Boiler Short Cycling

[bagaceratops]

If all rads are producing their rated output then rad demand ~ 9.4kw.
UFH, 8LPM? at say a dT of 8C= 4.45kw. total, assuming no DHW = 13.85kw.
But, by calculation, boiler output at a flowrate of 1.42m3/hr @ dT of 12C, (70-58) is 19.82kw, 1.42x16.67x60x12/860, so again something not adding up, I suspect the pump is not now providing the correct data as one, its head at 3.7m can hardly even overcome the boiler Hx loss and two, by calc, the pump at those values is running at a efficiency of 54.85%, seems unlikely IMO, so again maybe reduce d.0 to a few kw below what your present calculated demand is, from pump flow&boiler dT and the flow temp should fall a bit below its SP, this will show up something.

Hi,

I have gone through all of my radiators and their output is 9.4kw at Delta 50, 7.5kw at delta 40.
My underfloor heating has in total 7 loops manifold 1 and 6 loops manifold 2. They are all configured for flow rate of circa 2 litres per second. I believe this gives 7.2kw with dT of 8 degrees, but they are probably running at 10.

Assuming I use delta 50 figure this gives me 16.6kw plus the hot water take - How to calculate that - 250 llitre tank, fully steel enclosed and cold to touch on outside.

My system if un vented, with the condensing boiler which is circa 2007.

I set the boiler to d0 21kw and it appears to be behaving better - Is this due to my load being no where near 30kw?

I have tried to balance the radiators with a 20 degree difference, although should it be 20 or lower?

Many Thanks for your continued responses and help

Andrew

 

[ShaunCorbs]

Your load is 18kw you don’t add hot water re heat capacity in as you stagger your times eg an hour before the heating comes on the hot water comes on etc

I’m guessing the ufh is lpm so puts around 10kw at dt5 much better to run like this

You can balance the ufh as well with two clamp probes you want a difference of 5- 10 degrees so adjust the flow rate via the meters to sort this

Yes correct trying to shove 30kw down a 18kw load will cycling half the time roughly

Dt15-20 on the rads is good

 

[John.g]

Hi,

I have gone through all of my radiators and their output is 9.4kw at Delta 50, 7.5kw at delta 40.
My underfloor heating has in total 7 loops manifold 1 and 6 loops manifold 2. They are all configured for flow rate of circa 2 litres per second. I believe this gives 7.2kw with dT of 8 degrees, but they are probably running at 10.

Assuming I use delta 50 figure this gives me 16.6kw plus the hot water take - How to calculate that - 250 llitre tank, fully steel enclosed and cold to touch on outside.

My system if un vented, with the condensing boiler which is circa 2007.

I set the boiler to d0 21kw and it appears to be behaving better - Is this due to my load being no where near 30kw?

I have tried to balance the radiators with a 20 degree difference, although should it be 20 or lower?

Many Thanks for your continued responses and help

Andrew

Your rads, with a 70C flowtemp & dT of 20C, output 7.03kw, 7.61kw at dT15C & 8.2kw at dT 10C.

Your UFH, if you have 7loops (UFH1) at 2LPM & dT8C will output 7.8kw & 6 loops (UFH2) at 2LPM&dT8C will output 6.7kw, total 14.5kw??.

If you run the rads alone at a dT of 20c then the boiler flow/return temps will be 70/50C. Boiler demand 7.03kw. flowrate 5.04LPM, 0.30m3/hr
If you run the rads+one UFH loop then the boiler flow/return temps will be 70/45C, dT 25C assuming manifold temp of 45C. Boiler Demand 14.83kw. flowrate 8.5LPM, 0.51m3/hr
If you run the rads+both UFH loops then the boiler flow/return temps will be 70/43C, dT27C assuming manifold temp of 45C. Boiler demand 21.53kw. flowrate 11.42LPM, 0.69m3/hr

DHW coil, if you run it alone then the boiler dT is the coil dT and use the pump flowrate and the calculation to get its output, can then add that to the others to see what comes up.

Going back to your original problem, is the boiler running better now because it reaches its SP of 70C before the dT reaches 14C or what?.

 

[John.g]

My underfloor heating has in total 7 loops manifold 1 and 6 loops manifold 2. They are all configured for flow rate of circa 2 litres per second. I believe this gives 7.2kw with dT of 8 degrees, but they are probably running at 10.

Can you clarify the above, has UFH1 got 7 loops flowing ~ 2LPM/loop, total 14LPM and has UFH2 got 6 loops flowing ~ 2LPM/loop., total 12LPM.
If so, then UFH1 output is 7.8kw and UFH2 output is 6.7kw.
You give a total output of 7.2kw which implies a total of ~ 7 loops?.

 

[bagaceratops]

Can you clarify the above, has UFH1 got 7 loops flowing ~ 2LPM/loop, total 14LPM and has UFH2 got 6 loops flowing ~ 2LPM/loop., total 12LPM.
If so, then UFH1 output is 7.8kw and UFH2 output is 6.7kw.
You give a total output of 7.2kw which implies a total of ~ 7 loops?.

Can you clarify the above, has UFH1 got 7 loops flowing ~ 2LPM/loop, total 14LPM and has UFH2 got 6 loops flowing ~ 2LPM/loop., total 12LPM.
If so, then UFH1 output is 7.8kw and UFH2 output is 6.7kw.
You give a total output of 7.2kw which implies a total of ~ 7 loops?.

Sorry yes 13 loops in total @ 2LPM

 

[John.g]

When you ran with rads+UFH1 can you remember the boiler flow/return temps and the (pump) flowrate?.

 

[John.g]

If you run only one UFH then a boiler temp of 70C will result in a boiler flow of only 3.39LPM, 0.2m3/hr and a boiler dT of 33C which will probably cause the boiler to trip, if you reduce the boiler temp to 55C then the boiler flow is 6.21LPM,0.37m3/hr and a boiler dT of 18C. Running both UFH with boiler at 55C 55c will result in the same boiler dT of 18C & a flowrate of 11.55LPM,0.69m3/hr.

 

[bagaceratops]

When you ran with rads+UFH1 can you remember the boiler flow/return temps and the (pump) flowrate?.

No, But I will take some measurements later. Thanks.

Is it likely that this is just a configuration / set up issue or boiler fault?
My engineer is coming back next week, but I am really not sure where to direct him and dont want to pay labour for nothing. I got the feeling when he was last here that he did not know where to look.

Thanks

 

[John.g]

If the fault is this "14C dT" issue then would appear to be a boiler fault, is this your only issue?.

 

[bagaceratops]

Hi Pretty much

Just fired up the CH, DWH off from relatively cold.
It seems to be cutting at dT of around 15 to 16 today
running at 3.2 head and 1.1 m3/hr

this was with d0 at 20
Dropped d0 to 10, but burner barely running and water not really heating

Guess I will need to get engineer back and have him call Glow worm for advice.

thx

 

[John.g]

Don't know if you made any progress but something strange (again).
If/when the rads only are on at 9.4kw then the dT should be ~ 7.4C at a flowrate of 1.1m3/hr so wonder if boiler sensors are OK even though you seem to prove that that they are/.

 

[bagaceratops]

Don't know if you made any progress but something strange (again).
If/when the rads only are on at 9.4kw then the dT should be ~ 7.4C at a flowrate of 1.1m3/hr so wonder if boiler sensors are OK even though you seem to prove that that they are/.

Not really, otherthan it works much better with d0 set at 21. Although concious that this is not enough power when everything is on and when it is cold.

Just tried now from cold (was at 28 degrees)
Boiler headed up to 70 and settled at
flow - 70
return 57
3.47 head
1.18 flow

I put temp probes on the flow and return into the boiler and the readings pretty much (<5 degrees) matched what d40 and d41 were saying.

I have my heating person coming back next week and have e-mailed glow worm who have given me a number for him to call and do some tests. I asked if I could do these tests etc, but was told that need specialist equipment?

Thanks again

Andrew

 

[ShaunCorbs]

How fast did the return get to 57 ?

 

[bagaceratops]

Not really, otherthan it works much better with d0 set at 21. Although concious that this is not enough power when everything is on and when it is cold.

Just tried now from cold (was at 28 degrees)
Boiler headed up to 70 and settled at
flow - 70
return 57
3.47 head
1.18 flow

I put temp probes on the flow and return into the boiler and the readings pretty much (<5 degrees) matched what d40 and d41 were saying.

I have my heating person coming back next week and have e-mailed glow worm who have given me a number for him to call and do some tests. I asked if I could do these tests etc, but was told that need specialist equipment?

Thanks again

Andrew

Did not time it, maybe 20-30 mins

 

[John.g]

Not really, otherthan it works much better with d0 set at 21. Although concious that this is not enough power when everything is on and when it is cold.

Just tried now from cold (was at 28 degrees)
Boiler headed up to 70 and settled at
flow - 70
return 57
3.47 head
1.18 flow

I put temp probes on the flow and return into the boiler and the readings pretty much (<5 degrees) matched what d40 and d41 were saying.

I have my heating person coming back next week and have e-mailed glow worm who have given me a number for him to call and do some tests. I asked if I could do these tests etc, but was told that need specialist equipment?

Thanks again

Andrew

< 5C might be significant if it resulted in your dT being 8C (13-5), this, assuming flow rate correct results in a heat demand of 11kw not a whole lot away from your rad demand assuming you didn't have the ufh or cylinder calling for heat.
Hope your heating person at least brings a multimeter with him/her to measure the flow/return sensors resistance and confirm them with Glowworm, probably did that at the last visit anyway.

 

[bagaceratops]

< 5C might be significant if it resulted in your dT being 8C (13-5), this, assuming flow rate correct results in a heat demand of 11kw not a whole lot away from your rad demand assuming you didn't have the ufh or cylinder calling for heat.
Hope your heating person at least brings a multimeter with him/her to measure the flow/return sensors resistance and confirm them with Glowworm, probably did that at the last visit anyway.

Thanks

No it was just rads on today, checked DHW off and UHF off.

No he visually looked, saw that there was a slight leak on a condensate pipe and suggested that may be the problem so the condensate trap was replaced. When that did not fix it he said it must be the temp sensors. Not unhappy with this because it was leaking and hence did need replacing.

It was me getting the manual and going into the settings that I pulled off the sensor values and put on probes and then saw that it was tripping at DT14 - although earlier today with d0 at 21 it seemed to do it at dT 18.

Am happy to replace the temp sensors (although they are £30 each), but do not want to willy nilly replace parts.
Looking at the manual there does not seem much else to replace, apart from the main PCB.

It all seems to be working ok at present, but that is with d0 at 21. What concerns me is when it is really cold again and then the boiler goes into its cycling and it takes ages to heat the house up - Which is what was happening in the mid december really cold spell. Everything came on then and it would heat up maybe 5 degrees, cut, heat again and take forwver to get to 70.

You may be asking why I am using this chap. Long story, but the fan failed about 12 months ago and I could not get any local plumbers out. Eventually resorted to paying £300 for Glowworm engineer to come out (Given fan was £200 was not unhappy with this), but Glowworm refused to work on my boiler on H&S grounds. The boiler is in my attic next to the hatch so you have to stand on the loft ladder whilst working on it. No Big deal, but it broke their Health and safety rules. Most annoying.

Thanks

Andrew

 

[John.g]

If you are using thermocouple probes taped to the pipes then they should be very accurate IMO, I have a Proster dual channel thermocouple thermometer and its very very accurate on the very few occasions that I've used it ,
However you can do another independent test with rads only on and boiler firing constantly, take a photo of your gas meter and exactly 3 minutes later take it again, X this consumption X 220 which will give the boiler input, X this by say 88% (boiler efficiency) and that will be your heating demand. (kw)

 

[bagaceratops]

If you are using thermocouple probes taped to the pipes then they should be very accurate IMO, I have a Proster dual channel thermocouple thermometer and its very very accurate on the very few occasions that I've used it ,
However you can do another independent test with rads only on and boiler firing constantly, take a photo of your gas meter and exactly 3 minutes later take it again, X this consumption X 220 which will give the boiler input, X this by say 88% (boiler efficiency) and that will be your heating demand. (kw)

Hello, resurecting this again.

I had my plumber on site today to try and diagnose this.
He had two long calls with Glow worm

First call I dont think they really understood it and said it was thermistors or PCB, but they asked him to get the system cold and try again.
He took the thermistors out and measured the resistance at the values given by Glowworm - They tallied.
Also he has his professional meter on flow and return and it pretty much tallied with D40 and D41 as did my cheap £20 monitor
They also suggested pump pulling too much flow. So it was tried on lowest PP curve. Did same thing and was not really pushing enough water.

He called back Glowworm when cold and went through more with Glow worm, this time he got someone more knowledgeable who seemed to sugggest gas valve not correct. Unfortunately his gas analyzer failed so I have to wait for him to return.

I said that when I drop D0 that the boiler does not cycle quite so much, but takes much longer to heat up - This could make the gas valve setting more plausible.

He spent 3 hrs here today and not sure any further forward.

I could get a PCB myself and fit but I am not convinced thats the issue.

Any more thoughts?

Thank You

Andrew

 

[John.g]

So, the problem still is that once the dT reaches 14C that it cycles?. possibly PCB as it probably should cycle if the dT reachs ~ 30C but certainly not as low as 14C, even 20C is a quite normal dT on any gas fired boiler. Don't know why they suggest that the pump is circulating too much, the higher the flowrate the lower the dT for any given output.

 

[bagaceratops]

So, the problem still is that once the dT reaches 14C that it cycles?. possibly PCB as it probably should cycle if the dT reachs ~ 30C but certainly not as low as 14C, even 20C is a quite normal dT on any gas fired boiler. Don't know why they suggest that the pump is circulating too much, the higher the flowrate the lower the dT for any given output.

Hi John,

Seemed to vary today at around the 15-16C dT figure. Glow worm said it would if dT was 20 or more, but never saw that figure.
Yes it keeps this cycle - eventually gets to set point, then modulates fine.

If you turn d0 down it seems to run better and accepts wider dt, but takes really long to heat up.
PCB looks easy to replace but they are circa £180, although one on ebay for £65 (new in box) so I guess could be worth a punt.

Thanks

Andrew

 

[bagaceratops]

Finally (I hope) got to the bottom of this today.
Glow worm told my heating engineer it was prob gas valve; unfortunately his analyser broke so had to wait for him to return. Today he returned and tested gases and all good. He said gas valve was prob next thing to replace. However I had decided (on off chance) to get a new PCB (which I obtained BNIB from Ebay for £60, rather than the new £200 price). Had him fit the PCB today to see what happens. So far it seems to be behaving properly, but time will tell.

Thanks for all your inputs

Andrew