Valiant System Boiler - Hot water?? - High Gas Usage Investigation!

2 Aug 2009
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United Kingdom

We have a Valiant system boiler and water tank installed several years ago following a large home renovation to our 5 bedroom house. An Evohome installed with 3 zones including a UFH using a room stat on each floor. Our gas bills usage is showing at the moment nearly 234Kw per day (approx £8.30 a day...£252 a month!).

I'm now at the point where I am trying to understand where the gas is being used. So now watching the smart meter for when gas is being used (only the boiler is our gas appliance, everything else in house is electric based). At the same time watching the honeywell evohome that tells me which zone is calling for heat.

What I originally observed is even though there is no demand from any of the zones, there is gas being used according to the smart meter. When I go to the boiler I can hear it switched on and even for 20 minutes or so its going through a series of status symbols from "egg timer" flashing, "radiator" flashing and then "radiator" solid. All pipes are cold apart from the hot water feed to the tank which is hot.

So reading up there was a suggestion that maybe the problem is a sticking value on the hot water that is keeping the boiler running. I "tapped" the motorised valve to see if that helps (sticking valve possibly?).

Normally we have the hot water to be on each morning from 5-6am. But as an experiment, I turned off the timer also however I've noticed over the past 3 days now we still have hot water showing from 61 to 64 degrees (according to the evohome controller screen). Is it possible that heating radiators also will by default warm up the hot water????
I do notice that even when there is no heat demand, the boiler still sounds like its on?? could it be that it using Gas but not supplying heat anywhere???

Sorry for lengthy question.....looking for some answers!

Thanks in advance.
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Possibly the 24volt RT link within the boiler control board hasn’t been removed, this will cause the burner to fire 24/7.
Possibly the 240 volt RT plug has been used wth the 24volt RT link still in situ.
Lastnight did check again. The gas smart meter reset at midnight so back to 0Kw. Around 1am the boiler came on and stayed on until 1:45am. The “hourglass” symbol flashing and status “pump overrrun”. I noticed again that the pipe going to the water tank is hot. I’m wondering if it’s releasing hot water to hot water tank to get rid of it?? (Protecting the boiler?). I did notice that during that time it consumed around 5kw of gas. This morning before the heating came on around 8am the gas reading was still at 5kw so tells me that the boiler did not come back on again. Is this normal behaviour??
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Did you look at post #2 ??

Yes, this was checked and is okay. I've also been doing some background reading and understand that the behaviour I witnessed is the system cycling which is often does through out the day to maintain heat in the downstairs zone throughout the day. I'm now just wondering if its the fact we have a really high KW boiler (Valliant 637) which might be contributing to the higher bills?

Our setup is:

Vaillant 637 system boiler
300l Water tank
Wet UFH (floor area 15x4m) in new rear extension (family area, kitchen and dining) - used all day (1 zone). Unknown but calculated by SOLFEX from what I recall seeing by the heating engineer at the time.
Existing build Downstairs, existing build upstairs each with own zone (2 zones).
5 bedrooms (1 guest so hardly used), 3 bathrooms.
17 Aluminium radiators in total 25Kw in output excluding UFH (i don't the KW what this actually is)
Honeywell Evohome with Y87 wall stats in each zone
The past week I've also been monitoring evohome and I can see that the UFH once up to heat even in this cold weather spell (-1 degree) it maintains a lovely 19 degrees all through the day with just the odd call every couple of hours for a bit of heat (10% or so) and switches back down to 17 through the night (i can see that it does'nt drop lower than 17.5 through the night using some logging software so no heat demand).

The downstairs zone (non UFH - 9 radiators - approx 14KW - downstairs includes bedroom 5 and ensuite) seems to be running most of the day to maintain a temp of 19.5 degrees. I've witness it "cycling" every 10 minutes or so throughout the day.
Flow temperature is: 75C
Return temperature is: 65C
I think youve answered your own question there really. Your heating a 5 bed house all day. Having evohome controls through out the house, while being a very comfortable method of heating it, is only cost saving if you use it to keep areas cooler or off more often than not.

The type of boiler you have, realistically cant "over use" gas unless its burning very poorly (when was it last serviced?) and physics says if your burning fuel then the heat has to go somewhere, so assuming at this point the boiler combustion is ok, then you pay for what you use. try turning the temps of each zone down 2 degrees each day to see the difference.
As a specialist in Evohome I am incredibly careful about selling it as an energy saving system

It can be .... But not always You use what you use. Just make sure that the boiler has had the RT link removed.
Also a 637 is probably massively oversized so you will lose SOME efficiency there. Although not bank balance effectively.

If that makes sense..

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