quick yes/no question regarding programmable thermostats

[haf]

I have a traditional gravity fed central heating system with boiler in garage and the usual programmable timer in the utility room plus a simple room thermostat in the hall way. The timer has the standard 2 'heating on off' settings for morning and evening use.
Now - given the freezing cold winter nights i'd like to have the flexibility of having the heating come on during the night (to 10-15 degrees maybe) and then go back to normal 25 in the morning.
My brother has a combi boiler with a clever honeywell programmable thermostat that allows him to specify what temperature he wants the room to be at during day/night and and the heating comes on and off accordingly. So my question is can i replace my hallway thermostat with a clever programmable one or do i need to be changing my utility room timer box?
Thanks,

 

[stem]

It's not quite as straight forward as yes/no.

By gravity fed, I assume you mean the boiler is fed from a feed and expansion tank, not that the heating system operates by gravity.

So I'm guessing that the hot water cylinder is heated by gravity whenever the boiler is switched on and when central heating is required a pump is started.

Assuming this is the case, at the moment the pump will be switched on and off by the thermostat in the hall, if you replace it with a programmable thermostat, it will switch it on and off as required, however, the heating will still go off when the existing programmer switches off. You can sort of overcome this by setting the existing programmer to 'constant' and letting the new one take control according to its swanky new functions.

However if you have pumped central heating and gravity hot water, the hot water would also need to be on constantly for the heating to work. This is a restriction of the plumbing, so the controls alone can't be modified to overcome it.

 

[Boilerman2]

Thats a Good YES/NO answer!!

 

[haf]

stem,
that is exactly the setup i have and you explanation makes sense. i.e have main timer set to 'always on' and then let programmable thermostat do the rest.
I'm interested in your comment that i will need the hot water to also be 'always' on if i go down this road. As my timer has separate on/off/timed switches for CH and HW, then can I not set CH to 'on' and HW to 'timed' so i get best of both worlds?
appreciate your advice so thanks,

 

[stem]

My comment refers to the most common type of gravity fed system, where the hot water cylinder is heated by natural circulation.

With this type of installation, the HW 'on' from the programmer switches on the boiler, water is heated in it, and then circulates by gravity around the cylinder.

When heating is required, the CH 'on' starts only a pump that circulates the hot water around the radiators in conjunction with the thermostat. The hot water has to remain selected to 'on' to keep the boiler going.

Newer systems have motorised valves with an integral microswitch that is used to keep the boiler running if only heating is required. The issue is not what the programmer is capable of doing, but rather what the plumbing set up you have allows it to do.

On the other hand, if you can presently run the heating without the hot water being on, maybe I'm not understanding what you refer to as

"traditional gravity fed central heating system"

correctly. If you can run the heating on its own, you probably have a fully pumped system (not gravity) with motorised valve. If this is the case, you can ignore the comment about keeping the hot water on.

 

[bathjobby]

Haf, I have exactly the same set up as you and some years ago did as your last post suggests. So I have CH on the old programmer set to permanently on and the programmable thermostat cycles through the various temps I want at the various times of day. I use the existing HW function on the old dual (CH / HW) programmer to control when I fire the boiler for HW. Works a treat.

 

[mikely]

It is possible to improve the control of a conventional gravity HW/ pumped CH system by adding a tank thermostat interlock as shown below:

Gravity Interlock

• mikely
• 7 Jun 2010
• 3

In a standard gravity fed HW system, the boiler is on constantly when CH is on and the boiler cycles on its internal thermostat. With this interlock, the boiler will only fire if the tank thermostat is demanding heat or the room thermostat is demanding heat. You still need to have HW on whenever CH is required, so the programmer must still be set to gravity mode.

 

[haf]

stem,
My system is definatly a traditional gravity fed (10 year old 4 bed house) and i have a hot water tank in the airing cupboard with a motorised valve and T junction 22m pipes for routing between CH/HW/etc. I'm assuming that when my timer switch is set to CH on and HW off then the motorised valve does its thing so i get the right result.

bathjobby,
sounds like you are confirming my theory so I think i'll give it a go

Now the tricky part of which timer/thermostat I should get as I'm assuming i will need one that matches the wiring on my current one. Hopefuly one of you will say 'its a standard setup' and i can just go and get a honeywell xyz model...

really appreciate the help and I'm sure I'll save a bucket load of money as my current setup means that if the temperature in the garage gets below 5 degrees (froststat) then all the heating comes on full blast and stays on pretty much all night (hallway stat set to 25 degrees).

 

[45yearsagasman]

If you have a motorised valve like you described then you have a fully pumped system and not gravity.Its very misleading to refer to it in that way.

 

[haf]

hmm - can't say i know the difference between gravity fed or fully pumped but if you say mine is fully pumped then i won't argue.. question is 'does that matter' with regards to changing the hallway stat to a better/variable temp model..

 

[Gasman77]

yes

 

[mikely]

To answer your original question, yes you can replace your old thermostat with a clever programmable one. As has been said, just set your timer to CH permanently on and let the programmable stat control as you require. It sounds like you have a fully pumped Y plan system – not gravity fed.
A Honeywell CM907 wired programmable thermostat will be suitable to replace your existing thermostat. What is the make/ model of your current thermostat and what wires do you have connected to it?

From what you say, it sounds like the frost stat overrides the timer control and brings the house up to the temperature set by the room stat. If so then adding the programmable stat will improve the situation and stop you overheating at night.

Is the frost stat external to the boiler? If so, it should be interlocked with a pipe stat so that the system just heats long enough to warm the water in the heating pipes.

 

[haf]

mikely,
thanks for actually answering my query in a clear and concise manner as I was struggling to understand my position given all the other comments.
I will checkout my current thermostat model & wiring and also take a look at the CM907.

You are correct in that I have a frost-stat in the garage next to the boiler but no pipestat to facilitate switching off the heating after a short period. A pipe-stat was suggested to me to eliminate the overheating at night but as i'd quite like the heating on during the cold nights (at a lower temp than normal) so replacing the thermostat seems a better option.

I guess i could add the pipestat as well but can't see the point as the hallway thermostat would do an equivalent job.

 

[mikely]

When the heating has been brought on by the frost stat, is it actually controlled by the room thermostat? If not then obviously changing the room thermostat will have no effect on the frost mode operation.

The frost protection should be independent of the room thermostat because it may still be warm enough in the house, but the pipes in the garage could be freezing. Therefore the solution is to add a pipe thermostat interlock with the frost stat and replace the room stat to give you the flexibility of temperature control.

 

[haf]

mikely,
thanks again and you are probably right in that i need both the pipestat and a better roomstat.
Your comment about the frost-stat is interesting in that i just assumed that the reason the rads were red hot all night was due to the room stat being set to 25 degrees and my hallway struggling to get to that temp when outside is below zero. As you suggest, it could well be all down to teh fact that the garage is below freezing all night so the frost stat keeps the boiler going.

Thanks to everyone for the excellent advice and I will go for both options.