New Room thermostat/programmer

[wrighty208]

Hi, I have a standard open-vented system with an old back boiler behind a fireplace downstairs feeding 7 radiators. I have a programmer in an airing cupboard upstairs along with a pump and 3-way motorised valve (for HW, CH or HW&CH). I then have a cable (1.5mm2 Twin and Earth) going downstairs to a room thermostat in the hallway.

I believe this room thermostat isn't working properly now as it's covered in paint (previous house owners!) and it clicks at about 14 degrees when I know it's actually about 19/20 degrees. Inside the room thermostat there are 2 wires connected.

Can anyone recommend a suitable replacement thermostat? also I'm thinking of changing the programmer as I'd like one that can set the HW to come on for 1 hour, then the CH automatically via timer (as the Rads get hotter when the HW is off). My current programmer has settings for HW and CH - On, Once, Twice and Off but the timer switches both together.

I would quite like the controls downstairs but it may not be possible as I only have 1 twin and earth cable going downstairs (and don't really want to lift floor boards to run another!) - but it's not the end of the world.

I may just end up replacing the thermostat - would digital be better than analog?


thanks for your help!

 

[tryingnottobeafool]

i would put thermostatic valves on radiators

rip out thermostat on wall use a timer clock with pins, soooooo much easier than digital

http://www.alertelectrical.com/Gras...nel-electro-mechanical-timeswitch-GRASQM1.asp

 

[wrighty208]

Hi, thanks for your reply.

I have thermostatic valves on most radiators (not in the hallway where the thermostat is) but if I take the thermostat out surely the boiler will be working all the time (whislst the time permits)?

 

[tryingnottobeafool]

no, as the room heats up to temp, the thermostatic valve will close switching off heating

you may need one radiator without a thermostatic valve, so say toilet upstairs if possible

set bedroom to say 2 on radiator, so you dont sweat in bed

4 in kitchen

3 to 4 living room

you should maybe consider oil heating

 

[wrighty208]

surely the TRV's will shut off the radiator but the heating circuit will continue to be heated up?

Sorry to be doubting - just trying to understand!

 

[tryingnottobeafool]

yes the pipes will still heat up but you wont get much heat loss really as the pipes will still be warm from heating the rads up in the first place


the trvs do away with the thermostat on wall........ plus kids playing with it , etc

I prefer heating clocks with the push out pins as easy to work and wife proof !! ok man proof !!


the thing with a thermosta on a wall is you could be heating a room that doesnt need it, a bedroom that doesnt need it ( if it hasnt got a trv ) because the hall is cold

 

[tryingnottobeafool]

you could possibly put timer on wall where thermostat is currently situated depending on what wires are there

its not much current / amps required and its really just an on/off switch

 

[D_Hailsham]

the trvs do away with the thermostat on wall

This is only true if you have a boiler which can react to changes in the pressure in the pipes or to the changes in flow and return temperature.
In all other cases the boiler will continue to run until it reaches the temperature set on the boiler thermostat. That is why the Domestic Heating Compliance Guide and the Good Practice Guides recommend a boiler interlock, which requires a wall mounted thermostat (and a cylinder thermostat for a non-combi boiler).

I recently updated the thermostat in my son's house. They have a similar installation to yours; back boiler, pump etc in the airing cupboard, wall stat downstairs fed by a twin + earth. The stat was wired so the earth was acting as the switched live Like you, it was not feasible to pull up the floor to run a four-core cable to the old thermostat and locating the stat anywhere else would have been just as complicated.

Our solution was a Honeywell CM927 wireless thermostat. The receiver is located in the airing cupboard near the pump etc and the control unit/transmitter downstairs. If you don't need a 7 day programmer, the CM921 is the 24 hour version.)

If you go down this route, you will need to retain your old time switch to control the Hot Water times, which can be completely different to the CH times. Further info can be found in the Honeywell Application Guides.

 

[wrighty208]

Thanks for your replies above.

I'm starting to think I'm going to just simply replace the thermostat with a new one and keep the existing timer.

Can someone to me the difference between the 2 wire and 3 wire thermostat (and before you say it I know it's an extra wire!!!)

I only have the 2 wires and have heard something along the lines of "without a neutral the anticipator within the room thermostat does not work" Would I be better off with a digital thermostat with batteries in this case?

http://www.plumbworld.co.uk/honeywell-t6360b-spdt-room-761-1361

or
http://www.plumbworld.co.uk/honeywell-dt200-digital-room-761-1362

or even this(although maybe a tad overkill!)
http://www.plumbworld.co.uk/honeywell-cm907-7-day-761-17210


maybe???

 

[D_Hailsham]

Can someone to me the difference between the 2 wire and 3 wire thermostat (and before you say it I know it's an extra wire!!!)

The three wire mechanical thermostat has a smaller temperature difference between ON and OFF. This means it tends to keep the room more comfortable

I only have the 2 wires ...Would I be better off with a digital thermostat with batteries in this case?

Yes, the Honeywell DT200, if you just want to replace the stat and keep the existing timer. But it won't let you have different HW times, which is what you asked for originally

If you want separate CH and HW times, you have two choices:

1. Change your existing timer to a two channel version. Danfoss-Randall make one with separate times form CH and HW. Also fit a new DT200 thermostat.

2. Keep existing timer for the HW and install a CM900 series programmer for CH time and temperature.

This assumes that you are happy with the current thermostat position.

Personally, I would go for option 2 as I have got one and it is much easier when the timer and thermostat are in the same place.

 

[wrighty208]

Thank you so much for your help guys! I really appreciate it! I've learnt a lot!

I think I'm going to go with the CM900 series programmer for CH in place of my existing thermostat as I only have 2 wires - I guess I would have to set my original programmer CH setting to always on!

Well, I've been busy today, had all 7 radiators off 1 by 1 and taken each one into the garden to flush them out! Whilst doing this I also changed all the radiator valves and added TRV's!

All valves (both sides of all rads) are 100% open and it's very hard to tell what order the rads are on the circuit as they are all getting hot so I guess I don't need to balance?

Thanks again for all your help! I've actually enjoyed learning all this!

 

[wrighty208]

Hi guys, just another thought!

Is there a wireless programmer/thermostat unit that controls CH and HW? Reason I ask is the wire to my existing thermostat runs along a wall and looks horrible, now if I'm going to replace the thermostat anyway maybe I could go wireless and have all the controls where I want?

I'd quite like to stick with Honeywell but can't find a programmer/thermostat that controls CH and HW.

Failing that, how about just a wireless programmer/thermostat for CH and I use my existing timer for HW (as mentioned above) - which model and would it work with my system - I've seen the Honeywell CM67 RF and also the CM900/927 - not sure of the differences or if they would work with my system.

Any help would be great!

 

[D_Hailsham]

Any wireless stat will work with your system.

The CM921 and CM927 are just the updated versions of the CM61 and CM67.

I don't know of a combined CH and HW wireless programmer although Danfoss-Randall do make a wireless cylinder stat, which costs about £100

Keep the old timer for HW, it's the cheapest option and how often will you need to change the HW times?.

Instead of leaving the CH on continuous, disconnect the CH connection.

All the info on wiring etc can be found here: Honeywell Application Guides. Either No 109 or 110, depending on whether you have one 3-port or two 2-port motorized valves.

There is an error in the wiring on 109. The schematic (above the red "Please note etc" box) shows a connection between the Grey, HW off and Cylinder stat 2. This is correct. The wiring diagram (left of the warning) does not show this. You will need to put a wire from HW OFF on the old timer to terminal 7 of the connection strip.

Make sure you check the communication between the transmitter (wall unit)and receiver (boiler end) before fixing the receiver in place. I wired mine temporarily to a length of cable with a 3 pin plug at the other end. It's all in the CM921 instructions.

 

[wrighty208]

Thanks for your advice! You've been a great help!