Honeywell CM927 Thermostat - control pump or burner?

[gm4jjj]

I have a rather old Stelrad Ideal Mexico Gas Boiler. Gravity fed HW and pumped CH. TRV on all rads except Towel Rail. Timer is also old electromechanical model.

I would like to add a Honeywell CM927 wireless programmable thermostat to the existing boiler.

Should the CM927 unit be wired to control the circulation pump or the burner? - or both?

Any advice much appreciated.

 

[Boilerman2]

You should consider upgrading the whole control system, using a 2 port valve in the gravity cct, then you would be able to stop the boiler cycling, untill then your CM can only control the pump so not as efficient as it could be

 

[gm4jjj]

Thanks BoilerMan.

The present system uses a primatic type HWC, is that still suitable for conversion as you suggest?

 

[gm4jjj]

A few more questions, assuming that I ca use this system with the primatic.
Is what you are proposing called the C Plan?

Do I also require an automatic bypass with that system?

Will the wireless programmable stat be suitable allowing me to do away with existing dual timer?

In the C Plan arrangement will the stat still only control the CH pump?

Appreciate any help.

 

[Boilerman2]

You will still need a time clock and Cylinder stat to control the HWS, however some clever wiring using the Three wire auxillary switch in the HWS Zone valve, will allow the boiler to shut off when both hot water and Heating are satisfied

 

[gm4jjj]

I am slowly getting the picture now, thanks.

So, it looks the parts I require are a 2 port motorised valve, a tank stat and the programmable stat and some wire and junction box to connect all.
The existing timer just for HW would be kept in place.

I will get a pro to do the plumbing side, but am happier with the electrics DIY myself.

Thanks again and please let me know if I have overlooked a anything.

 

[dfaz]

please be carefull check with your installer first putting a zone valve on a gravity circuit could cut off the expansion pipe.

 

[gm4jjj]

Good point. I will need to check if the zone valve should go in flow or return to the cylinder. Found the wiring details for the C Plan now, so have a better understanding of how that works now.

 

[Agile]

David, I am sure you can get your head around your existing system if you spend a few minutes on it.

Its old and your boiler is ideal for replacement and might qualify for the scrappage when its finally introduced in Scotland.

You have to take a whole view of the system but simply the normal connection for a two wire stat would be in series with the heating output of the Ch channel of the programmer.

You might need to add a relay to ensure pump and boiler are both powered. You dont need to pump the HW although it would give faster reheat iof you did.

Tony

 

[mikely]

If you don’t want to go to the trouble of re-plumbing you can insert a boiler interlock by adding a tank thermostat and wiring as shown below. Once the tank is up to temperature, the boiler will only fire when the room thermostat demands heat. However, you will need to keep the timer so that the HW output is still on whenever heating is on and so that you have timer control of the hot water. You can use the CM927 as the room stat, but the timer CH output will have to be on whenever you require heating.
View media item 11845

 

[gm4jjj]

Its old and your boiler is ideal for replacement and might qualify for the scrappage when its finally introduced in Scotland.

Tony

Tony - thanks for that, most helpful.

I am slightly resistant to changing the boiler because it is in a cupboard downstairs( with the cylinder), and flue goes straight up through the ceiling to the roof. The piping is microbore and it is all underfloor, no underfloor access because house is built on a concrete 'raft'.

Most of the people I know that took advantage of a Scottish scheme for older folks a few years ago, got their boiler replaced (free), but they also had to get all the radiators and pipes changed at the same time and that would be a major thing in this house, ripping up floors everywhere, no thanks.

The old gas boiler is simple/reliable and costs almost nothing to maintain, a few thermocouples and a gas valve and water pump over the last 20 odd years. By comparison my mother's "modern" boiler is always on the blink. OK it might save a bit of gas, but the extra costs of maintenance would outweigh that. I'm certainly not going to pay thousands to replace this system to save a few pounds a year on gas. Hence the idea of just updating controls a bit on existing system.

Thanks to the helpful people here I have a good idea of how to proceed now.

 

[gm4jjj]

If you don’t want to go to the trouble of re-plumbing you can insert a boiler interlock by adding a tank thermostat and wiring as shown below. View media item 11845

Brilliant, that looks ideal to me. OK it doesn't stop the DHW getting too hot when the CH is on, but apart from that it allows the sort of extra control I am looking for.

I need to investigate how to fit a cylinder thermostat now. The cylinder is all covered in foam, perhaps there is a cutout or I have to make one?

 

[mikely]

Brilliant, that looks ideal to me. OK it doesn't stop the DHW getting too hot when the CH is on, but apart from that it allows the sort of extra control I am looking for.

That's right, when the heating is running the tank temperature will also depend on the boiler thermostat setting.

I need to investigate how to fit a cylinder thermostat now. The cylinder is all covered in foam, perhaps there is a cutout or I have to make one?

You need a cutout about 1/3 up from the bottom of the tank. You can get wireless tank stats if wiring is a problem, but wired is preferable.

 

[gm4jjj]

OK, that's great. I got the Honeywell wireless stat today and bought a cyl stat at B&Q as well as control centre terminal box, so today's project is now in hand, will report back.

 

[Agile]

You need a cutout about 1/3 up from the bottom of the tank. You can get wireless tank stats if wiring is a problem, but wired is preferable.

Its probably cheaper to use/bodge a standard wireless stat.

However, I dont remember seeing a dedicated wireless stat for a cylinder. Can you post a link to details of one?

Tony