Boiler short cycling CH

[anthonyag]

We are experiencing an issue with CH short cycling on a Worcester Bosch Greenstar 25i ErP boiler.

From a cold start the boiler fires for only ~25 seconds before exceeding the set CH flow temperature and entering it's 3 minute short cycle programme. As soon as the short cycle programme kicks in the flow temperature drops rapidly. This cycle repeats indefinitely.

The CH flow temperature is set to 45 deg C. The Maximum CH output is set to 7Kw (the minimum the boiler will modulate down to).

The boiler in plumbed to an underfloor heating manifold with 2 no. ~80m long 16mm dia loops. There is no switching or blending on the manifold. There are no other CH circuits plumbed to the boiler.

Adjusting the pump speed to maximum and fully opening the flow valves on the manifold (giving a significantly higher flow than required, ~6 L/min total) does not have an effect.

Adjusting the CH flow temperature to above 75 deg C results in the boiler not short cycling and running continuously. This flow temperature is of course not suitable for underfloor heating. This was WB's only suggested solution when they sent an engineer out to look at the boiler under warranty.

Whilst we are aware that the 7kw minimum CH power that this boiler can modulate to is larger than required by the heating circuit, but we would have expected it to be able to run a little longer than 25 seconds from a cold start before short cycling?

Is there an issue with the boiler or is it simply that our expectations are incorrect, and the boiler is not able to modulate down low enough for a heating system of this size?

(A quick note to explain the logic behind this boiler and underfloor heating combination - the boiler was existing to the property when we purchased it and we are undertaking a staged renovation & extension. We have removed all the existing radiator based heating system and renovated 2 no. rooms including the installation of underfloor heating loops. The remaining rooms and extension to the property will also be heated underfloor and work will commence on these in the spring. At this stage I am simply looking to get the boiler functioning as efficiently as possible this winter with the currently small underfloor set up)

 

[JonathanM]

Is there any other route by which the water could be returning to the boiler, without going through the UFH. Like some sort of bypass?

Adjusting the pump speed to maximum and fully opening the flow valves on the manifold (giving a significantly higher flow than required, ~6 L/min total) does not have an effect.

Do you know what the flow rate is when using those maximum settings?

Adjusting the CH flow temperature to above 75 deg C results in the boiler not short cycling and running continuously.

What happens to the temperature when you do that? Does it shoot up initially, then stabilise, before gradually climbing? Or does it just keep shooting up?

 

[JonathanM]

Is there an issue with the boiler or is it simply that our expectations are incorrect, and the boiler is not able to modulate down low enough for a heating system of this size?

If you know the water capacity of the system in litres, it should be possible to calculate how long it takes to heat up all that water from say 20C to 45C.

So say the capacity is 20 litres, then my calculation shows it would take just under 5 minutes to heat this amount of water, from 20C to 45C, with a boiler output of 7KW.

 

[anthonyag]

Thanks for your reply.

Is there any other route by which the water could be returning to the boiler, without going through the UFH. Like some sort of bypass?

No, the only connection to the boiler CH flow and return is a direct piped connection to the flow and return of the manifold. As there are no radiators on the system and the boiler claims to be able to modulate down to a suitable flow temperature the system was designed to run with no intermediate mixer/bypass etc.

Do you know what the flow rate is when using those maximum settings?

At the boilers maximum pump speed the (fully open) flow meters on the 2 no. underfloor circuits read ~3 L/min, so ~6L/min total on the CH circuit.

What happens to the temperature when you do that? Does it shoot up initially, then stabilise, before gradually climbing? Or does it just keep shooting up?

It rises pretty quickly and stabilises. I have only left it running at this temperature for a couple of minutes (so as not to damage the floor finish) and it does not rise significantly past the set temperature.

 

[anthonyag]

If you know the water capacity of the system in litres, it should be possible to calculate how long it takes to heat up all that water from say 20C to 45C.

So say the capacity is 20 litres, then my calculation shows it would take just under 5 minutes to heat this amount of water, from 20C to 45C, with a boiler output of 7KW.

Yes, I would calculate that there is ~32 L of water in the system (excluding the manifold and pipes to the boiler). So on the basis of your numbers above it should be taking ~ 8 minutes to heat this amount of water. I had expected this to be the case, but as above the boiler is short cycling in ~ 25 seconds.

 

[JonathanM]

It rises pretty quickly and stabilises. I have only left it running at this temperature for a couple of minutes (so as not to damage the floor finish) and it does not rise significantly past the set temperature.

So, are you saying it goes all the way up to 75C, very quickly?

 

[anthonyag]

So, are you saying it goes all the way up to 75C, very quickly?

Depends on your definition of quickly. Within ~ 1 minute give or take.

 

[JonathanM]

Depends on your definition of quickly. Within ~ 1 minute give or take.

That would be quick by my definition! To heat the whole system content, from 20C to 75C, should take at least fifteen minutes, at 7KW

It does seem that not enough flow is going through the boiler. Unless there is some sort of bypass or return, somewhere, which is bringing hot water back to the boiler.

Did you say the pump is inside the boiler, and you have set it to maximum?

All boilers start off at a higher burn rate. The usual figure quoted is 70% of full output. So your boiler might actually start about 17KW. And then what we don't know, is how long it takes to drop down to 7KW. Some do that almost instantly. WB should be able to tell you.

EDIT: are these the options you have for pump speed? If so, which have you chosen?

 

[Johntheo5]

Yes, I would calculate that there is ~32 L of water in the system (excluding the manifold and pipes to the boiler). So on the basis of your numbers above it should be taking ~ 8 minutes to heat this amount of water. I had expected this to be the case, but as above the boiler is short cycling in ~ 25 seconds.

The boiler fires up at alot higher output than 7kw and then modulates down, 7kw gives a
17C (16.7) dT, assuming a starting water temp of 35c then it requires 14.7 kw to give say a 35C dT which is possibly what's happening, range rating the boiler may help but probably won't.
A LLH may help.