Venturi keeps melting

[mowflow]

I've seen a few old posts about this but i've not found any that mention the exact cause and solution.

I've got a Baxi combi 80 eco that's going through venturis (melting them) at the rate of one every 3 weeks. My friend is a corgi registered engineer and even he's stumped by what the cause is.

We thought the fault was being caused by dry firing as the boiler did seem to loose pressure over a prolonged period of time. After the last venturi change i made a point of checking the pressure regularly and topping up if necessary, it pretty much never needed topping up.

Last night when i came home the house was like an oven, i checked the boiler and it seemed the radiators were running at max temp although the thermostat was turned half way down. When i switched the timer to off the pressure gauge went crazy jumping up and down, the boiler started making some scary noises and it wouldn't go off. When i flicked the main switch to off the heating did switch off but when i switched it to hot water only the radiators/CH would come on full again, the pressure switch would go crazy and again the scary noises.

In fear of the boiler exploding i switched it all off and left it. When i later tried turning it on for some essential hot water the boiler wouldn't fire up at all. Surprise, surprise the venturi had melted again, i've replaced it along with the thermistor and the boiler seems to be working fine again ( no jumping pressure gauge or noise and the hot water switch only gives hot water). My friend thought the problems sounded like a jammed diaphragm or switch but surely that couldn't have been the problem if it's all fine now?

Anyone got any similar experience or diagnosis as i'm getting to the point of just ripping the thing out and replacing it?

 

[Agile]

Its difficult for me to guess whats wrong but chearly there is a serious problem.

I am also concerned that you apparently open up the combustion chamber and change venturis when it should be done by your CORGI friend.

As a start you should get your friend to fully check the APS is working properly but few seem to know how to do that properly.

Then get him to check the system flow switch to make sure the pin is not jammed or the switch stuck.

Tony

 

[mowflow]

My CORGI friend has just been round and changed the diaphragms and the micro-switch although the old micro-switch appeared to be fine. He also checked the venturi and the thermistor that i'd changed were properly fitted (the boiler has not been on since i changed them, i removed the main fuse).

He also checked the APS and the system flow switch as you suggested Tony, both were fine (thanks for your suggestions). The only fault he could find was a bit of a leak at the pump so he replaced the o-rings. I guess we've now just got to sit and wait to see if the venturi melts again.

I'm hoping the thermistor was the cause and the cure.

 

[giblets]

Did your CORGI friend gas rate the boiler, check the pressures and check to see if the fan was running correctly?

 

[Agile]

I dont see any way that a thermistor could cause the venturi to melt.

The water flow through the heat exchanger removes the heat from the combustion gases so that at the venturi the temperature is only about 150°.

If there is no, or limited, water flow or if the heat exchanger is seriously scaled up then insufficient heat is removed and the gases may be at about 200° and most plastics are likely to soften at that temperature.

The boiler protects itself with the water flow and the air flow switches.

Tony

 

[GodKnows]

The venturi's melt when there is less than 1.0 bar cold pressure in the system. Often if the system is just say on 0.5 bar or just below that there is just enough water pressure to start the boiler, BUT, the excess heat created in the chamber goes through the fan and melts the venturi fella... I have simplified the process for you to understand. Keep monitoring the system pressure, ensuring it is at the required pressure

 

[nickso]

alpha 240's do this if the pcb is faulty and the fan doesnt get the right voltage. perhaps something else to think of on your boiler.

 

[baxpoti]

Have seen this a few times, last time the eco boiler was used to heat a swimming pool. On full rate 24/7 I put this down to over use? Didn't ever go back as told customer the boiler is for domestic heating and hot water and not for heating swimming pools.

Sounds like yours is operating without being told to, this normally comes down to the dhw microswitch being worn.

God knows what 'godknows' is on about.

 

[GodKnows]

Got told that info off a baxi senior tech guy! Obviously, baxi's own don't know why the venturi melts either I only stated sortta what Agile said about the restrictive flow of water through the HE carries the excess heat up past the fan and melts the venturi, apologies for not being clear....

 

[Agile]

There is no reason why one of these models should melt the venturi specifically when used on a swimming pool.

In the typical pool setup the circulating flow is very low resistance and offers the ideal setup for boiler operation.

I have sometimes seen copper chloride deposited on the fins of the HE when, as usual, the water treatment chemicals are stored close to tbe boiler. This will reduce the heat transfer and could cause the venturi to melt.

Equally though, I have seen boilers in perfect condition when chemicals are stored close by. It could well be associated with the air intake path though and might just be as a result of airborne chlorine from the pool surface.

In fact this is an ideal application for a boiler like this and I have seen many Biasi 24S, a couple of Suprimas and a C&M used this way.

There is a design flaw because the pool HEs are made in the US and even with a flow of 80°C they just do not take anything like the power transfer which their spec says they should.

I have seen a pair of 24 kW boilers with a flow temperature of 80° giving less than 12 kW into the load and thats with a water temperature of well less than 20°C The HE needs to be seriously over rated if the boilers are to give out much of their heat.

Tony

 

[DP]

Message reoved- Dan

 

[Agile]

My message reoved as well!

 

[BigBurner]

Its difficult for me to guess whats wrong but chearly there is a serious problem.

I am also concerned that you apparently open up the combustion chamber and change venturis when it should be done by your CORGI friend.

There is no law that says he cannot do that. His "CORGI" friend cannot figure out what is wrong.

 

[bengasman]

As suggested before, have your friend check the burner pressure and the gas input. If the injector is too large, you can get this sort of problems.

 

[mowflow]

Thanks everyone for all your comments and advice. This gives me a good list of things to have checked out should the venturi melt again. Here's hoping it doesn't