Boiler Short Cycling

[John.g]

The pump curves are in your attachment, I reckon that you would get ~ 1.1m3/hr @ 2.1m on PP B and ~ 1.4m3/hr @ 3.6M on PP C.

This doesn't solve your problem however, theoretically PP C should reduce the dT from 14C to 8C and allow the boiler to get closer to its SP temperature before recycling and might reduce the 20C dT to 11.5C even though no problems with the boiler once it reaches its SP.

Have you reduced d.0??.

 

[bagaceratops]

Thats a amazingly low head, anyway, by calculation, using the pump flow of 0.8m3/hr and the boiler dT of 21C gives a boiler output of 19.53kw, the 8 rads at rated output = 9.4kw but because of the flow/return temps will be outputing ~ 74% of their rated output, say 7kw. So 12.53kw going elsewhere??.
If the pump values are still similar suggest reducing d.0 to 15kw, if the above boiler output is valid then the flow/return temps should reflect a 15kw boiler output and would confirm that d.40 & d.41 are reading accurately.

Interesting,

I dropped D0 to 15 and set the pump to PPB and it stopped cutting out. But it did not appear enough to heat the water. I had rads on and one UFH circuit at the time.
Then all circuits started calling for heat so increased d0 to 25, It started its cutting out. Gradually reduced D0 to 21 where it appears not to cut out, this is with most of the circuits on, but it is taking much longer to heat up, although not cycling.

Pump is showing circa 3.5 metres head and 1.5 m3 flow this is on curve B

Should I be using curve C?
Not having any heat loss calculations for my house again dfifficult to know what boiler should be on.

This is really interesting stuff, my plumber / heat engineer never discussed any of this.

Thanks

 

[ShaunCorbs]

Easy enough to do go round and calculate the size of your rads and find similar ones on the internet there output will be listed so Calc all the rads and total it up

Then give us the flow rate of each ufh loop via the flow meter and we can work out the rough load ufh wise

 

[John.g]

Interesting,

I dropped D0 to 15 and set the pump to PPB and it stopped cutting out. But it did not appear enough to heat the water. I had rads on and one UFH circuit at the time.
Then all circuits started calling for heat so increased d0 to 25, It started its cutting out. Gradually reduced D0 to 21 where it appears not to cut out, this is with most of the circuits on, but it is taking much longer to heat up, although not cycling.

Pump is showing circa 3.5 metres head and 1.5 m3 flow this is on curve B

Should I be using curve C?
Not having any heat loss calculations for my house again dfifficult to know what boiler should be on.

This is really interesting stuff, my plumber / heat engineer never discussed any of this.

Thanks

Does the cycling still only occur when the dT reaches 14C?.

You can calculate the boiler output at any given time by
(m3/hr16.67x60xdT/860)kw, At 14C dT, boiler output is (1.5x16.67x60x14/860), 24.4kw. So if rads only on easy to calculate their combined output once you also note the boiler flow/return temperatures.

A flowrate of 1.5m3/hr, 1500LPH should result in a Hx dP of 3.6m, 2.38x(1500/1220)^2, but your pump is only developing 3.5m so something not adding up?, boiler internal or external bypass passing, or another pump hidden away some where?.
Even If the boiler has a internal bypass thats faulty presume the boiler will still measure the flow/return temps correctly.
The pump, judging by your original pump values for PP A seems correct, can you just post the power W + flow & head while running in PP B and we can soon tell if these values add up.

 

[bagaceratops]

Does the cycling still only occur when the dT reaches 14C?.

You can calculate the boiler output at any given time by
(m3/hr16.67x60xdT/860)kw, At 14C dT, boiler output is (1.5x16.67x60x14/860), 24.4kw. So if rads only on easy to calculate their combined output once you also note the boiler flow/return temperatures.

A flowrate of 1.5m3/hr, 1500LPH should result in a Hx dP of 3.6m, 2.38x(1500/1220)^2, but your pump is only developing 3.5m so something not adding up?, boiler internal or external bypass passing, or another pump hidden away some where?.
Even If the boiler has a internal bypass thats faulty presume the boiler will still measure the flow/return temps correctly.
The pump, judging by your original pump values for PP A seems correct, can you just post the power W + flow & head while running in PP B and we can soon tell if these values add up.

View attachment 80657

Hi,

So boiler settled down and is modulating ok now
flow = 70, return = 58
Pump is running at 26 Watts, 3.7 metres head and 1.42 litres per second
I have rads on, DHW is on, but not sure if calling for heat
Two UFH zones are on (on seperate manifolds)
UFH 1 is about 50 metres and 2 litres per min
UFH2 is 3 loops of 80,80 and 50 each running at 2 litres per min

There are 3 pumps on my system
Main circulator plus each UFH manifold has its own pump

 

[bagaceratops]

Hi,

So boiler settled down and is modulating ok now
flow = 70, return = 58
Pump is running at 26 Watts, 3.7 metres head and 1.42 litres per second
I have rads on, DHW is on, but not sure if calling for heat
Two UFH zones are on (on seperate manifolds)
UFH 1 is about 50 metres and 2 litres per min
UFH2 is 3 loops of 80,80 and 50 each running at 2 litres per min

There are 3 pumps on my system
Main circulator plus each UFH manifold has its own pump

and d0 is set to 21 KW

 

[ShaunCorbs]

Won’t be 1.42 lps as that’s 110kw at dt 20

21kw is 0.25lps / 15lpm at dt 20
Or at a dt 10 it’s 0.5lps

 

[bagaceratops]

Won’t be 1.42 lps as that’s 110kw at dt 20

21kw is 0.25lps / 15lpm at dt 20
Or at a dt 10 it’s 0.5lps

My mistake not litres per second, m3 / hr

 

[ShaunCorbs]

So 0.394 lps so 16.54kw the pump is moving

 

[John.g]

Hi,

So boiler settled down and is modulating ok now
flow = 70, return = 58
Pump is running at 26 Watts, 3.7 metres head and 1.42 litres per second
I have rads on, DHW is on, but not sure if calling for heat
Two UFH zones are on (on seperate manifolds)
UFH 1 is about 50 metres and 2 litres per min
UFH2 is 3 loops of 80,80 and 50 each running at 2 litres per min

There are 3 pumps on my system
Main circulator plus each UFH manifold has its own pump

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.

 

[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.