Vaillant boiler hot water temperature variation..

[SRW]

I have a Vaillant 828/2E turbo max plus boiler and it has worked okay for nearly two years. Since recently the hotwater temperature on the taps keeps going hot and cold. It appears the boiler heats and cuts off and fires up when the temperature drops.

I have changed the NTC sensors one at a time using a new one, without any success.

Any advise is much appreciated.

 

[ChrisR]

Often a sign of blocked/scaled heat exchanger(s). Or sticky mod coil on the gas valve, or pcb problem, even pump underpumping.
Check what water temp & flow you're getting - on average is it as much as it should be?
Check temp drop across heat exchangers.
Is modulation OK on CH?
Hard water area?

 

[SRW]

Hi Antartica,

Thank you for the reply. Re blocked/scaled heat exchanger - is this likely after two years ? I will try to check this tomorrow.

As for the mod coil, the boiler flame seems to change - i.e. when you increase the temperature control for hot water/CH, the flame increases in size.

When it's come to central heating the pump seems to be doing well. Flow rate from the tap is also very good.

Is there a easy way of checking the temp drop across heat exchanger ?

Many Thanks for your reply - SRW

Often a sign of blocked/scaled heat exchanger(s). Or sticky mod coil on the gas valve, or pcb problem, even pump underpumping.
Check what water temp & flow you're getting - on average is it as much as it should be?
Check temp drop across heat exchangers.
Is modulation OK on CH?
Hard water area?

 

[ChrisR]

If it is "nearly two years" then it's still under guarantee - call em out.
Pipe temps are easy to measure with a surface contact thermometer - not something you'd have. If it's OVER 2 years and you feel brave you could take the secondary heat exchanger out and look in to see if there's scale or sludge - and use a chemical to clean it

If the installer didn't do basic things like flush and add inhibitor, and used a bad flux, it is possible to have corrosion problems in 2 years. Is your tap water particularly hard? Do you have the tap water temp on max (promotes scaling)?

Could you MEASURE the flow rate of hot water, in litres per minute with a marked bucket, and guage whether its say shower temp(38 ), bath temp(45), hot washing up temp(50+)

I have seen things like cracked tracks in temp adjustment potentiometers (the thing the knob's connected to) which could cause your sort of problem, but common problems tend to have common causes!

 

[SRW]

Hi Chris,

Thank you very much for all the useful information. I took out the heat exchanger and a certain amount of rust fell out when I tapped it out and flushed it out with water. I reassembled and flushed the whole system out with a cleaner and it is working fine now.

Many THanks once again for your help.

Regards

Sheran(SRW)

If it is "nearly two years" then it's still under guarantee - call em out.
Pipe temps are easy to measure with a surface contact thermometer - not something you'd have. If it's OVER 2 years and you feel brave you could take the secondary heat exchanger out and look in to see if there's scale or sludge - and use a chemical to clean it

If the installer didn't do basic things like flush and add inhibitor, and used a bad flux, it is possible to have corrosion problems in 2 years. Is your tap water particularly hard? Do you have the tap water temp on max (promotes scaling)?

Could you MEASURE the flow rate of hot water, in litres per minute with a marked bucket, and guage whether its say shower temp(38 ), bath temp(45), hot washing up temp(50+)

I have seen things like cracked tracks in temp adjustment potentiometers (the thing the knob's connected to) which could cause your sort of problem, but common problems tend to have common causes!