Any Central Heating Experts?

[JtB]

We've just had our old condensing boiler replaced and I notice that when the domestic hot water is switched "off" the radiators warm up nicely, but as soon as the domestic hot water is switched "on" then the radiators cool down until they are just lukewarm. The temperature at the bolier is fine so the problem seems to be elsewhere with the central heating system (note: we have a hot water cylinder and not a combi boiler).

With the old boiler the radiators would take slightly longer to heat up if the domestic hot water was switched "on" but they would still heat up fully. The only thing the boiler fitter changed with the central heating system (apart from the boiler itself) was some re-plumbing in the airing cupboard (something to do with helping to stop air from building up in the system), so I'm wondering whether this might be contributing to the problem in some way.

Any ideas or suggestions would be most welcome - many thanks.

P.S. Just wondering whether it makes any difference if the radiators are balanced with the domestic hot water switched "on" or not.

 

[screenman]

Could it be the diverter valve?

 

[JtB]

I was wondering that, but it seems strange I did not notice this problem before the boiler was replaced.

 

[screenman]

I was wondering that, but it seems strange I did not notice this problem before the boiler was replaced.

I know it seems odd but often I have found things like this play up when something else on the system has been changed. I think on some you can adjust them manually. Unfortunately our pesonal plumber now live sin the South Of France, he is also our middle son, bit far to ask him to pop around and have a look.

 

[sight-pin]

Sounds like the hot water circuit is taking most of the flow with it probably being hot water priority, does the heating cut back in once the cylinder stat has switched the hot water off?

 

[JtB]

Yes, once the hot water shuts off the radiators go back up to full temperature.

 

[sight-pin]

Give the plumber a call if it's been recently installed as it could be a few things; even a simple airlock which a bleed and balance may do.

 

[JtB]

Should you balance the radiators with the hot water switched on?

Also, is it possible to adjust the flow to the hot water cylinder like you can for each radiator?

 

[sight-pin]

Really need to balance the rads when the system is cold. from cold turn heating on walk round all the rads feeling the flow pipe into each rad seeing which is taking the most heat first, then turn down the lockshield valve a little (say a quarter turn only at a time) on the rads that get hot. first, But if all the rads are heating up and at roughly the same temp it's probably sounds ok anyway...... Don't forget to bleed them all first though and make sure the flow valves are all turned on full to balance. (Balancing is only needed if some rads are not getting to temp)
The flow and return to the cylinder normally can't be adjusted nope.
Edit; If your not sure leave to someone who knows as you could put it out of balance. BTW I'm not an expert; i just know a little about plumbing through my own experience.

 

[JtB]

Thanks all for the comments.

When I balanced the radiators a while back I did it with the hot water cylinder turned off and I got them all balanced nicely. But I was wondering whether maybe I should have balanced them with the hot water cylinder turned on.

 

[sight-pin]

I wouldn't think it'll make much difference as the the flow to each individual rad would be the same i'd of thought.

 

[sight-pin]

Balancing the rads won't cure the flow problem you having when the cyl is calling for hot water though.

 

[Piemaster]

Sounds a similar setup to ours. Not a problem we've noticed but the hot water cylinder is only set to warm up a couple of times a day and not usually when theres much demand for heating. The cylinder is only a year old and so well insulated the airing cupboard is now actually useless for airing clothes as it doesn't leak any noticable heat, so any water heating is more of a 'topping up' more than anything else after use when everyone has gone out.

 

[JtB]

I've just worked out what the re-plumbing in the airing cupboard was for. The bypass which was previously taken from the central heating side of the diverter valve was re-routed and taken from the input to the diverter valve instead. I'm assuming the bypass was previously only needed in case all the radiator valves were shut, however since the new boiler uses "pump over-run" then the bypass is now also needed when the diverter valve is completely shut (to both the radiators and cylinder) and the pump is still running to draw heat away from the boiler. I don't see how this would contribute to the problem we are seeing, instead I think I will go with grit in the diverter value or an airlock as suggested by a few people above. I also notice there's a gate valve on the flow to the cylinder, unfortunately though I don't think gate valves can be used to regulate flow.

Edit: Thinking about it, if there is a blockage in the central heating side of the diverter valve, then re-routing the bypass (as above) would probably highlight this problem even more.

 

[sight-pin]

Sounds like the new bypass from the zone valve is taking most of the flow from the heating then when the system is also calling for DHW; You could try the closing the gate valve just a little to restrict the flow to the cylinder seeing as it was already installed prior to the recent installation, that's probably why it was installed in the first place anyway i would think, careful though as old gate valves can be a problem and break easily as you probably already know.