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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> One heating circuit - two thermostats(?) https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1433004768 Message started by aeg on May 30^th, 2015, 5:52pm

Title: One heating circuit - two thermostats(?)
Post by aeg on May 30^th, 2015, 5:52pm

I'm considering the possibility of installing two room thermostats to control the same heating circuit and switching between them with a two way switch...

Problem: the programmable room thermostat I already have controls the heating with the same set of on/off times every day of the week whereas, occasionally, when I may be out for much of the day, I would prefer to have a different set of times.

Idea: if I installed another programmable thermostat and was able to switch between the two, I could select whichever set of times was most appropriate.

Proposed plan...
Connect live from programmer to common terminal of two way switch.
Connect from S1 of 2-way switch to thermostat 1
Connect from S2 of 2-way switch to thermostat 2
Connect from outputs of both thermostats to zone valve via wiring centre.

Although I can see no reason why it shouldn't work, I would appreciate the opinions of experts.

Title: Re: One heating circuit - two thermostats(?)
Post by thescruff on May 30^th, 2015, 11:05pm

If you used a two channel programmer you could use T4 HTG on, and T3 DHW on. you'll need a relay where t3 and t4 become common

Title: Re: One heating circuit - two thermostats(?)
Post by aeg on May 31^st, 2015, 7:04am

thescruff wrote on May 30^th, 2015, 11:05pm:

If you used a two channel programmer you could use T4 HTG on, and T3 DHW on. you'll need a relay where t3 and t4 become common

Thanks for this suggestion.
I already have a two channel programmer that is set up as normally intended for heating and hot water circuits, and it is to T4 HTG that I was intending to connect the common terminal of the two way switch.

Perhaps I don't fully understand what you are suggesting, but I don't see how your method would control different temperature settings at different times of the day.

Further clarification of what I am seeking...
Example:
Normal daily setting (someone at home all day): Thermostat 1 in control...
Raises temp. setting to 20^oC at 06:00 and drops back to 12^oC at 22:00

Occasional daily setting (house empty for parts of the day): Thermostat 2 in control...
Raises temp. setting to 20^oC at 06:00 and drops back to 15^oC at 08:00, then
Raises temp. setting to 20^oC at 12:00 and drops back to 15^oC at 13:00, then
Raises temp. setting to 20^oC at 17:00 and drops back to 12^oC at 22:00.

Title: Re: One heating circuit - two thermostats(?)
Post by woodsmith on May 31^st, 2015, 8:16am

I can't see why what you are proposing won't work technically, the only problem will be remembering to switch it over when you go out. Have you looked at smart thermoststs, like the Nest, not cheap but it may solve your problem in a more elegant fashion?

Title: Re: One heating circuit - two thermostats(?)
Post by thescruff on May 31^st, 2015, 9:14am

Ok first I assumed you had a combi.

What motorized valves do you have.

if you had 2 channels for the heating you could program them different and use the program that best suits you daily need.

https://www.tlc-direct.co.uk/Products/HOH37.html

Saying that a programmable stat should operate independently of any programmers.

Perhaps some information on makes/model of what youhave at present.

Title: Re: One heating circuit - two thermostats(?)
Post by thescruff on May 31^st, 2015, 10:36am

Thinking about it, if you have 2 programmable room stats you could use a simple changeover switch for the call wires.

http://www.modellers-mecca.co.uk/electronic-components/1226-280-11-spdt-miniature-changeover-switch-centre-off-3-postion.html

Title: Re: One heating circuit - two thermostats(?)
Post by aeg on May 31^st, 2015, 3:35pm

Thanks to woodsmith for your contribution; the Nest thermostat appears to be something that would appeal to me but not necessarily other members of the household!

Thanks also to thescruff for your contribution; I couldn't open the link you provided in your latest post but what you've suggested appears to be very similar to the idea I've proposed, except that my two way switch solution (also spdt) has no off position.

The programmer I have at the moment is very similar to the Drayton LP241 but with British Gas branding on it.
I have two 2-port valves, one for hot water and one for heating.

The idea I've proposed came to mind when I was about to replace a Landis & Gyr Chronogyr Rev10 thermostat with an unbranded touchscreen thermostat (eBay). The Landis & Gyr still works OK but has a LCD display that is difficult to read and adjust in low light conditions, whereas the touchscreen stat has a backlight and a more easily readable display. It then occurred to me that keeping both thermostats would provide the advantages I've mentioned previously.

From the advice I've received so far, it appears that my plan should work, so I will probably go ahead with this and fit the two thermostats on either side of a two way switch. I shall rotate the switch 90^o from normal orientation so that the rocker position will indicate which thermostat is active.

Title: Re: One heating circuit - two thermostats(?)
Post by thescruff on May 31^st, 2015, 7:08pm

would still like to know what room stats you have/intend to use.

What you have to remember is you'll be powering the brown (MV) from 2 different sources, and can be highly dangerous to anyone working on the heating in the future unless you can include a common isolator.

Title: Re: One heating circuit - two thermostats(?)
Post by Lectrician on Jun 1^st, 2015, 6:18am

Live from programmer into both stats. Output from one stat to L1 of switch, output from the other to L2. COM to the brown of zone valve.

It will do what you want, but not very conventional.

There will only ever be one stat feeding the zone valve.

Title: Re: One heating circuit - two thermostats(?)
Post by aeg on Jun 1^st, 2015, 9:27am

thescruff wrote on May 31^st, 2015, 7:08pm:

One of the room stats will be either...
http://www.ebay.co.uk/itm/Digital-Under-floor-Heating-Programmable-Thermostat-Room-Temperature-Controller-/151601600371?pt=LH_DefaultDomain_3&hash=item234c28d773
or...
http://www.ebay.co.uk/itm/Digital-Underfloor-Heating-Programmable-Thermostat-Room-Temperature-Controller-/251347129284?pt=LH_DefaultDomain_3&var=&hash=item3a8574d3c4

The other stat may be the same as above but, initially, I'll probably use the existing Landis & Gyr Chronogyr REV10 (an old (mid 90s) digital thermostat that still works) to test the idea and its usefulness. http://www.forum-chauffage.com/images/thermostat-landis-gyr.jpg
Interestingly, the Landis & Gyr stat partially does what I'm trying to achieve as you'll see from the image but I haven't been able to find a modern stat that does the same thing, which is the reason for my proposed solution.

The thermostats will be located near each other with common connections made locally so the MV will only see one source. I'm aware that this arrangement will be unusual so I shall be very careful to avoid leaving nasty surprises for others who may work on the system!

Lectrician wrote on Jun 1^st, 2015, 6:18am:

This appears similar to my proposal but with the two way switch after the stats instead of before them. I'd be interested to know why you think that would be a better arrangement. It's good to know that you think it will work OK so I just need to try it.

Although this arrangement is not very conventional, it is innovational and could become conventional if enough others needed to achieve the same result. A better solution would be for thermostat manufacturers to include the features I'm seeking in their products, similar to those incorporated by Landis & Gyr in the mid 90s (see above). There may be some on the market of course, but I don't know of an easy way to find them.

Thanks for your help.

Title: Re: One heating circuit - two thermostats(?)
Post by Lectrician on Jun 1^st, 2015, 12:32pm

wrote on Jun 1^st, 2015, 9:27am:

Lectrician wrote on Jun 1^st, 2015, 6:18am:

You are choosing programmable stats, and placing the two way switch before them would kill power to them, meaning you could not make adjustments to them.

Not all work this way, most have a live and neutral supply to operate, and then volt free contacts to switch the heating load. Some do not though (the eBay one) so I suggested after the stat.

Title: Re: One heating circuit - two thermostats(?)
Post by thescruff on Jun 1^st, 2015, 6:27pm

The two way switch would have to be after the stat as you are using the switching terminals to power the MV.

What you will also need is a permanent L&N to each stat, ideally from the same fused isolator.

Title: Re: One heating circuit - two thermostats(?)
Post by aeg on Jun 2^nd, 2015, 8:37am

Lectrician wrote on Jun 1^st, 2015, 12:32pm:

wrote on Jun 1^st, 2015, 9:27am:

The Landis & Gyr stat is battery powered so the programming of that one will not be affected by the proposed switch, but I take your point about mains powered stats that don't have permanent mains power so I shall fit the switch after the stats as you suggest. Thank you for pointing that out.

[quote author=thescruff link=1433004768/11#11 date=1433179641]
What you will also need is a permanent L&N to each stat, ideally from the same fused isolator.

I considered having a permanent L&N supply but don't think I can. There are only 4 connections on the eBay stat: N and N1 that are linked, L and L1, where L1 is the switched live. The supply to L must surely come from the programmer, otherwise the stat may try to operate the MV regardless of whether the programmer was in heating mode.

Title: Re: One heating circuit - two thermostats(?)
Post by Lectrician on Jun 2^nd, 2015, 9:52am

With programmable stats, it's usual to either bypass the time control, or turn it on 24/7, letting your stats take control.

You should supply all equipment from the same supply/fcu.

The eBay stats would need three cores plus earth.

Title: Re: One heating circuit - two thermostats(?)
Post by aeg on Jun 4^th, 2015, 8:34am

Lectrician wrote on Jun 2^nd, 2015, 9:52am:

Yes, I do sometimes have the programmer on 24/7 with the stat handling all of the timing (such as when we are away during the winter) but most of the time I use a combination of the two for additional flexibility.

Well, it's all connected and working as intended! I already had a three core+earth supply from the same fuse point as the boiler and controls so it was really just a case of fitting the components and making the connections.
The eBay stat can only be programmed whilst the programmer heating channel is on, of course, but having the switch after the stats was good advice because it does mean that I can still adjust the programming, regardless of which stat is in control. Thanks again for your help.