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?.