How much to replace a second (DHW) heat exchanger on a combi

[Participant]

How much to replace a secondary (DHW) heat exchanger on a combi

Hi all

We have a Ravenheat CSI 120 T Low Nox combi boiler. It's awful and we're getting it replaced next year when we do the kitchen, but in the mean time...

Our hot water is cycling hot and cold. Observing the boiler I can see that it's shutting down then kicking back in again repeatedly, which is apparently a classic symptom of a blocked DHW plate heat exchanger (probably caused by a filthy system).

I've called a heating company out to come and take a look, but how much is reasonable for a job like this so I know if their quote is OK?

The part alone is about £150 . There's a pattern part on the good old internet for about £40!

Can anyone tell me a sensible ball park guide price? The manual makes it sound like a fairly simple job.

Thanks.

 

[Participant]

Thanks. I'm very tempted to have a go myself, and have already ordered the part and removed the front and bottom panels.

My fear is, although I've done a few reasonable plumbing and automotive jobs, I have no idea what I'm doing when it comes to boilers.

I don't know what valves to close, how to drain the boiler, whether or not I need to drain the system, and what I need to remove before I can safely remove the secondary heat exchanger. The manual just says to undo the three nuts, when there are four nuts going into the exchanger!

I've found the part in my boiler (at the back through a network of pipes I have no idea about):



This is a factory image of the part:



...but I'm slightly fearful I'll cause a massive leak or create more issues than I solve.

 

[Participant]

If your asking these sorts of questions and don't know how to drain etc even though if you know how you can get away with it then pay someone to do it.

So I had a plumber out to investigate in the week. They diagnosed (correctly) a faulty diverter valve.

However their assessment was that the entire valve unit needed replacing, quote for parts labour and VAT: £368.

I did a bit of Googling and it seemed that nine times out of ten, it's actually the diaphragm that fails rather than the valve.

Taking the valve apart revealed the state of the diaphragm:



Yep, broken.

£8 later on eBay and 3 hours work, I now have instant hot water again. £360 in my pocket compared to if I'd taken your advice .

Internet 1 - 0 ripoff tradesmen, once again. Although I obviously paid the chap for the 40 minute call out, during which he spent roughly 15 minutes arguing with his daughter over whether she should SORN her car or not (£54).

I'd describe it more as a learning curve than difficult...of course the trade would have you think otherwise .

The hardest parts were getting my wrenches at the funky angles required, and draining down the boiler without spraying water all over the PCB.

 

[Participant]

Well done. However if it wasn't for your plumber coming round to diagnose the fault you would be replacing a part that wouldn't have needed replacing! (Heat exchanger) so give some credit to people in the trade, they are not all out to get you...

In your case the plumber probably doesn't want to come round after replacing just the diaphragm, plumbers are belt and braces they don't want to come back to a job so will replace the whole thing with a genuine part.

I paid the guy for the diagnosis, which he correctly performed, but £400 to investigate and replace a diverter valve is nothing but daylight robbery. I'd expect a plumber to only replace the necessary parts, not a £150 part when a £10 part would do it, but maybe I'm just old fashioned.

I replaced the heat exchanger too since I bought it and the old one was looking decidedly dirty.