Discuss Honeywell ST9400C wired correctly? in the Central Heating Forum area at PlumbersForums.net

Honeywell ST9400C – is there any set up where the hot water slider would control both hot water and central heating? That’s the simple question, the rest of the info is:

Mother in laws house. Large 4 bed detached house only 12yrs old. Radiators upstairs, underfloor heating downstairs controlled by 7 programmable, wired room-stats. The gas bills are enormous! It’s always hot because the under floor heating in one room or another is always on.

It’s been looked at by numerous ‘specialists’. One said it needed a new boiler – 7k thank you very much – still the same. The last said you need two new roomstats – no difference. I walked in late Saturday night and said what I always say “heating’s on”, because it’s like a punch in the face when you walk in the door. And what the mother in law always angrily does is drag me to the programmer and show me the slider on the heating side set to ‘OFF’.

Sunday morning I had a look. With the heating slider set to ‘OFF’, I go to the boiler – it’s not running. I set the nearest thermostat sky high – ‘clonk’ from thermostat and boiler starts. Go back to the programmer and set the hot water slider to ‘OFF’ – boiler switches off. Both the hot water and heating are definitely being controlled by the hot water slider.

I know why the heating is always on because every man and his dog have interfered with the heating and I will always find a thermostat that’s been set to 27 degrees. But everyone always assumes that the heating can’t run because it’s set to off, but it does because the hot water is set from 6am to 10pm (I’ll be changing that).

I can only see:

1 – For some reason this system has to be wired this way and you rely on the roomstats to control on/off and temperature

2 – It was wired incorrectly from the start

3- The programmer has an internal fault

I need to get to the bottom of it because it’s driving me mad. I can easily get someone to rewire / replace the programmer. I can then set the times and temperatures and lock all the programmers! No more money can be spent on specialists as the boiler replacement cost 7k, and the gas bill is over £100 each month!

Please help!
Some clear pictures of zone valves and underfloor manifold set up would be useful.
The underfloor heating should be controlled by it's own thermostats with correctly set maximum and back set temperatures. You do not operate it like a radiator system switching it on and off via a programmer!
Currently looks like you have a permanant live through hot water side that needs finding
Needs someone competent with a multimeter.
You need a suitable engineer to check things over with a multimeter. By the sounds of it it was never wired properly from the word go.
I wouldn't get an electrician to look it over because most are not familiar with heating system wiring. You need a decent heating engineer.
What part of the country are you? Myself or others might know someone who can help.
Thank you all. As I said, there have been numerous "specialists", as they called themselves, look at it already and so far changed a boiler at 7k, and 2 stats (they were only asked why is the heating always on with such high bills - never a fault with the old boiler). There have been more, at least two more heating engineers take a look, that I know of, there could have been more. That's why I now need to direct what is done.
I gave as much info as I could, and I was worried you might have questions about the set-up.
The boiler is mounted to the back wall of the garage, and the manifold and everything else is mounted to the back wall which has been panelled over in such a way that it lead me to believe they never wanted anyone to see behind there as none of it is removable - not without destroying it that is...
The reason I suggested an electrician is because I know one that would do it as a favour to me.

I won't be back there for a while but when I do I'll come back with more info, and photos. It's been like it for years, I don't think a few more months will make any difference.

We can only comment on the description you've given so it sounds like a control/electrical fault or issue having said that many so called engineers don't really know how to set up UFH properly especially when combined with a radiator circuit but we can't comment on that without seeing it. With a large hike in gas prices coming the sooner you can upload more details and pictures the setup the sooner you might get the correct steer as to how to correct or diagnose the problem.

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