Grant Combi 90 External losing pressure and switching off

[Leila]

In 1999 the combi and 4 new radiators were put in, we still have two old ones upstairs. The pressure has been dropping and the high limit stat has been tripping for the whole time I have owned it (just over a year). I have replaced the expansion vessel, repaired all leaks in the boiler, replaced a leaking radiator valve and most recently replaced the thermostat. Everything is fine for a few days then the pressure starts to drop again although never lower than 1 bar. It also seems to be intermittently switching itself off, when running water for a long period of time (deep bath etc). I don't understand this as the only mechanism it has for switching off is a high limit stat and if I have a new thermostat why would the temp ever go above this.

I am at the end of my tether and very poor, i have another engineer coming tomorrow and I don't know what to do next, can anyone help? I can't bare another winter of putting the pressure up in snow!

 

[simond]

Amazing how many women post on this site, with an excellent grasp of technical boiler terminology. When we want to recruit female boiler engineers there never are any.

If I was being cynical I would ask whether it was a front for a husband or boyfriend looking for some advice. Rather like the way I send my wife down to the builders merchant.

Anyway, if you have been losing water in the past and the boiler locks out when in hot water mode over a longer period eg: bath fill, it is quite possible that sludge has accumulated in the heat exchanger (the smaller secondary one about the size of a small tissue box).

This sludge would have been created in the central heating circuit by the regular addition of oxygenated fresh water you have been adding to top up the leaks you have described.

The oxygen-rich new water corrodes the living daylights out of the insides of the rads and other metals, forming a thick black sludge, called magnetite, which then tends to get trapped in the smallest apertures in the boiler circuit. These happen to be in the hot water heat exchanger.

If you are a bit tasty with the spanners you could take it out and inspect it, otherwise it's time to call an oily person in.

 

[Leila]

eh, I think that is a compliment! I sell underfloor heating so know a bit about mechanical matters unfortuately I sell systems from the boiler out so its not my expertise. The info about the heat exchanger is very interesting, its not something that anyone else has mentioned though from reading through forum responses today it has come up a number of times, I have a heating engineer coming tomorrow as he thinks it is a faulty high limit stat but I don't. I will show him your response and ask him to check it.

I'll let you know, cheers!

 

[JohnGuilbert]

HI Leila,

Any luck - I have the same problem with my Combi.

Thanks.

John