Any ideas on why my Main HE Boiler might not be staying on?

[Spanky2k]

Hi all,

Still trying to keep this old boiler alive for another year or two when we hope to build an extension and move the boiler location (would rather get a new one then). It's a Main HE system boiler. We have separate hot water tank in the cellar.

We've had an intermittent issue with the boiler where sometimes there doesn't appear to be any hot water. I noticed this a week or two when it was pretty hot here but by the time I had a chance to look into it more, the problem had disappeared. This morning again though, we've had no hot water. I turned on the hot water boost mode and the boiler fired up just fine. However, a short while later it turned off again. Then on again. It seems to cycle on and off with mainly being off. The lights don't indicate any fault. My guess a couple of weeks ago was that it might be overheating somehow but it's not been that hot today so now I'm not so sure. I had the fan replaced and pressure switch in late 2017 and the flue elbow in early 2018.

Any suggestions?

 

[Agile]

You have not advised what kind of system/boiler you have.

Most Main boilers are combis but some of your words suggest that you may have a system boiler and a hot water cylinder.

This forum does not allow giving advice on work which should only be done by registered gas engineers and we would not be able to advise on replacing fans and air pressure switches for example.

Cycling on/off and not being on enough is usually a fault which usually requires significant diagnostic work!

 

[Spanky2k]

Oh apologies, it's not a combi, it's a system boiler with a hot water cylinder.

 

[CBW]

It could be an issue with a motorised valve or the circulating pump. If the boiler isn’t overheating I’d say look at the motorised valve 1st. This will probably be one or two, depending if you have an s-plan or a y-plan system.

 

[Spanky2k]

I've done some more investigating and I'm pretty sure the pumps are working. At least, I can feel them vibrating. There's one by the boiler and one in the cellar next to our water tank. I *think* the motorised valve is ok because I thought that the default on them is hot water only instead of hot water + central heating. It's in 'normal' position, i.e. not in the override one. Doing some experimentation, it seems like if I turn the central heating on, the boiler will then stay on just fine. It's only when I have the hot water on on it's own that the boiler will cut out. I thought this could suggest a pump failure and maybe the boiler is turning off because the water is not moving and it thinks the water is hot enough but as I said, the pumps appear to be working fine. The pump by the boiler is only about a year old. While we do live in a hard water area, we've had a water softener for just over two years now so there shouldn't be any new limescale buildup causing any blockages.

 

[CBW]

If you’ve got 2 pumps, then it’s a little out of my depth, as I only deal mainly with small systems, and vast majority are combi boilers. Check the cylinder thermostat as well, make sure it’s functioning correctly and attached to the cylinder.

 

[ianmcd]

post pics of your two "Pumps" would be unusual to have 2 in a domestic house , you may have a secondary HW circuit