Back boiler - is this working right?

[markdon]

Hi.

We have a Baxi Bermuda back boiler, it was fitted in 1998, we have been in this house two years now. I have replaced the old manual thermostat for a new digital type, the gas valve, burner and heat exchanger hae also been replaced and its been serviced.

The problem i think i have is that the pump is out of stnc with the system? Though im not sure if anything can be done about this?

Basically the temp drops below what is set on the stat, i hear a click from the stat, then can hear the pump kick in. 20-30secs later the boiler fires.

Now this isnt a problem in itself, but once the boiler has been on a while, it starts to kettle. This only happens at the end of the cycle. I think when the stat clicks off, the pump cuts out but again theres a delay for the boiler to cut out, so no water flowing so it boils?

Not sure if this sounds right, or if there is anything i can do about it? Sure i wired the stat up right, but perhaps not?

Thanks for any help.

 

[twgas]

Try turning the boiler stat down, a boiler of that age kettling is quite usual but not worth spending loads of money on to cure.

 

[markdon]

Boiler stat is on one, switched it down yesterday, but still doing it.

 

[gas112]

By kettling at end do you mean gurgling and a bit of thumping ?
When it makes noises as it goes off it is usually because boiler stat is on its way out and is allowing boiler to get a lot hotter than normal.
Standard kettling , bit of hissing and slight noise can be cured with senitnel x200

 

[markdon]

Can hear the water bubbling and going up the vent pipe, bit of banging too.

 

[gas112]

Your boiler stat has gone needs changing

 

[markdon]

Quick update, ive ordered a new stat, should be here last friday.

Anyways, for the first time in ages i selected hot water instead of c/h, as i need some jeans for work tomorrow and the rad in back bedroom heats with h/w for some reason.

Anyway. No bubbling or banging? Noticed this so switched to c/h, 5mins later it starts bubbling. Knock it back to just h/w and all is well again.
Mso whats the difference?? Will the stat make a difference?

 

[andytw]

If the stat doesn't fix it then make sure you have an intact spreader in the heat exchanger. You did say the heat exchanger has been replaced, was a spreader even fitted at this time ?
I had similar banging and thumping at the end of the heating cycle and it took ages to resolve, parts (thermostat !), and flushes etc.
In the end I bought a replacement heat exchanger and only when my RG engineer and myself were fitting it did we find the broken spreader. Replaced spreader and all sorted.
The spreader makes sure water goes down all chambers of the heat exchanger equally, without it there is limited flow down some chambers which boil.
When the spreader is supplied with a new boiler it is in a bag with a gravity flow injector tee so there has been mention in forums that the spreader is only for use with gravity flow systems. This is wrong. In fact my RG engineer said the same thing to me, but I showed him the book. The repair further convinced him. The instructions clearly state that the spreader should be fitted for gravity and pumped installations.

andytw

 

[gaswizzard]

The spreader is used to prevent the return water short circuiting the hex , the same deal applies with the distribution tube on the old Ideal concords along with many others.

 

[markdon]

Looked in all the paperwork and can't see anything about a spreader?

Have fitted new stat, but think I have a new symptom.

The stat hasn't fixed the problem, but it has pointed me in the right direction I think.

Turn heating on, boiler clicks, fires, perfect. Turn room stat below current temp, few secs later stat clicks, few secs later pump stops, but this is where something goes wrong, the pump has stopped, but the boiler hasn't. So cue bubbling and banging.

All I need now is for a talented individual off here to tell me what's gone wrong and I will be very happy!

Thanks in advance.

Mark.

 

[markdon]

Anyone help ?

 

[Agile]

I think you should have had a gas reg engineer to replace all these parts and properly set up your boiler!

 

[markdon]

They replaced everything apart from the thermostat, i looked in the paperwork that came with the boiler and was confident i could change it. Was easy enough.

Just when the replacement exchanger didnt make a difference i wanted to know if anyone had any other ideas before i throw more money at someone coming out and possibly not fixing it.

It must be electrical? When they replaced the room stat, could that be wited up wrong to create a delay from the pump switching off to the boiler switching off?

Thanks.

 

[stem]

I think it will help, if you understand how your system works. The delay between the boiler and pump is fine. From your posts, I am assuming that you have a boiler heating a stored hot water cylinder by gravity (not pumped) and then a pump circulating hot water around the radiators. If this is correct, read on.

When you select hot water 'on' at your programmer / time switch, the boiler will fire up and heat the water inside it up to the temperature set on the boiler thermostat. When it reaches the set temperature the burner goes off. Meanwhile, the water inside the boiler will circulate around the hot water cylinder using the principle that hot water rises and cooler water sinks. This is a slow process and the boiler will fire up from time to time to maintain the temperature of its contents at the level set on the boiler thermostat.

When you select central heating 'on' and the room thermostat is calling for heat, the pump will be started and will circulate the hot water around the radiators. If the boiler is already full of hot water, it will not re-ignite until the contents have cooled, hence the delay you refer to. The central heating controls only operate the pump. The boiler is operated by the hot water 'on'. This is why it is not possible to have central heating on on its own. With this type of system, the hot water has to be 'on' at the same time to operate the boiler.

I'm not sure how bad your kettling is, but I don't think I've ever come across a back boiler that doesn't kettle to some extent.

 

[markdon]

Thanks for explaining that, tried to understand it before but it never made sense until now.

When we moved in 2years ago it wasnt this bad, its gradually got worse and worse.

Its loud enough to wake our 19mth old son up at 6am with it banging and clattering, hence wanting to fix it so we can get some better sleep!

Thanks.