Boiler Short Cycling

[bagaceratops]

Hello,

I wonder if anyone could offer some advice please?

I have an Ultracom 30HXI that serves DHW, Radiators and UFH manifolds. The boiler is experiencing short cycling problems and I have had a heating engineer attend twice now and he is unable to diagnose. The problem is as follows: When there is call for heat the boiler fires and heats the water, but only heats the water until there is a 14 degree flow/return difference - I have checked the values via the boiler diagnostic menu. It then cuts out, the flow temp drops and then within seconds it fires up. Heats the water again until a 14 flow/return difference. It continues in this cycle of fire, cutout fire etc until it reaches the flow temp set (in this case 70 degrees) so long as the return is not greater than 14 degrees difference. The boiler will then modulate its power and continue running; however once more than 14 degrees it cuts and continues its cycle again and again. The upshot is that it takes ages to fully heat the house. Are you able to advise on likely problem (or is this as design) such that I can direct my heating engineer to replace the correct part. The condensate trap has been replaced (as it was leaking) and the boiler has recently in the last six months had a new fan / service.

I have also posed the above to Glowworm technical support

Thanks

Andrew

 

[ShaunCorbs]

What happens if you just turn the rads or hot water on ?

 

[bagaceratops]

What happens if you just turn the rads or hot water on ?

Hi,

Thanks for the reply.

If I start from cold and just put the radiators on it does exactly the same. Heats till 14 degree difference, shuts off and then back on again in a few seconds. Repeating until it reaches set flow temp so long as difference no more than 14 degrees.
I went into boiler diagnosis mode where it would display the actual flow and return temps it is receiving.

If I leave just rads on and let it keep doing its cycles until it gets to max temp it modulates fine. If I then switch on the UFH and introduce some more cold into the return it goes back to its cycles. Eventually return increases and it will again modulate at the 70 degree set flow temp so long as difference does not go more than 14 degrees.

Thanks Andrew

 

[ShaunCorbs]

That defo doesn’t sound normally have you measured the temp on the pipes also how old is the pump ?

 

[bagaceratops]

That defo doesn’t sound normally have you measured the temp on the pipes also how old is the pump ?

Hi,

Pump is new as thought I that was the problem. It is better with the new pump - I think my previous was slightly undersized.
Yes have measured temps on the flow and return and whilst the reading is not the same as the boiler showing they are within about 5 degrees. My heating engineer was going to replace both the flow and return thermistors, but I am reluctant given they are £30 each plus his labour - albeit an easy thing to replace.

 

[ShaunCorbs]

Yea within 5 is fine have you tried lowering the flow temp to 65 ?

 

[bagaceratops]

Yea within 5 is fine have you tried lowering the flow temp to 65 ?

Hi, Yes have tried both increasing and decreasing the boiler max flow temp, but it still does the same thing. Although it gets there quicker with a lower set temperature..

 

[ShaunCorbs]

What speed is the pump on ?

 

[bagaceratops]

What speed is the pump on ?

I have tried all the pump speeds and does not seem to make a difference. It is on the proportional pressure curve b

 

[John.g]

What status codes "S" and "D" are displayed during this cycling?.

 

[bagaceratops]

What status codes "S" and "D" are displayed during this cycling?.

Hi,

There is nothing on the display other than the flow temp and burner flame.
or do you mean the state list of this boiler, which I have not worked out how to access.

Thanks

 

[John.g]

Presume you got the boiler return temp from d.41? through Diagnostic menu Level 1 so you should be able to access the S parameters by selecting d.99, this should give all the S parameters?

 

[bagaceratops]

Presume you got the boiler return temp from d.41? through Diagnostic menu Level 1 so you should be able to access the S parameters by selecting d.99, this should give all the S parameters?

View attachment 80539


View attachment 80538

View attachment 80537

Ah yes D99 - It is showing S04 and then when it cuts goes straight to S03 and then S04
I did try reducing pump speed again today to PPA curve. It is still cutting out, but stays on longer and lets the flow/return differential go much higher, circa 25 degrees

 

[John.g]

So only 2 of S codes, S.03 & S.04 which are ignition on and burner on, a bit strange that no other S code up when the burner stops, normally if the burner cuts out on say the boiler flowtemp exceeding its SP by 5C (where the boiler min output is greater than the heating demand) then you will get a S.08, the anticycle time.
You might look for a F (fault) code when next it stops, one would think though that if a fault code is flagged that you would have to reset the boiler?.

Not sure if available at your boiler output but you might try setting d.0, Heating Part Load, to say 10 or 12kw and see if any difference.

Where do you get these settings? "pump speed again today to PPA curve"

 

[bagaceratops]

So only 2 of S codes, S.03 & S.04 which are ignition on and burner on, a bit strange that no other S code up when the burner stops, normally if the burner cuts out on say the boiler flowtemp exceeding its SP by 5C (where the boiler min output is greater than the heating demand) then you will get a S.08, the anticycle time.
You might look for a F (fault) code when next it stops, one would think though that if a fault code is flagged that you would have to reset the boiler?.

Not sure if available at your boiler output but you might try setting d.0, Heating Part Load, to say 10 or 12kw and see if any difference.

Where do you get these settings? "pump speed again today to PPA curve"

Thanks, No F codes and just S03 and S04.
Will try the d0 setting, it is set to max at present

My pump is a Lowara Ecocirc L-plus 25-8/130 which has 3 fixed speeds, 3 constant pressure curves, 3 proportional pressure curves and an auto adapt curve

 

[ShaunCorbs]

Why such a big pump how many rads ?

 

[John.g]

Does that pump display the head(M), flowrate(m3/hr) and power(W)?, if and if d.0 is settable then if the max output is set lower than the heating demand then we can calculate the dTs etc from this and compare them with the boiler readings.

 

[bagaceratops]

Does that pump display the head(M), flowrate(m3/hr) and power(W)?, if and if d.0 is settable then if the max output is set lower than the heating demand then we can calculate the dTs etc from this and compare them with the boiler readings.

Hi,
Thanks for the continued responses.
Yes the pump does display all of that and yes according to the manual d0 is a writable value - Given this is a large house though what should I set to?

Boiler is a 30kw boiler.
I have 8 radiators which are each circa 4000 BTUs each at delta T 20.
I have 2 underfloor manifolds - each manifolds supports 7 pipe runs that are each circa 80 metres.
One manifold is fed with 28mm pipes as it is a long run
The other manifold is fed with 22mm pipes
The UFH is set at 45 - 50 degrees on the mixers
Plus DWH with a 250 litre tank.

Thanks

 

[John.g]

Just now, if (when) running continuously, note those pump values and also d.40 & d.41 and we can compare them.
Preferably with rads only on.

 

[ShaunCorbs]

Are the ufh manifolds got there own pump set ?

 

[bagaceratops]

Just now, if (when) running continuously, note those pump values and also d.40 & d.41 and we can compare them.
Preferably with rads only on.

Thanks will have a look and post.
As well as the 8 rads I have 3 large towel rails that run off the radiator circuit too.
When I say large that are each about 6ft tall.

 

[bagaceratops]

Just now, if (when) running continuously, note those pump values and also d.40 & d.41 and we can compare them.
Preferably with rads only on.

Hi,

Boiler is modulating fine at present.
Just rads calling for heat (and maybe DHW), but UFH off

Flow is 70 degrees, return is 49
Pump is reading - 8.2 Watts, 1.2 metres head and 0.8 m3/hr
I checked d0 and it is set to 30

Thanks

Andrew

 

[John.g]

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.

 

[John.g]

Does your system have a low loss header or close coupled Tees as the very low head seems to indicate this and the pump is on the lowest PP setting A, I can't see any PP or CP settings, apart from A,B or C for this pump (for interest), do you have any values/ranges for them?.

 

[bagaceratops]

Does your system have a low loss header or close coupled Tees as the very low head seems to indicate this and the pump is on the lowest PP setting A, I can't see any PP or CP settings, apart from A,B or C for this pump (for interest), do you have any values/ranges for them?.

Dont believe there is a low loss header and no close coupled tees. Just a standard bypass valve

Re the pump again not really sure which setting it should be on, assume PP as I have rads with thermostatic valves, but pump also has the eAdapt setting, constant pressure and also fixed speeds

If I run the pump on curve B or C then the head and flow increases.
I found this for the specs on the pump

https://www.pumpsukltd.com/pub/media/bss/productattachment/191027281_c_w06-2020_ecocirc-50hz_en.pdf

 

[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.