which thermostat?

[william.dossett]

Hi,

I've been trying to work this out for over a week now... plumbing/heating engineer hasn't been much help as I think this is a bit complicated for him and I need to have a wireless room unit and he hasn't had much experience with that.

I have a boiler with a 3 port midpostion honeywell valve. This feeds the HW cylinder and CH. I would like one wireless thermostat that combines timer function for when the boiler is turned on and also a thermostat that controls the 3 port valve throwing into the heat position. I already have a stat on the cylinder that will be wired into the valve for the HW control.

Can anyone tell me which, honeywell preferrably, product will do this? I bought a wireless thermostat, but it only has one on off function... well, actually, it appears on the diagram as a SPDT arangement, but I need two seperate relays. One to tell the boiler when it can be fired up, via the timer function, and the other to contol the 3 port valve to tell it when to open port a for the CH.

Hope this makes sesne, I'm more of an electrician than plumber, but if I could get the right equipment, then my heat engineer will hook it up.

Then again, If I'm completely wrong about how I'm trying to go about this, please let me know.

Thanks for your help.

[Roger Mills]

It makes no sense whatsoever.

Assuming you you have a conventional Y-Plan system (which is what you would usually have with a 3-port mid-position valve) you only need *one* room stat, plus a cylinder stat. The necessary control logic is built into the valve actuator. Depending on what is demanded by the programmer and stats (nothing, HW-only, CH-only, both together) the valve automatically moves to the appropriate position *and* turns on the boiler and pump when needed. [That's not *totally* accurate - but is a good enough explanation for most purposes. See Y-Plan in

for how it needs to be wired up].

[william.dossett]

I've been looking at the Y-Plan for several days... and as far as I can tell, there is a separate timer function that controls when the boiler can be turned on.. this is not part of the programming unit. That is why I want one programming unit that does both timer and room stat functions. You need two separate on off relays controlled by the room control unit if you want it to control the time and the temperature don't you? Surely you need two on off functions... one which is controlled by time, the other is controlled by temperature. Both must be on for hot water to enter the CH. I could leave my ancient timer where it is and just have the new honeywell relay unit contol the valve, allowing hot water into the CH system when the thermostat in the unit requests it... but if it's saturday and I'm downstairs and its cold, the thermostat will say come on.. but the old timer up in my boiler area which is not intelligent, won't allow the boiler to come on. There is a timer function in my honeywell unit that is very sophisticated, allowing full programming... if I link that to the boiler, it will turn the boiler on... but with only one relay in the contol box it talks to, what will move the valve to CH?

Sorry, I'm really having trouble understanding this as we used to have separate HW and CH boilers and now it's all being done by one, I'm having trouble understanding the logic. The Y plan obviously works, but it requires two units, a timer and a thermostat. As I am doing this wireless, I want a wireless contol unit that does both timer and thermostat and talks to a box with two relays in it one controlling the boiler and one controlling the valve... don't I?

[Roger Mills]

What gives you that idea?

No.

This is normally a programmer for time control and a thermostat for temperature control - wired in series. However, these functions can be conbined in a programmable thermostat - see later.

NO!!

The whole point of modern control systems - be they Y-Plan or S-Plan - is that they provide what is called 'boiler interlock'.

This means that the boiler *only* fires when either or both CH and HW are calling for heat - and *never* fires just to keep itself hot just for the hell of it when there's no external demand. So you *don't* need any separate control for the boiler - it is controlled by the stats and valves.

If you study the Y-Plan wiring diagram which I referenced earlier, you will see that when there is a HW demand (on at programmer and cylinder stat) the boiler and pump are switched by the cylinder stat. Whenever the HW demand is satisfied, they are switched by the orange wire from the mid-position valve which - in turn - is controlled by the programmer and room stat. [When there is a CH demand, the programmer and room stat tell the valve to move either to the mid position or to the CH position, as appropriate. When it does so, a switch inside the actuator closes, supplying mains to the orange wire. In effect, this is the relay which switches the boiler on - so you don't need any external relays.]

A conventional timer (like the one shown in the diagram) allows independent switching of HW and CH, but not many allow them to be programmed at completely different times. However, there is a very simple solution to this - which will suit your purposes. That is to replace the room stat with a programmable thermostat. You then set CH to 'constant' on the main programmer, and use the programmable stat to control both when the heating comes on and off *and* the required temperature for each 'on' period. [Most programmable stats provide for several on-off events per 24-hour period, with the ability to specify different temperatures at different times of the day if you so wish.] Programmable stats come in 2 varieties - hard-wired and wireless. Wireless stats enable the bit with the temperature sensor and display to be separate from the bit which does the actual switching - without needing to run wires betwee them - which sounds like what you want.

[william.dossett]

Roger, thanks a great deal for your help.. I know I'm beging a bit dim about this and am highly embarrassed by it as normally I am very good with electrics of all kinds.

omit the timer section of it ? And the timer on the room thermo-stat will take it's place? Does this mean that the HW would be on constant, or does the timer on the room thermostat also take care of that? If the H/W was on constant, it wouldn't concern me too much I guess as I have a dual coil cylinder and hope to heat that with solar in the near futher, so that means the cylinder stat would seldom ask for heat.

Thanks again for your help on this.

Bill

Roger Mills wrote:

[william.dossett]

further to that... if I don't have the timer... there would seeem to be no electrical feed at all to the room stat on terminal 4 or to the cylinder stat on terminal 6.... I assume the output from the timer labeled HTG and HW On is a Live? though that looks as the the thermostat could connect throw 2 and 1 together creating a short between live and neutral... or does that never happen? just looks a little scary to me.

[e-mail] wrote:

[Roger Mills]

No, I didn't suggest omitting the timer. One subtle feature of Y-Plan is that you must have a live HW-off signal when HW is not required (pin 7) and this is what powers the system in CH-only mode. I suggest you do what I do, which is this:

I have a conventional - albeit electronic - programmer/timer which provides Off/Timed/Constant options for HW and CH separately - but only has one clock so that if CH and HW are both set to Timed, they come on at the *same* time as each other. I also have a programmable thermostat for the CH. Now, here's the clever bit . . The main programmer has HW set to timed and CH set to Constant. So the timing of the HW is controlled by the main programmer. But the timing of the CH - which can be at *different* times from the HW is controlled by the programmable room stat.

[Roger Mills]

Yes indeed, you would need to feed these directly from the FCU. But see my other post about the need for a HW-off signal.

Yes.

No, you're mis-reading the diagram! The type of stat shown is an old mechanical type which has a built-in accelerator heater to improve its responsiveness. The neutral connection is the return for the heater - and live never gets directly connected to it.

Electronic/programmable stats do not have accelerator heaters - and don't need a neutral connection.

[william.dossett]

Ok, think I got it now... I just need to get a timer system that controls hot water and central heating, but allows seperate settings for both and set the CH on all the time, so that in the room stat will actually control this, but the HW will be controlled by the timer and then I can follow the diagram exactly.

Thanks hugely for your help on this, I was floundering I must admit... can you possibly suggest a timer that would do what I need, would prefer an electronic one. I already have the RF Room system, both ends of it, it's Honeywell... I was afraid I had bought the wrong thing and thankfully I haven't!

Thanks again

Roger Mills wrote:

[Roger Mills]

Precisely.

Just one caveat! The diagram shows the boiler and pump lives directly connected to each other - and assumes that the boiler has no pump over-run feature. If the boiler *does* need pump over-run (to carry away the residual heat after it stops firing), you'll need a slight mod to the diagram, as follows:

The boiler will have additional terminals - Permanent live as well as (switched) live, and pump output. Permanent Live needs to be connected to pin 1 of the wiring centre, and Switched Live (the one which actually tells the boiler to fire) needs to be connected to pin 8 (as per the diagram except that it's just shown as L). The pump needs to be connected to the boiler's pump output terminals rather than to pin 8.

Mine's more or less like this

(but an earlier version). It's worked ok for the last 15 years or so.