~ 110C.
What diameter is this 50M piping and what are the UFH flow&return temps?
Hi John.
The pipe diameter 33mm copper from boiler until it branches off to manifolds and rads where it drops to 28mm composite pipe before going into 16mm rads and manifold. With boiler thermostat set to 60 degrees Flow at end manifold is 66 degrees, return 54, rads are flow 70 return 55 and trouble manifold is 48/50 flow, 36 return. I timed the boiler heating troubled manifold this afternoon from cold, boiler fired for 93 minutes without stopping only getting to flow temp of 51degrees. Return 36, 🤷🏼♀️ Thank you.
Pipe sizing is not a problem at any rate.
If the boiler cycles on/off with 2XUFH + rads then if the boiler is firing continuously, the excess output must be absorbed somewhere as the problem UFH cannot absorb full boiler output on its own? and with such a slow rise in boiler temp points to the excess output going somewhere else. If the system is not absorbing it the only thing that makes sense to me is that a PRV is lifting somewhere and emitting ~ 8LPM to drain, highly unlikely, would also mean that the boiler has a auto filling system
Theoretically, but this would apply in all modes, if the E.vessel is on the suction side of the pump, then the pump discharge will be 3.6bar (1.8+1.8), don't know if this would lift the PRV or not.
If the boiler cycles on/off with 2XUFH + rads then if the boiler is firing continuously, the excess output must be absorbed somewhere as the problem UFH cannot absorb full boiler output on its own? and with such a slow rise in boiler temp points to the excess output going somewhere else. If the system is not absorbing it the only thing that makes sense to me is that a PRV is lifting somewhere and emitting ~ 8LPM to drain, highly unlikely, would also mean that the boiler has a auto filling system
Theoretically, but this would apply in all modes, if the E.vessel is on the suction side of the pump, then the pump discharge will be 3.6bar (1.8+1.8), don't know if this would lift the PRV or not.
Hi John
Just carried out your calculation and came to 21 kw. I have flushed manifold yesterday thinking it could be full of air, there was abit in one circuit but not bad. I have again stood and watched boiler, while heating troubled manifold, temp rose from 45deg to 58deg in 1/2 hour and sat at 58 for the next hour, return 44. Then turned on Other
manifold and it heated to 66 degrees return 58 in a matter of minutes 🤷🏼♀️ You are so helpful, thank you.
Just had another thought, could the expansion vessel in the boiler be to small? No sure how to calculate total water content of system. Boiler has a 12 litre vessel set to 1 bar.
Hi again, a funny thing happened today, I cheated the system and made the other thermostat on the manifold call for heat and popped the actuators off the other thermostat and believe it or not the boiler heated to 68 degrees although the thermostat is set to 60, why would cheating the system like that work, I thought a thermostat only told the boiler to turn on and off 🤷🏼♀️😂
Good Morning/Night
Are you running the UFH at very high flow/return temps? if not, what are the (mixed) flow/return temps and
what is the flowrate through each loop?
My basic calculations assume that you are using 100M, 16mm loops which will have a ID of 12mm, a 12mm ID pipe will contain 0.113L/M so total of (9+8)x100x0.113=192L in UFH pipework, say another 85L for the boiler+12 rads = total system contents of 277L. Assume max mean temperature of 60C if whole system in service. At 60C, expansion is 1.62%, a 12L E.Vessel with a precharge pressure & a filling pressure of 1bar will result in a final pressure of 2.2bar, if mean system temperature is 55C, expansion 1.36%, then final pressure is 1.92bar. You should be OK IMO, you can see the boiler pressure with a hot system anyway.
| (From water at 15C) | |
| Water | |
| Temp | Exp |
| Deg.C | % |
| 50 | 1.124 |
| 55 | 1.36 |
| 60 | 1.62 |
| 65 | 1.83 |
| 70 | 2.18 |
| 75 | 2.48 |
| 80 | 2.8 |
| 85 | 3.13 |
| 90 | 3.49 |
| 95 | 3.85 |
| 100 | 4.24 |
| 100 | 4.64 |
| 105 | 5.05 |
Strange all right re boiler stat.
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Mixed flow is 48 return is 36/37 on the trouble manifold flow rate is 2.4 lpm per min on 3 loops, 2 litres on 6 loops. When flow meters are opened up fully they never drop to 4 Lpm they only show 3 so not sure if they are reading correctly. Wow you have a lot of knowledge. Thank you very much for all this information.
That loop is emitting 16.1kw, you would think all right that the dT might be less at 2.0/2.4LPM, also still doesn't explain why it takes so long to get the temp up to 50C, the loop contains ~ 90L and assuming a cold water temp of 20C, would only require only 3.2kwh so should only take ~ 10/15 minutes as the loops are emitting very little until the flow temperature rises up a bit. If both UFH+rads on then will be very slow to heat up with a boiler output capped to 26kw, should be OK once rooms up to temperature.
You said the pump is noisy when you try to reduce the head, have you actually reduced it or is it still at 1.8bar?, if you post the other UFH details like above, loop flows+ manifold flow/return temps we can see the total flowrate required (with rads i/s as well) and see if the old pump was/is still suitable.
You said the pump is noisy when you try to reduce the head, have you actually reduced it or is it still at 1.8bar?, if you post the other UFH details like above, loop flows+ manifold flow/return temps we can see the total flowrate required (with rads i/s as well) and see if the old pump was/is still suitable.
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