Fitting new programmable CH room thermostat

[Brownie]

I did Google prior to this post but was a bit confused by a variety of answers.

I have a gravity fed CH system with standard boiler with a programmable timer. Temperature for heating is set by those commonly found Honeywell mechanical large-dial room thermostats. I work it so the CH comes on and off when I want and when it is on, the room thermostat controls the temperature. i.e. boiler runs until room temperature matches set temperature on thermostat. If CH is not in an 'on' program, room thermostat does nothing.

It all works fine except it would be great to have more temperature control so the heating could be on low overnight or when the house was unoccupied and rise just before waking up or coming home. I am hoping that replacement of the mechanical thermostst with something like the electronically programmable Drayton Digistat 2 or 3 is the answer?

Is this just a simple swop over for a novice? I am seeing references to 2 or 3 wire systems and wires having various controls of the boiler. Surely the mechanical system I have now simply sends to the boiler 'below temperature = on (makes circuit)', 'at or above temperature=off (breaks circuit)'? Doesn't the end result of the pragrammable version just do the same?

I was thinking it would be simply an easy case of swopping over the boxes?

[Richard Savage]

When I was asking similar questions the Sunvic TLX 6501 seemed to come out top. I can email the instructions to you if you wish.

HTH

Richard

[to]

I have a Danfoss TP5, Screwfix I think. Works perfectly over the last year, and what a blessing it is.

I pondered for 2 years on changing mine, as the old mechanical unit was 5 wires, and the new electronic 3 wires, and I couldnt find out how to wire it. I got a mate to do it that is knowledgable in these things, and it took about 5 minutes to change.

toad

[Andy Hall]

In the case of bimetal strip thermostats, some have an accelerator heater to improve operational behaviour and so a three wire connection is used - live from the programmer, neutral and live out to (normally) a/the motorised valve.

You can see the standard hookups at

where a simple electromechanical thermostat is used, only two wires are used and connected.

There is a wiring scheme used with some types of motorised valve where three wires are used, but consist of the switch common (coming from the programmer) and then the outputs are normally on and normally off, i.e. live out on one when heat is required (opens valve etc.) and the other giving live out when it is not (closes valve). These are not very common, because most installations use spring return motorised valves needing a live to move them one way and the spring moves them the other.

If you have a two wire hookup, then it is fairly obvious how to replace. If it's three wires, then it's important to check what the wiring arrangement is.

From the perspective of an electronic thermostat or room programmer, the electronics has to be powered. Some require a permanent live and neutral as well as the switch contacts. Others like the Digistat 3 that you mentioned, have batteries inside to power the electronics and can thus work on a two wire arrangement.

To have night set back and differing temperatures during the day, you could use the Digistat 3 or 3i - the latter has an optimised start to delay starting the heating on warmer days.

Others to look at are the Honeywell CM67 series and Danfoss Randall TP75 series (now superceded).

Another approach is to use wireless (RF) models of these. In this case, you can have a receiver close to the boiler or wiring centre, where it is usually easy to pick up the wiring connections needed, and then the room unit is battery powered.

.andy

To email, substitute .nospam with .gl

[Michael Chare]

I assume that your central heating is pumped.

Do you have a 'gravity' hot water system, typically with 1" diameter pipes going from a floor standing boiler to hot water tank, or a fully pumped system typically with a wall mounted boiler?

In the former case you might want to consider the Danfoss TP9 which would replace you existing programmer and thermostat. Last winter I could not find any exact competitors. The unit comes in two parts, programmer and room temperature sensor. I bought one, and it works well in this situation.

In the latter case you can probably consider replacing the existing room thermostat with a programmable uint such as the Honeywell CM67. You would leave your present programmer CH on all the time, the CM67 would call for heating according to your temperature profile. There are a number of competitors.

In either case you can download the installation instructions from the manufacturers web sites. If you can follow the instructions, it is not a difficult DIY job, if not, you may be better finding a suitably qualified person to do the installation for you. The modern electronic progammable thermostats typically provide a more precise temperature control than the old mechanical ones.

Michael Chare

[corvette]

Richard,

I have the Sunvic but have lost instructions. Would be grateful if yo could e.mail me a copy.

Thank

-- corvette