Boiler short cycling on hot water

[bhm1712]

Hi,

Not sure if this is a new fault or indeed a fault at all. ive changed the time when our hot water comes on from an early morning time to an evening time due to a change in work shift pattern.

When the system starts up on hot water for the first 30 minutes it works fine and the boiler settles down to a very low modulation rate. As the cylinder approaches 60 degrees however, the boiler begins to short cycle. It will fire up at about 58 degrees flow temp and within 20 seconds it's achieved 72 degrees which is the boiler thermostat setting at present. It then shuts down and doesn't fire back up until 58 degrees again. It continues to do this until the cylinder stat is satisfied. I've measured the flow and return temps with clamp on thermometers and when it starts cycling, the flow and returns are around the exact same temperatures - no differential at all. There are no manual valves on the hot water pipework, it's straight from the 2 port valve up to the cylinder and then back down to the boiler.

System is a British gas 330 at 24kw, s plan.

As I have changed the times then obviously I am noticing this behaviour as I'm awake whilst it's doing it, but I suppose it probably was doing this when the time was set to early mornings too.

Anything to worry about or is this normal behaviour?

 

[fezster]

I had a similar issue and had to increase my boiler flow temp. The boiler needs to be a higher temp than the cylinder stat to satisfy it.

 

[bhm1712]

The boiler stat is 12 degrees higher than the cylinder stat.

 

[bhm1712]

If I turn the boiler stat up the behaviour continues , just at a higher temperature.

 

[fezster]

Hi,
I've measured the flow and return temps with clamp on thermometers and when it starts cycling, the flow and returns are around the exact same temperatures - no differential at all.

This would indicate no heat is being transferred to the cylinder. Given a sufficiently high enough flow temp, you should see a differential. I'd definitely try the boiler at max temp to check - but maybe you need to turn your cylinder stat down?

 

[bhm1712]

Can't really turn it down anymore as it's already on 55 degrees at a third of the way up the cylinder.

 

[Steverod]

Pump speed needs increasing?

 

[bhm1712]

It's already on three. The heating works fine it's just got water and when it gets towards 60 degrees on the cylinder temp

 

[Steverod]

Are you in a hard water area? How old is your cylinder?

 

[bhm1712]

Not really classed as a hard water area but the cylinder is pushing 40. The whole reheat process only takes about 45 minutes from cold, it's just that last 15 minutes or so where it does this.

 

[Steverod]

I reckon you're not losing enough heat into the cylinder for the boiler return temperature to drop sufficiently, hence your boiler reaches a point where it can't modulate down any lower and thus cuts off the burner.

 

[bhm1712]

Sounds about right to be honest and confirms kind of what I was starting to think. I bet the coil in that old cylinder is only 5 or 6kw on a 24 kw boiler.

 

[Steverod]

What happens if you have the central heating on at the same time as hot water? Does the boiler still short cycle? Does the cylinder reach target temperature?

 

[bhm1712]

If the heating is on at the same time, no it doesn't short cycle at all, and the cylinder stat will still be satisfied but it does take a little longer to get there than solely hot water. Around 1 hour to 1 hour 15 depending on how cold the cylinder is to start with

 

[Steverod]

Ok. Over to the pros...