Baxi bahama no hot water + banging

[ChrisR]

And I haven't seen a transparent one yet. A PATH through it may be available, the other 90% blocked.

 

[Dick n Dom]

Sorry to jump in on this one .But had one the other day exact symptons turned out to be the flow switch.

 

[irelandy]

Hi Tony the heat exchanger I've checked is the water to water one at the bottom of the boiler. I haven't looked at the one inside the burner...do you think the problem could be arising in there? One additional piece of information is that about 3 weeks ago the system was drained and refilled during a complete service....and I'm thinking this may have dislodged some gunk from inside the rads or the system which may have blocked the micro vessels in the exchanger...as suggested earlier, what do you think...once again I can't thank you guys enough out there, Andy.

 

[themaster]

in my experience, u have bought probably the worst ever combi brought out, exchange it for a 105he and youll have no bother.

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i definitely think your expansion vessell has gone(this is from experience of working on these boilers), when the boiler changes over to domestic hot water the volume of water in the boiler is less than 2 litres, this is why u get the rapid increase in pressure, (pressure is always related to volume) which consequently results in the expansion relief valve opening.

 

[themaster]

i definitely think your expansion vessell has gone(this is from experience of working on these boilers), when the boiler changes over to domestic hot water the volume of water in the boiler is less than 2 litres, this is why u get the rapid increase in pressure, (pressure is always related to volume) which consequently results in the expansion relief valve opening.

 

[irelandy]

The problem has to be the top end of the secondary heat exchanger...the one which circulates the 85,000btu's into the hot water bit. It was blackened when we took it off...but did not seem clogged...as suggested the water has found an easy path through it without allowing the heat to dissapate into the block of the exchanger...so when the hot water is switched on, the diverter valve closes off the rads...leaving a small amount of water circulating which cannot dissapate its heat....this causes overheating...pressure blow out, and flame lock out. My friend proposes to put fernox into system and run the rads for a day or so to clear the blockage in the secondary heat exchanger..the in theory it should be ok...what do you think...will this work? or do you have any other suggestions to clear this without replacing the secondary heat exchanger?

 

[kevplumb]

try it but keep running the hot water or it wont have any effect
by keep running i dont mean constantly

 

[ChrisR]

Could take the secondary h/e off, and tip a hot strong concentration of DS-40 into it. Repeat until there's no more fizzing. Jobs a goodun.

 

[themaster]

as ive said, its the expansion vessell, now exchange that or add another one to the return and be done with it.

 

[kevplumb]

shoemenders you are talking out of an orifice

 

[irelandy]

Well its all over and working again. We put the fernox in with most of the rads closed to keep it concentrated, and ran the rads for about an hour. Then turned on the DHW for a couple of mins to get it into the secondary heat exchanger, left that for 15 mins with rads still going, and gradually built up the time spent using the DHW. After a few goes, we were up to 5 mins of continuous hot water. I noticed the secondary heat exchanger was of a uniform temperature top and bottom....indicating the blockage had gone enough to allow efficient heat exchange.
This site and all the help posted has been invaluable assistance in getting this boiler back on the road and I would like to thank everyone who took the time and trouble to post advice for my guidance,,,,Well done give yourselves a collective pat on the back and a virtual pint on me!!!!