Glow Worm 30 hxi advice

[Amuia]

Hi

Moved into a house that has a Glowworm 30 hxi boiler - never had an open vented system before. It was recently serviced but we've only just noticed that the boiler comes up to temperature, stays there for a few seconds, and then goes off. The pump stays on (it's on the highest setting).

Is this how it should be operating (i.e. cycling on and off) or should the boiler stay lit until the room thermostat reaches temperature? It's the same if we just have it set to hot water. Both the heating and water are coming up to temperature (eventually, we have a one-pipe system) but I wasn't sure if the boiler is working correctly?

 

[kittedup]

How big is the house?

30kw open vented is a large boiler.

Could be over sized.

Ted

 

[Amuia]

We thought it might be! We have 2 small rads, 5 large rads, and 2 towel rails on the system over two floors....

 

[Dan Robinson]

Sweet Jesus.

Spinal Tap boiler sizing at its finest.

 

[Agile]

"just moved in" usually implies a tenant.

Your boiler seems grossly oversized. A typical three bed semi usually needs about 10-12 kW boiler.

The boiler should heat the boiler up to the set flow temperature of about 70 C and then modulate back to its minimum power output and then turn off if this is still too high as will be the case when so oversized. On most one pipe systems that will make it worse. What is the one pipe size?

Tony

 

[Amuia]

Hi

It's a 4 bed detached and we're not tenants! Pipe diameter is about 15mm. Appreciate the boiler's oversized but not sure what our best approach is to using it....

 

[kittedup]

You need to lower the KW rating on the boiler. It is all in the instructions which hopefully you have or they are available online from Glowworm.

I would suggest start at 10kw and see how that goes.

Ted

 

[Dan Robinson]

15mm?

Oh dear.

 

[Agile]

Hi

It's a 4 bed detached and we're not tenants! Pipe diameter is about 15mm. Appreciate the boiler's oversized but not sure what our best approach is to using it....

If its fairly modern like 1980 onwards then it will have good insulation and the heat loss could be as low as 12-14 kW. If solid wall Victorian perhaps 18-22 kW.

The problem with 15 mm single pipe is that the flow resistance is so high that its not possible to pump fast enough ( with normal pumps and noise levels ) to get very much heat into your rads.

I do hope you were warned before you bought it of the one pipe problems?

The best solution is to repipe it a normal two pipe. But lesser solutions like creating TWO one pipe loops in parallel can help a lot.

Might not suit in your case, but in small houses I often overcome the limitations of one pipe by fitting a large parallel rad centrally in the hall to create a low resistance path for the boiler and create a few kilowatts of heat centrally to aid the usually poor heat output of 15 mm one pipes.

Tony

 

[Amuia]

Hi Tony

Thanks for the advice, sounds like a big job but worthwhile in the long run. It is an old house...and the pump isn't that new either.

 

[Agile]

You can get an idea of how well its flowing by measuring the flow and return temps on the one pipe loop with a contact thermometer.

For heat output the lower then better! For boiler condensing the closer to 20 C the better!

Tony

 

[kittedup]

Or lower the kw all for free

 

[Amuia]

After all that, the PCB needed replacing - boiler now working fine!

 

[RigidRaider]

Well that's what the service engineer told you and you paid for! They don't diagnose things any more, they just throw new parts at the problem at your expense in the hope of fixing it. Same as car dealerships.