2 boilers, 1 controller

[Jon Connell]

My new house has two boilers heating different parts of the house, but lousy programmers inconveniently located next to the boilers and no thermostats. I'd like to be able to control when each of the boilers can come on as well as have thermostats in each part of the house to regulate the temperature in those two zones.

I *think* what I need is a 2 zone programmer like this:

Does that sound plausible? The wiring diagrams don't make it clear that this will control two boilers; they're using different motorized valves for the two zones.

Jon

[Dave Liquorice]

As two independant systems or are the boilers connected on the wet side as well?

More or less but as you are putting in thermostats get programmable ones, the temperature *and* time control then moves to them. So you don't really require an on/off programmer as well but one can be useful to force the system off. Programmable stats have "holiday" modes but they can be a bit awkward to set up compared to adjusting a programmer to OFF.

Assuming the two boilers really are independant of each other you wire each one as as normal:

Mains -> Programmer -> Thermostat -> Valve. Valve switch -> Boiler.

[Jon Connell]

They're completely independent with no water connections.

I suppose there's the minor inconvenience of having to control the timing from two locations, though thinking about it, the whole idea is to make the system more automatic rather than the current poor level of control.

Pity that the Nest doesn't work in the UK yet. They've hinted that they will bring it to the UK, but nothing concrete that I have seen yet.

Gotcha, thanks.

Jon

[Jon Connell]

In parallel, I emailed Honeywell to ask if they had a wiring diagram for a 2 boiler system and they emailed me one back within 15 minutes. That qualifies as great support to me.

Jon

[fred]

The neg on using a controller is that you only get simple on-off timed control whereas if you use a pair of programmable stats you get to run the system at a setback temperature of your choice during those 'off' periods. This is very handy for old thick walled piles in winter (and summer).

Honeywell stats are very easy to use and set up, and have very simple day copying options to let you make changes very quickly. Some hard wired ones can be used with remote sensors too so you can have the controls together by the boiler or elsewhere away from fiddling fingers with the temperature sensors in the main rooms if you wish. One neg however is that Honeywells only run on proportional control, insisting on cycling the controls multiple times per hour (min 3, default 6 per hour) which wears out the motorised valves and in my view cycles the boiler unnecessarily. On a very large house you could find that a remote rad on low flow is only starting to warm up when the stat thinks it is time to cycle off again, result, you're paying to heat pipes not rads.

[Jon Connell]

Old thick walls. Yeah, got them.

[dennis]

I think you need two programmable stats to control the two zones, these have built in programmers.

You may want a programmer to time the hot water but you can probably use the existing one.

[John Rumm]

Have you considered simply fitting a prog stat for each boiler in an appropriate place in the respective zone?

You could basically set the programmers to "always on" then, and let the programmable stats deal with the timing and variation of temperature.

[Jon Connell]

I was hoping for a single point of control to keep things simple, but this does appear to be the consensus.

I'm going to play wait-and-see over the summer to see if Nest complete their trial in the UK and launch a UK-compatible product before the winter. If not, I have a programmable stat that I failed to get round to installing in the previous house that I can bring into service for the zone that currently has the most woeful control (a bust on/off controller).

Jon

[Dave Liquorice]

Once you have programable stat and set it up you don't need to keep tweaking the set point or times as it does that for you, unless one has a very irregular schedule. Ours only gets tweaked when it gets cold during the day and we are in that part of the house anyway.

Wozzat?

[Dave Liquorice]

Yes, you don't need one that says "Hot Water" it's just a switch, the electrons don't care. Just re-label it, I've reused a HW/CH controller here for the two heating zones.

[John Rumm]

The difficulty you will have is that you need one stat per zone, and they need to be located somewhere in the zone to be effective. Since most stats require local adjustment, you arrive at two prog stats as a simple solution.

There are some controls that will allow you to program both stats from one location. Vaillant control centres can act as a prog stat and also control a slave one for a second zone. However they are quite boiler specific, and they assume that both zones are being driven from the same boiler.

[fred]

The Honeywell CM907 wired stat has a remote sensor option, just a 10k thermistor IIRC but you need to buy the Honeywell one to get the custom terminal block that fits in the main unit to connect to the auxiliary pushfit pins (although I was tight and didn't). I ran the sensor pair over Cat5e twisted pair that was spare and I have the stat by the boiler in the kitchen, away from impulsive fingers in the lounge. There's programmable offset too in case the wiring upsets the set point.

I wouldn't expect wireless to work through too many thick walls but I have run the CM927 (wireless) through 3 or so thinner solid walls.

I've also modded some non remote sense stats by simply clipping out the thermistor and remoting it over twisted pair. An easy mod and working well on my current install after the Honeywell croaked.

Useful info ta.

[Stephen H]

I have two heating zones myself on two different Prog stats.

While I appreciate the issues in having two timeclocks to set date and time of, I overcame this by ensuring the stats got their date and time from either Rugby/Anthorn or from Mannheim.... I do not need to set the date and time on either of them.

I used Salus ST620's one on each floor.

[Jon Connell]

Cheers