Combi problem

[nrh]

Is a combi boiler supposed to modulate its burners on and off every 15-20 secs, even when on a full hot water draw at the tap? High flow rate too.

Cheers N.

[Andrew Gabriel]

Might help if you gave the combi model. "modulate" and "on and off" are rather opposite things.

A combi cycling on and off on full hot water draw can be caused by the secondary (plate) exchanger becoming scaled up, so it can't transfer the full power through the plate exchanger. The primary circuit keeps climbing to max temp, requiring burners to cut off. I recently descalled a friends Baxi which had this problem. (It got left like this long enough that the frequent cycling of the gas valve was probably a contributory cause of the gas valve burning out.)

[nrh]

In news:439391bf$0$20537$ [e-mail], Andrew Gabriel scribed:

Thanks for the reply Andrew, and sorry for the confusion re modulate. I do mean that the burners turn off completely, not just decrease/increase in strength.

I have a Vaillant Combi (Model VCW GB 282 EH) which was installed in Nov

1997.

The suggestion you have made about the secondary exchanger becoming furred up sounds plausible to me, although as I live in a fairly soft water area, it does surprise me a little. That said, in Feb 2001, I had to replace the 'Water Section' unit on the DHW side due to the seizing up of the flow rate adjuster control, and there being a small leak

Although I have had a combi-care cannister fitted since new, there was a period owing to family commitments in which I had forgotten to replace the cartridge, and I think this is what contributed to the early demise of the DHW water section. I have noticed the hot water efficiency decline slowly over last couple of years, to the point where it is barely possible to run a good hot bath. You certainly do not need to add any cold water to the mix now, that's for sure.

Wondering if it might be a faulty temperature sensor of some kind, I located the Overheat limiter sensor located on the DHW outlet pipe, and saw there are 3 wires connected to it. One goes to an earth. The other two are connected to the same spade/lug on the sensor. According to the wiring diagram, one of these goes to something called an NTC Resistor and the other goes into the electronic control box.

As an experiment, I removed just the latter wire (leading to the control box), and found that the gas burners remained lit, and a good continouous flow of hot water was obtained at full draw, albeit accompanied by quite audible boiling noises similar to a kettle. However, after a couple of minutes drawing hot water, the flame lockout reset button popped out on the electronic control box, and the boiler shut down. It was then I noticed that the temperature gauge on the Central Heating circuit had shot up to around 100 deg/c, which presumably caused an overheat on the CH circuit.

This then tends to confirm your diagnosis, I guess. If so, what would be the remedy here? Should I attempt a descale, and if so how?

My very grateful thanks to you.

N.

[Andrew Gabriel]

Don't know that model, but the instructions should say how to remove and reinstall the secondary heat exchanger. Descaling can be done with kettle descaler or plumbing descaler such as Furnox DS-3 (much cheaper and better than kettle descaler, although it might only be available in large size).

[nrh]

In news:4393fc29$0$20537$ [e-mail], Andrew Gabriel scribed:

Ah right. Yes, the manual does include instructions on how to drain down and remove the exchanger. I was just unsure whether or not the descaling could be done with the exchanger still in situ.

Not to be pedantic, but do you mean Furnox or Fernox? I haven't heard of the former. Can one buy it 'off the shelf' at a plumbers merchants.

Many thanks for this. N.

[nrh]

In news:2LTkf.2780$ [e-mail], nrh scribed:

As you were. I have Googled it! Fernox it is.

N.

[nrh]

In news:sTTkf.2785$ [e-mail], nrh scribed:

Update!

I have managed to complete a repair, but ended up having to buy a new secondary heat exchanger for £96 inc VAT.

When I stripped down the old one to descale it, I noticed that the internal pathways seemed crystal clean on the DHW side. However, the CH connection points were full of black particles, similar to chips of rust! There was also quite a bit of 'scale' build up around the venturi where the pipes from the boiler connect. However, when I introduced a very strong solution of DS-3 in boiling water, it had absolutely no effect whatsoever. No bubbles of CO2 gas, nothing. Tried it in the DHW side as well, and same result. No descaling action at all. (and I know the stuff's good, because I descaled my kettle at the same time!) I tried force flushing through with cold water from the mains, but the sludge was stubborn, and water could be heard sloshing about inside, even though I had shaken it 'dry' and no more water could be seen coming out. So, that was it, and I didn't want to take any chances with it.

I am now getting a full draw of hot water with no intermittent cycling of the burners (hurrah!!!), but also wondering if the sludge that had collected inside the exchanger had come from the main CH exchanger instead? I think I may be needing to replace the boiler sooner than I had thought, if the main heat exchanger is silting up. I then gave it a force back flush through with mains water, and a little black deposit came out, but nothing to really shout about. That said, the burners don't cycle when the CH is working, and temps are good to the rads etc. So who knows. Probably my water pump disintegrating now! :-((

Anyway, fingers x'd. Just want to say thanks again for the help, and hope this long ramble helps someone else.

N.

[Andrew Gabriel]

Most likely coming from the radiators, due to lack of inhibitor in the system. The plate exchanger, being the thinnest part of the system, has been acting as a filter. Really you should flush the system out, and then refill using appropriate amount of inhibitor. You can get in-line filters to trap these particles, but washing them out of the system and preventing further corrosion should be done anyway.

[nrh]

In news:4395f30a$0$20536$ [e-mail], Andrew Gabriel scribed:

That's a thought. I've had some Fernox Copal in it because of a couple of fancy aluminium rads in the circuit, but that was since new around 8 years ago! Should have been changed at 5 years IIRC. Naughty me. I'll get right on to it, and thanks again for that heads up Andrew.

N.

[Ed Sirett]

The black bits are general corrosion form the heating circuit. If there are so many that you can get to the point where the heatXer is blocked then it's probably time to have a thorough clean out of the heating system. Running a desludging chemical for a time would be a good start.

[nrh]

In news: [e-mail], Ed Sirett scribed:

Yes, I did that when I had the new boiler put in, but admittedly, that was around 8 yrs ago! And look what time of the year it is. Not looking good for a spring clean for at least a couple of months, assuming we don't get the harsh winter the forecasters are predicting. Brrrr! :-(

I think that is what I shall do immediately. Can you recommend one, please? My system holds about 50 litres of water in a 3 bed two-storey maisonette with 8 rads, two of which are aluminium.

Cheers and thanks again for your interest. N.

[Ed Sirett]

Sentinel X400 would be a 'standard' product. I've not tried ferroquest but others have used it ok.

[nrh]

In news: [e-mail], Ed Sirett scribed:

Muchos Gracias, Ed ;-)

N.