Vaillant turbomax cuts in and out when filling bath

[teddywood1]

hi our Vaillant turbomax is heating water when filling bath but now and again the gas goes out but then a few seconds latter relights and heats the water again can anybody help

[Andrew Gabriel]

Could be many causes, but most common is that the secondary heat exchanger is scalled up, so it can't transfer heat across fast enough and the primary heating circuit inside the boiler keeps going over its expected temperature.

If the boiler displays the primary heating circuit temperature, does this hit a peak just before the burner cuts out?

[teddywood1]

hi thanks for the prompt answer I will look at the temperature when fill the bath , how do I descale it would it pay to get a new one eddie

[gremlin_95]

If it turns out to be the plate heat exchanger, it is quite easy to remove and clean yourself. When I did my nan's I used an acid based descaler.

[teddywood1]

thanks I will give it a go asap and let you know

[Andrew Gabriel]

The descaler to use is Furnox DS-3, available from plumbers merchants in a tub. Make sure you fully dissolve the crystals before pouring the descaler in - it's not good to end up with crystals left in the heat exchanger afterwards. The descaler has an indicator dye in it which is yellow when it's usable, and blue or green when it's used up. You want to keep pouring descaler through the plate exchanger until it goes green/blue, and then discard and mix up some more. Repeat this until the you can't get the descaler to go blue/green, i.e. there's no more scale left.

Do this in a washing up bowl or similar, so you can recover the descaler which spills out, to pour though again. The indicator dye in the descaler can stain some surfaces. The amount of scale in a plate exchanger can be enormous - you wonder how so much can even fit in, and it can use a lot of descaler (don't bother trying to use little packs for kettles). It works better if the water is warm, but it may froth initially and generate an acidic airosol spray which is not plesent to breath in - do it in a well ventilated area.

One other possibility is that the primary side is blocked with rust which has been circulating in the heating system, if it's been run without inhibitor or the inhibitor has expired. This may not be recoverable, requiring a new plate exchanger. Descaler is not intended to dissolve rust, although it might do so enough to allow it to be dislodged and washed out. Normally when this happens, the plate exchanger will need replacing, after cleaning out the rest of the primary circuit including all the radiators and pipework. If the system has radiators which have rusted on the inside, it will need an inline filter/strainer fitting to trap rust particles returning to the boiler, so they can't get into the plate exchanger.

[teddywood1]

thanks for the great write up I will follow the instructions you have given me eddie