Central Heating boiler safety cut out

[Andy Hall]

Danny, is the URL correct? I am not getting this as a site that exists (no domain name).......

.andy

To email, substitute .nospam with .gl

[Danny]

[Danny]

[Andy Hall]

Hmm.....

This doesn't seem to be quite as I'd expect.

Are you saying that the two pipes to the FE tank are 3" apart on the flow pipe coming from the boiler? i.e. the amount of flow pipe between the branches is only 3"? Normally if the distance between these two pipes is less than about 6", and there is nothing other than pipe between them, there should not be pumping over......

Are you sure about the arrangement of where the motorised valve is located? This doesn't seem right somehow because there is then nothing to control the operation of the hot water. Is there a thermostat on the cylinder?

Also, can you indicate where the return pipe from the CH connects......

There are certainly a few funnies that ought to be corrected and improved (e.g. putting in a diverter valve or zone valve for the HW and better controls). However, I can't see anything that obviously leads to low temperatures from the boiler. Short cycling where the boiler comes on for a very short time and then off, would be a symptom of poor flow or something related to it. However, if the boiler runs for several minutes and then turns off with its internal thermostat, it is most likely that the thermostat is the culprit. Is the thermostat set to full, and is the temperature of the flow less if you reduce it?

Would others like to comment here?

.andy

To email, substitute .nospam with .gl

[Danny]

Yes the pipe is approx 3 inchs apart, with nothing but pipe between them. The position of the motorised valve is correct, I assume the gate valve located prior to the inlet on the HW coil was to control the temperature inside the HW tank by restricting the water flow. The HW tank also has an emersion heater (switched off at present) and a thermostat located on top - this is set to 60 degrees

I'm not sure where the return pipe from the central heating connects - I can't get to them without lifting half of my upstairs floor up!

The boiler thermostat is set to half way, because on anything more cause the boiler to pop off. I was thinking that I may have sludge in my boiler - but the fact that the rads get hot really quick tells me that the water circulation in my system is good, ie, not blocked. But the overflow of water into the FE tank leads me to think that the pipe on the flow from the boiler is blocked just after the pipe which hangs over the FE tank. Any comments on this?

What is a zone valve, and what are the controls to make my HW connections more efficient?

Regards,

[Christian McArdle]

There should be a zone valve capable of shutting off the hot water flow through the cylinder coil. This valve would be wired to open only when (a) the programmer told you to heat the water and (b) the cylinder thermostat said the cylinder wasn't hot.

You say there is a thermostat "on top". Normally the cylinder thermostat is between the bottom and halfway up (but higher than the bottom coil tapping). Are you sure the thermostat you mention isn't part of the immersion heater?

Christian.

[Roger Mills]

This is almost certainly the stat which controls the immersion heater - when it is switched on - and is thus not doing anything useful at the moment.

"Zone valve" is a generic term for any automatically operated on/off valve which controls the flow to different parts - or zones - of the system.

You appear already to have one for the heating circuit - controlled by a room stat.

You need to add another one into the hot water circuit, and to control it by a stat strapped to the cylinder. Each of these valves should have a independent pair of contacts which close when the valve is fully open. These need to be wired up to control the boiler and pump - so that they only come on when the demand in either or both circuit is unsatisfied. This saves a lot of energy otherwise wasted making the hot water unnecessarily hot and keeping the boiler warm when it's not needed.

You'll see how this all works if you look at the S-Plan details in

[Andy Hall]

Probably, but it's a poor way to do it because there is no control to prevent the tank from overheating when the CH is running.

OK, then I think we have to assume that it does the obvious thing and rejoins the return after the cylinder.

What boiler is it BTW.?

Not necessarily. It can be that the heat exchanger is badly sludged but allows some water through. Generally, when this happens, the boiler sings or bumps when the burner is firing, or as in your case the safety cut out operates.

Well it starts to look like it but I just looked back through the symptoms in the whole thread.

I think forget the thermostat for the moment - the safety cut out is operating for a reason. Either it's faulty or the water flow is not what you think and there is localised heating when the burner fires.

Given this situation, I would focus next on making sure that the heat exchanger really is clean. Since you already said that you had sludging in the bypass and elsewhere, I am thinking that this is the next area to look and I would do that before buying spare thermostats and cutouts.

It would be a two port motorised valve similar to the CH one, operated by a thermostat on the cylinder. You then need to have appropriate wiring with the boiler, pump and timer to hook it all together. I would expect that the current setup that you have is very simply wired. Have a look at the Honeywell web site and S-plan configuration for two valves, or Y plan if you would prefer a single divert valve. I wouldn't bother with this until the boiler issue is resolved, though.

.andy

To email, substitute .nospam with .gl

[Danny]

I guess the thermostat that I've mentioned is for the immersion heater.

[Ed Sirett]

Sounds like any of the following:

1) Over sensitive over-heat cut out. This should operate around 85C + not 65C. 2) Pump over run thermostat faulty - followed by temporary overheating in the boiler.

It can't be the main thermostat 65 for a middling setting is just fine. Sounds like the general health of the system is other wise OK.

I doubt whatever the boiler is that the parts to fix this will be over 20 quid. This is about as difficult as delving into the washing machine it all depends on your level of experience with dealing with electrical appliances repairs.

HTH

-- Ed Sirett - Property maintainer and registered gas fitter. The FAQ for uk.diy is at

Gas fitting FAQ CH FAQ

[Danny]

OK, I think we're making good progress here. Any suggestions on the best/simplest way to 'flush' through the boiler so as I can clear it of any sludge, hot spots?

My main concern is getting the boiler to stay on, thereafter I'll focus on making the system more economical to run - the thermostat on the cylinder and use of zone valves is really good - I was actually wondering if I would have scale build up in my HW tank given the fact that the HW would actually heat above the recommended 60 degrees, I guess this has been answered!

My boiler is a Glowworm Ultimate 60F

How important is the clearance surround>

[Ed Sirett]

This looks like a very typical arrangement for an older system and correctly laid out. It does not have fully independant control of CH and HW and HW temperture is set by the boiler's thermostat.

[Danny]

The pump does infact over run - but only when the boiler thermostat is set to maximum (this is correct as per the boiler instructions)

I've priced up a new over-heat thermostat, I might try this option - it doesn't seem too difficult to install

[BigWallop]

Most boilers today are designed around the aptitude of a Monkey.....Sorry.....British Gas engineer, so the parts have to be easy to replace or BG wouldn't have any engineers that are able to this type of work.

Owww !!! That's better. Just had to get that off my chest after arguing with one their towrags on the phone. :-))

[Danny]

What is the best way of making sure that the heat exchange is clean? (sorry if this should be way too obvious!)

[Andy Hall]

Flush it through very thoroughly.

.andy

To email, substitute .nospam with .gl