ICOS HE24 / Megaflo / Horstman H37XL - upstairs zone

4 Jan 2008
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United Kingdom
I have checked the FAQ and scoured the forum.. hope you can help ?

Everything listed above works fine - 95% of the time. The system is well maintained, full of all the right inhobitors etc etc... EXCEPT, the upstairs heating doesn't work on it's own.

1) HW is fine
2) Downstairs heating is fine
3) The upstairs heating will work but only if the downstairs heating is also on (i.e. the neon on the 'stat is glowing).

I only discovered this recently when I commanded the upstairs zone to come on earlier than downstairs, and noticed that despite the thermostat being powered, and engaged, the rads were

When the downstairs stat clicks on, and the timer is correctly sequenced, then the boiler fires up and both up and downstairs get hot.

When I stuck my head in airing cupboard and ask someone to operate the upstairs stat, the diverter valve moves as it should

I have a new V9 control box fitted some 3 years ago to cure a totally unrelated problem.

The system is 5 years old

Everything else...works perfectly.

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Sounds like the upstairs stat is wired to drive the zone valve but the zone valve limit switch isn't driving the boiler. Either the switch is faulty or it isn't actually wired to drive the boiler. Can you supply an actual wiring diagram of your system since this is essential to providing an educated answer rather than simply guessing as to what's wrong.
I cannot (with regret) give you a circuit only the generic one that is supplied with the boiler. However, you have sparked off a train of thought.

In the zone valve (which moves) are you suggesting there is another switch that sends a "fire up now" signal to the boiler ?

I might look here to start

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Generally (always work on assumption that there are exceptions) zone valves have a built in microswitch which operates when the valve reaches the extreme position that power drives it to. Generally the switch contacts close once the valve is fully open (assuming a 2-port valve). This switch is then used to control firing of the boiler, or at least forms part of the boiler control features.

You can tell if such a switch is installed if you can see the termination block that the valve cable connects to. If there are 5 (or 6) wires then the valve does have a switch. (3 wires are for the motor and earth connections - the remaining 2 or 3 wires are for the switch) If you google the valve make and model you will turn up a circuit diagram which indicates which colours connect to what.

It's not uncommon for upstairs heating (bedrooms ?) to be "slaves" to the main house heating control clock. ie it only heats if downstairs clock calls for the boiler to run.

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