Another boiler issue.

[graduate_owner]

Hi all. Question for any plumbers out there. I have a Grand condensing oil boiler (not combi) fed from attic tanks. The previous Thermecon boiler seemed to work fine, it had 28mm fittings. The Grant replacement has 22mm fittings going into 28mm. My problem is the boiler is short cycling. Once it gets the rads up to temperature, it starts short cycling. It fires for about 8-10 minutes then stops for another 5 or so minutes before firing up again. The pump keeps running all the time the boiler is short cycling. This never happened with the old boiler and since changing boilers I have doubled the size of one rad so I think there is plenty of load on the system.
Could this be the boiler thermostat? Any other ideas? And is the regular purging and firing wastful of oil?

Thanks.

K

 

[baldkev]

My gas boiler was doing similar, was a faulty diverter valve. Im not saying yours is, minevwas a combi.
My advice is to email grant and outline the issue. Their tech department will help, they are reasonably good

 

[Richard_C]

Yes, try Grant first. I had a Grant boiler installed 18 months ago and had a question about noise from the flue. I emailed their tech team and got a full and helpful reply. Certainly worth a go.

 

[TomTheToolMan]

I'm not a heating engineer but looked into causes when my boiler was doing the same. If you have loft tank then you will probably have motorised valves like me so no diverter valve. 2 main causes that you can look at before you go near boiler as sounds like thermostats are actually doing their job. Not enough flow so boiler getting too hot or too much flow so boiler not seeing return temp at least 12 degrees lower than output temp. If you have an ir thermometer you can check temp on flow and return pipes at boiler and at each rad. If you have thermostatic they can shut off and restrict flow and that is why it is advised to have at least one non thermostatic in system. If you don't have a thermometer then I would try to rebalanced system. Can do this manually. In my case after doing all this I concluded the pump wasn't up to the job. I replaced with an upgraded pump as we had a lot or rads and the house has a lot of micropore pipe. £80 and an easy diy change. Been no problem for 7 years

 

[graduate_owner]

Thanks guys. I had a new pump installed at the same time as the boiler. The old boiler did not short cycle when used for hot water only, this one does when supplying hot water to the cylinder, and to the rads. It doesn't seem logical. There are motorised valves but they seem to be working because we do get hot water circulating to rads and cylinder. I shall try contacting grant anyway.

K

 

[Spectric]

Pump settings

 

[graduate_owner]

Roy - pump has 3 speeds. Same result on all three. So annoying.

K

 

[Spectric]

Are the bypass requirements the same for both boilers, once upon a time the pump was controlled by the demand signal but now it is controlled by the boiler so when the demand signal stops the boiler will continue to run the pump to prevent kettling in the heat exchanger. For this you need a bypass loop, again once upon a time it was just a lockshield valve but now with a std pump there are bypass valves, no good with smart pumps though.