Boiler seems to "idle" and won't run full power.

[RigidRaider]

It's a Glow worm Flexicom 24HX on a Y plan system, fully pumped, open. We still have the old inefficient HW cylinder (new, faster reheat cylinder to be fitted next month) so on the plumber's advice we have set the timer so that the boiler works on the HW for half an hour before the CH comes on.

If I come down early when the HW is already on and switch the CH on manually I've noticed that the boiler sometimes seems to be "idling" and doesn't react; the fan is running and there is warmth in the output pipe, the display shows 38-40 but the boiler doesn't fire up to full power. The room stat is calling for heat when this happens. If I turn off HW and CH, I hear the fan slow down and stop and then when I restart HW and CH the boiler fires up within seconds to full power, when it makes a much louder noise.

Any ideas as to why it behaves in this way?

 

[linkyplumb]

How big is the system? How many rads?
That is quite a big output boiler.
Had similar before, and adjusting the heatload, via the d0 code, did the trick

Graham

 

[RigidRaider]

Thanks, 4 bed detached house with 12 rads.

 

[D_Hailsham]

Thanks, 4 bed detached house with 12 rads.

OK, but it doesn't tell us very much.

You need to do the following:

1. Check your actual requirement using the Boiler Size Calculator.

2. Work out the output of your rads using the Stelrad Elite Catalogue. (You may not have Stelrads, but the result will be close enough.)

3. Post both results.

 

[Whitespirit66]

You don't say whether your hw and rads heat up as they should do.

 

[RigidRaider]

Thanks. The HW is less than ideal because we have a 23 year old cylinder with a slow reheat. We are having a modern, fast reheat cylinder installed in February.

The boiler is perfectly adequate for the house; when it's operating it heats the house fast and has reduced our gas bill by about 30-40%.

My question is about the strange behaviour of the boiler at that time in the mornings when it's already been working on the HW and I switch on the CH - why does it continue to idle even though the Y valve has opened and warm water is flowing to the radiator circuits? My understanding is that modern condensing boilers vary their output according to demand yet at that moment you have a room stat that's calling for heat and cold water entering the boiler from the rads so I would expect the boiler to respond vigorously, not just sit humming gently to itself.

 

[Agile]

Presumably you can obtain a "status" indication as you can on the Vaillants.

Look in the book and tell us what status you discover.

It seems likely that you are creating a fault condition because the return water creates an apparent too great a spread between flow and return and that triggers a long delay whilst it waits for the temperatures to settle down.

Tony

 

[RigidRaider]

Thanks again. That sounds to be on the right track. There was no F code showing at the time, just a flame symbol, a radiator symbol and the temperature.

I will try to reproduce the behaviour again and report back!

 

[D_Hailsham]

The boiler is perfectly adequate for the house; when it's operating it heats the house fast and has reduced our gas bill by about 30-40%.

Your boiler is, very likely to be considerably oversized for the size house you have. That is why I suggested you do the boiler and rad calculations. I know it means some work for you, but we would then be in a better position to advise you.

The Flexicom 24hx modulates between 9.4kW and 24kW and its quite likely that you house only needs about 12kW - and that's only in the depths of winter. When it is heating just the hot water it will be able to put out a considerable amount of heat as the coil in the cylinder can deal with it. But the maximum a radiator can handle is as the manufacturer specifies, plus about 15%. This means the boiler will have to lower its output when the radiators come on and probably idle for a while while the water cools down.

The water returning to the boiler will not be cold, just cooler, usually between 10 degrees and 20 degrees, than the water leaving.

 

[MIGEM]

The return to the boiler wouldnt be picking up heat from the surrounding hot water in the cylinder, by any chance?
Why were you told to have the HW on before the HTG? Not just a valve fault?

 

[RigidRaider]

Been thinking about it and I reckon you both might be on the right track; the problem might be that the cylinder (as I've already written twice) is 23 years old and has a small inefficient coil. Possibly the water in the coil isn't giving up as much heat as the boiler expects and is returning too warm, fooling the boiler into thinking it can carry on idling for a few minutes, at least.

Next time I recreate the conditions I'll try turning the HW off at the same time as I turn the CH on, to see what happens.

I rang Vaillant about it yesterday and they were just as baffled!

The boiler is programmed to work with a bigger, more modern efficient coil than ours. We know the bloke who built the house and he is notoriously tight-fisted, he installed the smallest cheapest cylinder he could find, there's only about an 8" distance between flow and return on the coil so there can't be much copper in there!

 

[D_Hailsham]

Possibly the water in the coil isn't giving up as much heat as the boiler expects and is returning too warm, fooling the boiler into thinking it can carry on idling for a few minutes, at least.

If you live in a hard water area, the coil in the cylinder could be caked in limescale, so reducing its efficiency.

In any case the boiler would modulate down so the heat it produces balances the heat lost in the coil. The flow temperature is the appropriate measurement. It could be that the boiler has modulated down as far as it can (9.4 kW) and the flow temperature is still increasing above the set level. In this case all the boiler can do is turn off and wait until the flow temperature has fallen below the required temperature. The boiler then enters an on/off cycle to maintain the correct flow temperature.

there's only about an 8" distance between flow and return on the coil so there can't be much copper in there!

The distance between the the flow and return connections is no indication how big the coil is as the top pipe can rise vertically within the cylinder before the coil starts.

 

[RigidRaider]

Thanks again, that sounds pretty logical. Our water is luvverly and soft here in Lancs but yes, after 23 years the coil could be a bit cruddy.

The cylinder is the last upgrade to what was a very rudimentary setup in the house. The builder spent all the money on some hideous fancy decor, stained glass, cornices, rag rolling etc. which we have ripped out, instead of getting the basics like heating, insulation and electrics done to a high standard.