Another Overheating Boiler

[mjspc]

Would appreciate if anyone could shed any light on the following overheating boiler problem.

The system consists of a Baxi solo 50 bolier (installed late last year) utilising an indirect cylinder with a 2-way zone valve installed in the heating circuit, prior to today there was no tank stat or room stat fitted resulting in the boiler cycling a lot and also very hot tap water besides this howver it appeared to work OK all radiators (7) warming up quickly and no overheats, since having the new boiler installed we have always had the controller set for both heating and water. There is a old manual thermostatic valve with a capillary tube attached to side on tank fitted in the hot water circuit but opening or closing this made bugger all differnce to the hot water tempreature.

Today using my humble DIY knowledge and several hours research on the Internet I fitted a tank stat and a wireless room stat (drayton RF3) . Powered everything back on set controller for heating and water and all seemed fine, both the room stat and cylinder stat working OK. Room came up to tempreature in around 30 mins and the room stat shut off the 2 way valve in the heating cirucit, within 2 minutes of this happening banging noises from the boiler and it shut down with overheat light. Let everything cool down and tried again exactly the same as soon as the heating valve shut down overheat in less than 2 minutes. Tried with just water and no heating and again shut down in under 2 mins.

The hot water is warming up, and there is a manual bypass valve fitted in the flow and return of the boiler.

Didn't have this issue on old boiler, there again it was 25 years old with a corroded heat exchanger and probably not giving out as much heat as the new one.

To me it seems like the hot water circuit can't take the heat away quick enough and hence the overheat, I'm a total novice at heating systems but the hot water cylinder itself looks a bit odd, all the indirect cylinders I've seen have the feed for the coil entering on the side of the tank this one (looks really old) has the feed entering and exiting via 15mm pipes via what looks like an immersion plug in the top of the tank, the pipe is reduced down to 15mm from the 22mm feed from the pump.

You can see a picture of the top of the tank at
http://www.mjspc.demon.co.uk/hwc.jpg

Is this just an old tank or has somebody bodged together some sort of indirect cylinder, seems strange it should only have a 15mm feed to the coil.

Any help or advice much appreciated.

Cheers.

 

[mjspc]

Forgot to mention the pump overrun facility is functioning OK and continues to pump for a few minutes after boiler flame shuts off, the overheat is happening whilst boiler still going and not during overrun.

 

[fitz1]

the 15mm is whats called a hot rod conversion. perfectly o.k. check ur expansion pipe not blocked.

 

[mjspc]

Thanks for the reply.

Is the expansion pipe to check the one that runs from the hot water feed on top of the tank then tees off into tank in loft and on to taps ?

Is there any easy way of telling if pipe is blocked ?

Thanks

 

[Bahco]

The 15mm is whats called a hot rod conversion.

What is a hot rod conversion

 

[doitall]

The 15mm is whats called a hot rod conversion.

What is a hot rod conversion

Its a gadget that screwed into the immersion heating socket and converted a direct cylinder into an indirect one

 

[Bahco]

Of course, silly me

Any comments Kev

 

[Bahco]

MJ

Have you tried opening your bypass valve a little more? The boiler will overheat if the hot water has nowhere to go.

 

[mjspc]

Is the manual bypass valve just there to provide a return path during pump overrun ? the overheating occurs whilst the boiler is fired and as there is no valve in the hot water circuit there should always be flow through it.

Is it feasible that the 'hot rod' conversion used to convert the direct to indirect cylinder is unable to dissipate enough heat output by the new boiler, when the flow to the heating circuit is shut off ?

Thanks

 

[doitall]

the hot rod conversion is for pumped primaries not gravity

 

[mjspc]

The 'Hot Rod' is fed from the output of the pump as well as the zone valve for the heating circuit.

 

[doitall]

Does the zone valve shut the pump down

 

[mjspc]

When the heating zone valve shuts off the pump continues to run as the hot water cicuit is still calling for heat, pump overrun also works when both the heating and hot water circuits shut off.

If I leave heating off and just switch on water boiler overheats in a couple of minutes, with heating circuit on (and heating zone valve open) no overheat and hot water heats up OK.

Boiler will overheat if it is only feeding the Hot Water Circuit.

Someone metioned in an earlier reply about checking for blockage in expansion pipe any other info on this would be usefull

Thanks again.

 

[croydoncorgi]

Sounds to me like the boiler is continuing to run AFTER the motorised valve has closed. If so, wiring problem. The valve should control the boiler! When you say 'two-way' do you mean '3-port' valve (one output to rads, one to cylinder, one input from pump) or a Zone valve, which has only one output?
If you have just one zone valve controlling the flow to the rads, how is the hot water temperature controlled???? If you've still got the (U/S) thermo-mechanical valve on the cylinder, what have you connected the new tank thermostat to?

On the information given, it sounds like you've only done half the work: you'll need at least another zone valve, by the sound of it.

 

[ChrisR]

Open the bypass wide then try it.