Boiler short cycling modulation problem.

[Mossy007]

My boiler keeps short cycling when the UFH (ground floor 5 zones) is demanding heat. Burner is on for 1 min and off for 3 mins constantly. I have CH (5 Rads) upstairs HW as well. Boiler is Potterton Tittanim 40kw. Boiler modulates perfectly when CH and HW are demading and burner remains constant. I have noticed that when the CH and UFH are demanding heat at same time that the flow to the UFH manifold remains constant and the burner remains on. There is a temp gauge installed on to the flow of the manifold and I have noticed it flactuates between 30 and 60 degrees when the UFH is demanding heat only and is short cycling. When the CH and UFH are demanding at the same time and the burner remains constant the temp on the manifold reads 60 degrees constantly. I have recently posted on here regarding my boiler overheating as the heat was not dispersin quick enough from the boiler and received some great information about an auto bypass which I have installed and this has helped the overheating issue. Just wondered how I can get my boiler to modulate correctly when the UFH is demanding heat. Do I have a electrical issue? Do I have a flow issue? Any help will be appreciated.

 

[ShaunCorbs]

Very common with ufh as the kw usage is low and the boiler can't modulate down far enough

Example time

Say you have 3 ufh zones calling
2 of them are 1.3kw each
1 of them is 0.5kw

So the total load is 3.1kw
The lowest the boiler can modulate down too is 7kw

So your under the min capacity of the boiler so it can't get rid of the heat fast enough

TBH your best option (there's two) is if you can

1 if you have any towel rails in your bathroom/rooms see if you can plum them in to the manifold direct and just leave them always on eg no heads

2 try and get a rad on the primary circuits eg somewhere that doesn't matter like an airing cupboard etc

 

[markfxy]

Old post I know but we always plumb the towel rail circuit into the manifold and leave it as an open circuit and always use a low loss header if more than one manifold

 

[johnduffell]

2 issues there one is minimum flow quantity which stops the boiler overheating and requires a bypass, you have solved that. the other is whether the boiler can produce little enough heat for your demand and the answer is no. If you have it firing constantly you're going to have to put the heat somewhere. It's not really an issue to modulate by switching off and on.
If you put a large rad in the garden the boiler will fire constantly but clearly that would be wasteful. But any solution to keep it firing would be wasteful and in fact against energy conservation.