Boiler short cycling

oldskoo1

Boiler made its first appearance for central heating this side of the year.

After many issues to fix this last year with it it seems like one still remains and that is short cycling.

It starts up and stays on for about 10 maybe 15 mins before falling into cycling mode of 1-2 mins on 1 min off.

No complaints with the heat it produces. The house does get warm but it takes some time.

Its an old ideal classic ff380, quite a high output with no smart modulation. It's a three storey house with 21 rads. Most have TRVs set to 2-3 with some on 1 but the downstairs don't have trvs.

Pump is on 2 and I'm just wondering if the largish boiler trv combination is causing this or perhaps I need to rebalance? If so should I be closing down some valves to slow the water flow down so the radiator can extract more heat?

I have no reason to suspect a blockage, pump is not making any noises and water is clean almost clear. I added the correct amount of sentinel x100 and x200 last year. I also installed a new pcb early this year so it isn't a relay problem.

sk240

Unfortunately I don't think there is much you can do, with the current outside temps the TRV,s will only be slightly open, and probably not for long.
As the weather gets colder, and the load increases it will do it less often.
That's a big problem with un-modulating boilers :-(

Cardew

Where is the wall thermostat and what temperature is it set at?

oldskoo1

I was hoping it was the high output non modulation but wondered if I could alleviate it with flow changes

My hall wall stat is set at 17c - big hall with half size double blade radiator

oldskoo1

Just realised that the boiler stopped after 8 mins as it reached its stat but the return was still cool, no where near the limit.

I've turned the pump up to 3 as I assume it needs the water out quicker.

Ideally I would prefer not to run on 3, in theory could I put it down to 2 and open the rad valves more to destrict flow ? Or does it not work like that?

matelodave

If the return pipe is still cool then there's not enough water circulating through the system and the boiler isn't getting the water back to cool it down.
You might have a blockage, a stuck motorised valve or all the thermostatic valves have shut down - you should have at least one rad without a thermostatic valve (usually in the room where the main thermostat is situated) to allow the water to get back to the boiler without restriction.
Try opening the valves on the hall radiator and if it's got a thermostatic valve open it completely as it will affect the operation of the main system thermostat.

Andy_WSM

Agree with Dave. One rad needs to be full on (Bathroom maybe?).

If you have already done this then it's either a blockage as Dave suggests, or the impellor on the pump has worn (they can corrode away eventually).

An option to having a rad on full all the time is to have an automatic bypass valve fitted at the furthest point of the heating circuit from the boiler. This adjusts itself to allow a continuos flow of water back to the boiler, without impeding the flow when radiators call for heat.

oldskoo1

ah right that does make sense

I did have some flow/return pump problems

When i moved in i realised the flow and return were on the wrong way round (coming out the top of the boiler).

Gas engineer came and swapped it, drained the system and reconnected the correct way.

After that all went well apart from this cool flow issue. Still miles better than before apart from the cool flow. He set the pump from 3 to 2 after he did the work which he said should be fine for the 3 floors as its a 15-60 pump.

So i'm wondering could the pump now be fitted the wrong way around after he swapped the flow and return on the boiler?

Or perhaps there is a blockage. Is this easy to spot and how would the engineer clear it? I've tried myself running the pump at full speed and back down again and all i hear at full speed is cavitation and the first rad in the loop just starts trickling.

Although on full speed i do get what i could describe as restricted flow, a audible change in pitch, a lower hum and then back to normal sounding flow.

/ps
my hall rad is fully open, no TRV. None of the downstairs or landing rads have TRVs and i opened them all this morning, previously the landing ones were just on minor flow through to stop the valves seizing but i opened them up so they get hot instead of lukewarm. Still no difference to cycling.

Pincher

You can turn down the boiler output.


Turn off all the radiators bar one. Test one at a time.

GeoffK

There might be another reason for this.
There is normally a fan above the boiler juts before the balanced flue to extract the hot gases. It's important that this is working so there is a safety cutout that is operated by measuring the small pressure difference between 2 points just past the fan. The small silicon pipes that link the pressure sensors to the flue at either point can age and leak. Result: no, or reduced pressure difference and the burners go off. Then, as the boiler waits a short time and restarts. The cycles just continue, getting shorter as the leak increases.
It can be hard to spot a leaky pipe unless you take them off but that is easy and they only cost a few ponds to replace if need be.

oldskoo1

Thanks guys

I don't think I can turn down the boiler output on this model. I read the manual and there is no way to do it. If I turn the stat down it just starts cycling much sooner.

I can test each rad thanks for the suggestion. I know the fan was replaced about 2 years ago in this 12 year old boiler. But that was done by the previous owners. I will try looking at flow some more and if I can't improve things I will ask my local gas engineer to come take a look and mention the fan tubes. Thank you