Combi heat exchanger still getting blocked despite phosphate doser?

[JimD]

I have a Brittony 80 combi boiler made by Chaffoteaux, which I installed about ten years ago. Almost every year the heat exchanger gets partially blocked, resulting in fluctuating temperature of the domestic hot tap water. I live in a very hard water area.

So a year ago, when I last renewed the heat exchanger, I also installed a phosphate doser on the water mains inlet pipe to hopefuly reduce limescale.

However, now, a year later, the heat exchanger seems to be playing up just the same as before: hot tap water coming out luke warm, then slightly warmer, but never really hot.

Is this the heat exchanger blocking up again, or could it be something else?

I can buy yet another heat exchanger tomorrow, but is ther anything else I should be doing to prolong its life? Water filter, perhaps?

Does anyone know where the filters (if any) are located in a Britony combi? I can't find any reference to them in the manual, but see to recall a plumber talking about them once. How often should they be cleaned?

Thank you,

Jim D

[JimD]

PS, the latest problems started just after I desludged the system a few days ago. Could that possibly have anything to do with it - or is it probably a coincidence?

Jim D

[Andrew Gabriel]

I don't know this model, but why not descale it?

I descalled a friend's one from a Baxi using Furnox DS-3. Dissolve some DS-3 in a large jug of hot water. Stand the heat exchanger in a washing up bowl to catch the descaller, and pour into the hot water side of the heat exchanger slowly. Be prepared for violent descalling spitting out the acid and acid spray. When you've poured it all through, empty the heat exchanger out into the bowl, transfer back to the jug, and the pour through again, until the heat exchanger stops fizzing. If the descaller goes blue, it's used up -- discard and mix up some fresh.

DS-3 is available from a plumber's merchant, and possibly larger B&Q's. I'm assuming the heat exchanger is stainless steel -- if it's something else, check compatibility with the DS-3.

DS-3 is sulphamic acid, i.e. pure kettle descaller without the sand and other rubbish you find mixed in with retail kettle descallers to dilute them. It has an indicator added so you can tell it's used up by change of colour, and a smell so you don't accidently drink it. A tub of it can be useful for other things too, but don't use it on plastic parts (or plastic kettles), as it can make them brittle.

[Andy Hall]

Following on from Andrew's comments, and you certainly should be able to descale the existing heat exchanger, if a phosphate doser isn't managing to handle the scaling then an ion exchange water softener is the solution. A water filter won't do anything about limescale.

Of course descaling will also tell you whether this is the problem.

[Jim D]

I did try descaling it once - but only with domestic supermarket descaler. It fizzed and bubbled but didn't unblock it. I've never tried "DS-3". Where can I obtain that in suitable quantities?

But I think these heat exchangers can also become blocked by rust and sludge on the other side (non-tapwater) side of the exchanger, can't they? That's what a corgi reg'd plumber once told me was wrong with mine, anyway. He pointed oit the bits of black stuff inside the exchanger and said "that's iron oxide that's blocking it" IIRC. Of course, he might have been feeding me a line of s**t just to flog me an overpriced heat-exchanger....

Thanks,

Jim D

[Jim D]

Sorry - I didn't see the lower portion of your response - you answered the above qn - thank you.

Jim D

[Jim D]

Thanks for the reply. It just occurred to me: scaling is only likely to occur on the side of the heat exchanger that passes water from the mains inlet to the sink taps, isn't it? In if that side was scaled up, then there would be a reduction in flow from the hot taps, wouldn't there?

Jim D

[Andy Hall]

Plumbing and heating merchants have it in small and large tubs.

There's really no reason why a properly maintained heating system should be suffering from sludge.

If it is a sealed system (no header tank on loft) then the only action that should be needed with a clean system is to dose it annually with corrosion inhibitor - cost about £20.

[Andy Hall]

Possibly some, yes.

If the system is sludged, then a different course of action is needed - basically clean it out and fill adding corrosion inhibitor to the system afterwards

[Jim D]

I have to confess, that is something I haven't done for about 3 years, ...until the other day, when I ran it with de-sludging solution and then flushed and filled with water+inhibitor.

Jim D

[Jim D]

That's exactly what I did, a few days ago, just before this problem started..

Jim D

[Jim D]

A couple more observations:

1) When I turn a hot tap on, the boiled does fire up and stays burning as long as a hot DHW tap is turned on. As an experiment, I pulled the contacts off the boiler's DHW flow switch while it was fired up, and this caused the burner to stop. As soon as I replaced the contacts, it fired up again.

2) I put my hand on the secondary heat exchanger while it was fired up. It was stone cold in all places.

Some might question the diverter valve at this point. However, I remember the tech help guy from Caffateaux telling me that the diverter valve never goes wrong on these boilers...

Thanks for any further suggestions...

Jim D

[Andy Hall]

Is it a sealed system or open vented with tank in the loft?

If it's the latter, then pumping over of heating water or sucking of air into the system because the relative positions of pump, feed/expansion and vent pipes can cause the water to become more oxygenated than it otherwise would be. This effectively "uses up" the inhibitor. You can diagnose this problem if the radiators need frequent venting and the gas released is air and not hydrogen, or pumping over can be seen from the vent pipe with the tank getting very hot.

EIther way, I wonder if you have managed to flush some sludge into the boiler....

[Jim D]

It's a closed, combi boiler system.

I have started a new thread called "Britony Combi 80 - symptoms restated"... to list all the current symptoms in one place and in one thread, for better clarity.

Thanks for your time,

Jim D

[<me9]

If it's a sealed system why dose annually? Once only should do (well perhaps every 10 years), unless you have a big leak.

[The Natural Philosopher]

Actually there is.

And that is the water company is sending you sludge.

Personally I'd fi a water softener meself..and then claim against the water company when their sludge blocks it.

[Doctor Drivel]

Every 4 years.

[Doctor Drivel]

Clean the heat exchanger with DS3 as instructed. Use descaler in the CH system after cleaning, drain, flush and fill with inhibitor. BUT!!!! add a Magnaclean filter on the CH return pipe back to the boiler. That should stop it happening again.