Roomstats

diditrain · Jan 3, 2010

I have a combi boiler at home without a roomstat connected, therefore i would like to fit one. At the moment it just has an integrated timeclock.

The wiring diagram states that a timeclock and roomstat are optional but all external controls must be voltage free.
I have checked for voltage when the boiler is not turned on for heating and i do not read a voltage. When i fire up the boiler i record 12 volts.

When i purchase a roomstat do i need to specify a voltage free one and is there anything else that i may need to consider?
Thank`s in advance for any replies
http://i777.photobucket.com/albums/yy57/screamingskull/untitled.jpg

Guest123 · Jan 3, 2010

Hi.

Yes you will need a volt free stat as some stats will only work on 230V and it sounds like you have 12V control voltage there.

Spudnik · Jan 3, 2010

Drayton RTS8 Combi-Stat FTW.

diditrain · Jan 3, 2010

Thank`s guy`s.
Is there a good reason why some boilers have this voltage free type of set up? Just curious

coatek · Jan 5, 2010

There are many different types of room stats and I suppose the boiler designers are giving the user the opportunity to use any they choose.(and they are safer) The volt free requirement is mainly stating that the room stat is a simple relay/switch type output. Some room stats are driven by 230V live in and the output goes 'live' when the room stat demands heat. Some nead live and neutral with a third switched output. Choose a room stat that has a AAA or AA battery in it with two connections coming out. These will be your relay normally open switch contacts. Don't introduce live in any part of the wiring.
You can easily check, with the room stat not connected to anything, put the battery in and program it as per manual. Set required temp to below indicated temp, measure resistance (by whatever means you are comfortable with) resistance should be very high ie: open circuit.
Set temp to above indicated, wait a while, should be a click, resistance should now be vey low, near 0 ohms ie: short crcuit.
Sorry if I'm treating you like a dummy but you have asked a question.

diditrain · Jan 6, 2010

No offense taken coatek, very good explanation.
Thank`s very much

iansparks · Jan 6, 2010

Just in case youve overlooked the need to repace the TRV on the radiator (room where the stat is going)

If not both stats will compete!!

diditrain · Jan 6, 2010

I have another query if you would be so kind.
When you install a roomstat, does this overide the stat on the boiler, if you understand what i mean.?

Just say that you had your boiler with no roomstat connected and you had the boiler temperature control set to 60 degrees. You then install a roomstat and set the roomstat temperature to say 25 degrees for instance. Does your boiler control setting become redundant.

Pat Tester · Jan 6, 2010

I think you will find the 60 refers to the water temp, the 25 on the room stat is the air temp, the water has to heated to a higher temp than the air or there would be no net temp gain in the air, turning the water up to high will burn off your skin, so be carefull.

coatek · Jan 6, 2010

I know what your saying! Basically, the room stat is wired in series with the boiler stat (forgetting about the hot water for now), so the boiler will only come on (for cental heating control) when both stats are closed. Whichever stat acheives its set temp, the boiler will turn off. (The room stat must be wired correctly in boiler).
So if it's really cold like now, you will find the boiler starts up and the room temp starts to increase but the boiler will probably hit 60 before the room hits 22 so it will turn off for a short while, the room stat will still be demanding heat but the boiler will be saying No can do. After maybe only a few minutes the boiler stat will say Ok, temp now below 60 so turn on again assuming the room stat is still demanding heat. If we lived in a perfect world, and you didn't have a room stat, the room would get to 60. Not nice, hence a room stat for more control.
Hope this helps further in your understanding. Just be thankfull you don't have the S or Y plan conventional boiler system with zone valves and header tanks!

diditrain · Jan 7, 2010

It sort of answers my question. So is there an optimum setting for the boiler to be on then if i have a roomstat in place?
I presume that it needs to be above the roomstat temperature settingl

coatek · Jan 7, 2010

Optimum setting? Probably no more than 70 in winter and 60 in summer. But by all means try 60 first and see if your rooms get up to required temp which is of course your choice. I set my rooms to 20 and when the wife moans, take it to 21.
Imagine if you set he boiler to 20, the rads would only get to 20, and due to heat loss in the room, it wouldn't get up to any sort of noticable temperature. Rads need to be hot, hot, hot (Not scalding hot).
Underfloor heating is better as you have a massive radiator covering the whole floor, this can be set to about 30. Much more efficient!

offshorealex · Jan 8, 2010

in simple terms, the boiler temp which should be set at approx 60 deg contriols the flames on the boiler, the roomstat controls the pump. The flames will only come on if the pump is running and able to take the heat away from the boiler. The pump can run though without the flames being on. For example, if the room is at 15 deg, and the roomstat is calling for 20, but the boiler has reached it,s set temp of 60 deg, then the pump will continue to circulate water round the system until it reaches the set temp of 20 deg. While this is happening the load of the rads will drop the water temp, and thee boiler will kick in and heat the water temp back up to 60 deg and so on until the room reaches it's set temp of 20 deg, at which point boiler and pump will knock off.

coatek said:
Underfloor heating is better as you have a massive radiator covering the whole floor, this can be set to about 30. Much more efficient!

Underfloor heating operates at an optimal 40 degrees, and the most efficient way to achieve this is with a ground or air source heat pump. (300% efficient!!)