Frost stat connection

Ajf100q · May 13, 2022

Iam looking for help to connect a frost stat to an S plan system.
My question is : when there are two zone valves present, one for up and one for down.
Will it matter which I connect to when the frost stat fires up?
Thanks

Avo Mk8 · May 13, 2022

No. You are actually connecting the frost stat between permanent live and switched live of the boiler. The switched live is from the two zone valves common connection.
You should also have a pipe stat on the return to the boiler set to 25 or 30 degrees, wired in series with the frost stat.
Otherwise your boiler may run for many hours once the frost stat kicks in (well, it will run continuously at long as the stat is closed, unless the boiler is providing heat to where the frost stat is, which is not necessarily the case!)

Ajf100q · May 13, 2022

Avo Mk8 said:
No. You are actually connecting the frost stat between permanent live and switched live of the boiler. The switched live is from the two zone valves common connection.
You should also have a pipe stat on the return to the boiler set to 25 or 30 degrees, wired in series with the frost stat.
Otherwise your boiler may run for many hours once the frost stat kicks in (well, it will run continuously at long as the stat is closed, unless the boiler is providing heat to where the frost stat is, which is not necessarily the case!)

Thanks.
My understanding was that I had to connect onto one of the browns of the valves, to fire the boiler up.
Wasn't sure if it mattered whether it was the up or down valve.

AJshep · May 13, 2022

Ajf100q said:
Thanks.
My understanding was that I had to connect onto one of the browns of the valves, to fire the boiler up.
Wasn't sure if it mattered whether it was the up or down valve.

You dont want to be opening valves its just to stop the pipes freezing.
as Avo says Frost stat turns on the boiler on when its cold then pipestat turns boiler off once pipes are warmed up, so the pair need wiring in series.

Connect between perm L- and boiler sw live.
Orange is usually tconnected to the boiler sw live in most common heating systems, but not always.

Ajf100q · May 14, 2022

AJshep said:
You dont want to be opening valves its just to stop the pipes freezing.
as Avo says Frost stat turns on the boiler on when its cold then pipestat turns boiler off once pipes are warmed up, so the pair need wiring in series.

Connect between perm L- and boiler sw live.
Orange is usually tconnected to the boiler sw live in most common heating systems, but not always.

Thanks.
I watched this John Ward video, and he seemed to say that the sw from the frost stat went with the brown on the heating valve?

Avo Mk8 · May 14, 2022

John Ward connects the frost stat/pipe stat combo to the L brown input wire of the heating 2 port valve (which comes from the room stat). That's a better method because it makes sure that the heating circuit is energised, rather than leaving the setup in hot water mode. But in your case, the frost stat will switch on the zone you have chosen to connect it to, but not the other one.

My immediate thought is that you need a frost stat with a 2-pole switch, or an additional relay in order to turn on both zones, unless you think turning on the lower zone should provide enough protection for the upper?

I'll have another think! My bad! Apologies

The simplest solution I can think of is to fit one pipe stat connected to L, connect that to two frost stats, and then each frost stat to each zone valve brown! Or use a relay!

Edit: If you have room stats for each zone, and they have both NO and NC contacts, I think there is a way of wiring those to the frost stat to activate both zones when the frost stat kicks in. But I guess feeding additional wires to the unused contacts in the room stats may not be attractive? (unless they are already there).

JW's wiring:

Ajf100q · May 14, 2022

Avo Mk8 said:
John Ward connects the frost stat/pipe stat combo to the L brown input wire of the heating 2 port valve (which comes from the room stat). That's a better method because it makes sure that the heating circuit is energised, rather than leaving the setup in hot water mode. But in your case, the frost stat will switch on the zone you have chosen to connect it to, but not the other one.

My immediate thought is that you need a frost stat with a 2-pole switch, or an additional relay in order to turn on both zones, unless you think turning on the lower zone should provide enough protection for the upper?

I'll have another think! My bad! Apologies
The simplest solution I can think of is to fit one pipe stat connected to L, connect that to two frost stats, and then each frost stat to each zone valve brown! Or use a relay!

Edit: If you have room stats for each zone, and they have both NO and NC contacts, I think there is a way of wiring those to the frost stat to activate both zones when the frost stat kicks in. But I guess feeding additional wires to the unused contacts in the room stats may not be attractive? (unless they are already there).

JW's wiring:
View attachment 97467

Thanks.
There is only about 2ft of exposed pipe work in this garage, I would have thought highly unlikely would ever be effected by frost anyway. So just opening the lower valve would be sufficient.

fourtytwo · Nov 27, 2022

Ajf100q said:
Thanks.
There is only about 2ft of exposed pipe work in this garage, I would have thought highly unlikely would ever be effected by frost anyway. So just opening the lower valve would be sufficient.

A bit late to the party I know but see my posting here where I have the same system as the OP describes (just a boiler & few feet of pipe in the garage. Running the whole heating system is very wasteful althought this seems to be the standard recommendation, such waste is no longer affordable!

Avo Mk8 · Nov 27, 2022

fourtytwo said:
A bit late to the party I know but see my posting here where I have the same system as the OP describes (just a boiler & few feet of pipe in the garage. Running the whole heating system is very wasteful althought this seems to be the standard recommendation, such waste is no longer affordable!

I believe the premise for a traditional frost stat function is that an unknown proportion of the system in an unknown area of the house (either heating or hot water pipes) might be in danger of freezing when a frost stat is triggered. If it were true the only freezing hazard was a pipe in the garage, a bit of electrically heated tape around the pipe connected to a frost stat would serve the purpose, so no need to run the gas at all!
But there may come times when other parts of the system are under threat of freezing. I guess the default recommendation is to cover all eventualities. If you are confident the hot water only solution suits your property then that's a good solution.
PS Sounds like you have quite a big system for it to take a long time to return warm water.

If you were the manufacturer doing the risk assessment for their product, to be suitable for everyone's system, you might choose the default solution. Surely up to installer/ customer to modify as they see fit, as you have done, and therefore now take responsibility for any consequences. The manufacturer and your insurance company now in the clear!

brianmoooore · Nov 28, 2022

Frost 'stat and associated pipe 'stat installed to protect a boiler should not open any zone valves. There should be a PRV, and often a towel rail, connected across the flow and return pipes before the zone valves to enable flow through the boiler.
If there is pipework, other than that closely associated with the boiler, that needs frost protection, then a zone valve might well be required to open, and the wiring should be adapted to suit.

fourtytwo · Dec 1, 2022

Unfortunately Brian much of the Honywell documentation (for Y-plan at least) tells you to connect a permanent live to the white 3 way valve wire resulting on the CH coming on. See attached pic inside 42005748 junction box lid and also T4360 frost stat installation manual, as a result not only my house but I suspect many others are wired up in this wasteful manner. Of course as Avo points out there might be SOME installations justifying CH on but I suspect they are vastly in the minority (holiday homes for example).

brianmoooore · Dec 1, 2022

That's possibly liability avoidance if the 'stat is connected to poorly designed systems, where there'd be no boiler flow if the valves were closed. Don't forget that these instructions are aimed at plumbers, and worse than that, plumbers doing electrics, so they can't really encourage innovation.