Glow-worm Flexicom problem

[jimmir]

I had a Glow-worm Flexicom HX15 boiler installed but I'm not convinced that it's working properly. It's been ok all summer heating the hot water but I've just tried the central heating and it's slow to heat the radiators.

The boiler fires and the temp display goes to 72°C then the burner shuts off as shown by the flame icon in the display. It overshoots by a few degrees to about 77°C then the temp display falls to 44°C before the burner fires again even though there is a call for heat from the room stat. The process then repeats 72°C > 44°C > 72°C etc.

The boiler also performs like this when it is heating the water in the hot water cylinder but the cylinder stat is satisfied ok so it's not a problem. It's a Y-plan and the mid-position valve is working ok.

The engineer who fitted it is Gas Safe registered and has spent all his life working on heating systems but doesn't think there is anything wrong.

I've watched a Glow-worm Ultracom and the burner stays on while there is a call for heat with the temp in the display hovering around 70°C so why doesn't my Flexicom do this?

Any comments appreciated as to why the boiler is performing like this and what is wrong with it.

 

[Dan Robinson]

Sounds like the boiler protecting itself from circulation problems.

Working on boilers all his life doesn't mean he has the foggiest of what he was doing.

System could be dirty, poorly balanced, or the pump could be on its last legs.

 

[jimmir]

Thanks Dan - the system is old but the hot water cyl is newish and the 22mm pipes to it are new. The system was cleaned with Fernox DS40 and power-flushed. The pump and mid-position valve are also new.

The boiler behaves as I described even when just heating the water but there shouldn't be a circulation problem on this circuit.

Thanks for comments though.

 

[Dan Robinson]

"Shouldn't" doesn't always equate to "isn't".

What is the return pipe temperature when the boiler is registering 70 degrees prior to shut down?

 

[jimmir]

I can't measure temp of return pipe but water has given up its heat as the pipes supplying the rads are warm and there is a bit of heat in the rads themselves. The heated water is definitely circulating but there isn't enough of it.

If the boiler would keep going the rads would heat up but there is such a long wait while the temp drops to 44°C before burner fires again.

 

[Dan Robinson]

You have shyte circulation.

 

[Garfeild]

D40 and D41 give you flow and return temps

 

[ScottishGasMan]

If the boiler is shutting off between 70 and 80 degrees that is at the flow outlet, if the rads are only just heating then it means one of two things.

1. There is poor circulation and the return temperature will be low as the return can only be equal to the temp coming from the radiator returns

2. The return temperature is hot and the boiler is shutting off as it cannot keep heating water past already hot, in which case given the heat is not returning from the radiators would mean you have a bypass of some description letting far too much circulation around it rather than the rads.


or as dan says Shiyte circulation. If the boiler was under powered it would run for ages and not get up to temp, if the circulation is poor then it will hit temp very quick and shut off, overrun, then start again and cycle like this with poor heating as a result.

 

[ScottishGasMan]

To add you say if the boiler kept going it would heat up - it would also likely over heat, and there is a factory delay before it restarts to prevent cycling like this, many people will blame the boiler in this case as it "should just stay on" but it sounds as if its doing exactly as it should, and the issue would be with the system, an HW is a heat only boiler, it will only work properly if the correct pipe sizes/pump size+operation/correct bypass/flow rates are all within specification, any one of these things being wrong will lead to issues with the boiler performing.

 

[jimmir]

Thanks for the help guys.

Garfield - thanks I will try to find D40 and D41

ScottishGasMan - thanks for your analysis and I understand what you say. I do have circulation to the HW cyl and when the boiler reaches 72C I get hot water at the heating coil very quickly, a distance of about 10m, so the pump is circulating the hot water. With a cold cyl the heat goes into the water and the return is cooler so this appears ok to me. I understand that I need to know real return temp to be conclusive.

There is a bypass but it's not flowing hot water, the hot water is definitely going to the heating coil.

Dan - I respect your opinion but I'm not convinced yet that it's shyte.

If I can post D40 and D41 will that identify a circulation or boiler problem?

I would like to understand the problem before calling a gas engineer. I do plumbing myself but obviously need a Gas-Safe guy for a boiler fault.

Thanks again - much appreciated.

 

[ScottishGasMan]

Sorry I realise I have made a few assumptions. If its an older style cylinder then the coil in it wont actually be suited to the boiler so rapid cycling would occur anyway even tho the water in the cylinder does get hot.

What I mean by this is that it is a 15kw boiler, potentially on a cylinder only capable of taking 3kw at a time so the boiler over shoots on temp and shuts off till the coil has lost some heat to the cylinder and then goes again. Where a new unvented cylinder could have a coil rating of 25kw. (yours almost certainly wont be this style due to being a Y-plan system)

My unvented cylinder (from cold) can have the boiler running at constant 22kw full burn for 15 min before it will start to wind down the gas burner and then when its hot hot, it would cycle a little till the cylinder stat was satisfied.


However it doesnt change the answer that yes at an absolout minimum you need accurate flow and return temperature readings taken from the one device at the same time.

If the flow and return when you run your Central heating are much more than 20*c difference (which I suspect they will be if you know theres no hot bypass water getting back up there) ie a temp difference of 30+ degrees C would be clear that there is circulation issues.

 

[Dan Robinson]

Dan - I respect your opinion but I'm not convinced yet that it's shyte.

Well - what do I know?

 

[Agile]

Everyone and the OP have overlooked that everything is probably working correctly! Even if correctly is not what you would like!

The boiler does NOT come back on when the temp is at say 60 C.

It DOES wait for the anticycling delay which could be set at three minutes or more.

During this time the rads or cylinder continue to take heat from the circulating water.

So by the end of the anticycling delay, the circulating water could well be down to 44 C.

The solution? This is more complicated! In theory you reduce the power output so the boiler takes longer to reach the set flow temperature, ideally never quite reaching it.

Easier said than done! The boiler is required to supply three different loads so no single power setting will be perfect for all of them.

I deal with this by lowering the power during the summer when only water is being heated and increase it during the winter when heating is required. Not perfect but much better.

Tony

 

[Dan Robinson]

Think I summed that up in the single first line of my OP.

 

[lightning]

Can you use DS 40 on a flexicom

Paul