Tthermostat/timer controller switch wiring question

Oct 08, 2009 9 Replies
M

Good afternoon all,



for a particular application I am looking for a combined central heating digital timer-controller/thermostat unit (wall-mounted), preferably mains-powered although I would consider one of those handy roving battery-powered ones if necessary...



My question relates to the wiring of the output switch. Traditionally a central heating controller has a SPDT switch providing both a "CH on" and "CH off" signal, and the pole of the switch is usually wired to live, to provide the mains supply to the zone valve, and so on.



However, I need to find a unit specifically in which the pole of the switch (and it must be SPDT, SPST) is not hard-wired to Live, such that I can wire it to something else.



Does anyone who has fitted such a unit recently know if it is usual for the manufacturers to hard-wire the switch pole to Live? Obviously for the battery-powered ones this isn't an issue but for the mains-powered controllers there is the possibility that the live power for the unit and the switch pole are connected internally, which scuppers my plans.



thanks,



Michael


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J

I believe many are termed "Volts Free" which means that the switching contacts are independent of the control circuit - most will be SPST.

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M

"Most will be SPST"? It strikes me as odd. For a typical Y-plan system with a mid-position valve, for example, the hot water circuit is required to have an SPDT switch. So why not make all controllers SPDT by default.

Surely most mult-zone controllers (i.e. one zone for HW and one or more zones for CH) have all their switches as SPDT, no?

I definitely need to have a CH zone with an SPDT switch on my main controller (which will have two CH zones and one HW zone).

Michael

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J

Perhaps I am wrong. Mine is SPST - the switch in the motorised valve takes care of other switching:

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Y

The Honeywell CM927 wireless programmable thermostat has a SPDT switched output which is independent of the receiver unit's mains supply. The Honeywell progstat itself is my favourite device for being pretty flexible and easy to install and program for the user to use.

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A

The Honeywell ST699 which was fitted here before I replaced it with my own computer control had a pair of SPDT volt-free contacts. They were labelled 240V 3A. When you followed the tracks back to the relays on the board, you find the relays were rated 120V 1A (or might have been 2A, I can't recall now), but not 240V at all.

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S

Not a problem if you find a unit that you like which has a hard wired live - just use the switched live to drive a relay with a mains rated coil.

Use one myself in a home brew lashup to release the three way valve when the CH goes off at night.

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D

The Honeywell I had before fitting a new system had a link from mains to the two SPDT relays - you could remove it if you wanted to switch say 24 volts.

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D

Probably couldn't be bothered testing them at 240 volt. Or simply didn't mark them as such. Many US firms are a bit insular. ;-)

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M

The Salus RT500RF is available on ebay at a very reasonable price (about

40 quid delivered). That has a spdt contact which comes with a removable link to allow volt-free operation. It's quite a nice little unit but a bit basic in operation (if that matters). It works in the 800MHz band, so no interference problems for most people with computers/networks.
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