Heating programmer replacement

[Swiss Tony]

About to replace an ancient switchmaster 900 electro mechanical programmer with a horstmann H21 which was recommended here

old system had a surface mount wire which plugged into a standard 13amp socket below it.

Looking at the install guide on the manufacturers website,

this links to a pdf) it says I have to link "L, 2 and 5 terminals" if I am using a mains voltage system. With-out sounding stupid, as its plugged into a socket just below, its going to be a mains voltage system rather than a "extra low voltage system"

One more question, how do I tell if I have a fully pumped or gravity link system???

Cheers Swiss Tony

[Roger Mills]

Yes, it's going to be mains.

If it's a gravity HW system there will be flow and return pipes from the boiler to the indirect coil in the HW cylinder which *don't* include the pump. The pump will only run when the central heating is on. Also it's unlikely that there will be any motorised valves, or a thermostat (other than one in any immersion heater you may have) on the HW cylinder.

If it's fully pumped, water for both HW and CH will go through the pump, and there will be one or more motorised valves and hopefully a cylinder stat in addition to a room stat.

So it's time to trace all the pipework!

[Michael Chare]

Typically the boiler will have 4 pipes going to it. Two large 28mm pipes for gravity H/W, two smaller 22mm pipes for C/H

I think all wall mounted boilers are fully pumped.

If you have a gravity system I suggest that you consider a Danfoss TP9 programmer/programmable thermostat.

This is a twin channel programmer which lets you vary the room temperature throughout the day. If desired, you can have a different program at weekends.

[Swiss Tony]

Right, I have knocked together an image which shows my airing cupboard which is where all the gubbins are.

at the top, go into the loft. There is a thermostat on the tank about 1/3rd of the way up from the bottom There is 1 thermostat downstairs on the living room wall. There are only 2 pipes to the boiler (myson velaire 17yrs old) both are 22mm (99% sure without measuring them) Red lines are wires the rest is pipework.

[Andy Burns]

have a model which does a similar task, just fitted one to replace existing programmer, very happy with it so far, since I don't have any 3-port valve I had to use a relay to combine the C/H and H/W demand to the boiler, while *not* combining the H/W demand to the C/H pump.

[Michael Chare]

You have a fully pumped system - not a gravity one.

Likely the wiring is either 'W' plan if you have a diverter valve or 'Y' plan if you have a mid position valve.

You can get the wiring diagrams from the Honeywell web site, other manufacturers have similar schemes.

I think that you have missed out a pipe. The H/W tank should have 4 pipes, 2 for the primary coil connected to the central hearting system, one for the water coming in at the bottom from the C/W tank and one for the hot water going out to the taps.

[Swiss Tony]

Thanks Michael On the Horstmann website is a conversion chart from practically every programmer so i will follow their instructions. Just wanted to be sure what kind of system I had. The drawing was very basic and I may have missed a pipe as I can only see about an 2 inch gap on the left hand side of the tank. Now waiting on parcel farce to deliver my new programmer. Thanks all

Swiss Tony aka Gareth

[Roger Mills]

As Michael says, it's definitely fully pumped and is either W-Plan or Y-Plan.

You can see schematics for the plans at

uses a diverter valve which gives either HW or CH but not both at the same time. This usually has HW priority - so you can't have any heating while the HW is being heated. This probably wouldn't be a problem with a fast recovery cylinder - but I doubt whether you've got one of those.

Y-Plan uses a mid-position valve which gives HW or CH or both at the same time. If you currently have a diverter valve it would be worth replacing it with a mid-position valve and converting to Y-Plan.

[Swiss Tony]

Got my new programmer today. Its now fitted and is working 100% Mega happy as I am warm again. Thanks for all the help guys Swiss Tony aka Gareth