Boiler Short Cycling

[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