Frost Stat Question

[Londoner2]

My old setup was like this, a fost stat set at 5c in a outbuilding and teh pipe stat set at 40c, however this did not prevent the heating to come on at freezing nights, we ran like this from 2004 to 2019 in era of cheap energy.

 

[Bon]

Usually with your type of system having motorised valves, the frost stat is connected to the central heating motorised valve (in series with a pipe stat). So when the frost stat and pipe stat are calling for heat, that opens the valve which then starts the pump and boiler exactly as it would when the heating was running under Tado control.

The frost stat would be fed from a permanent live, and not come via the Tado meaning it would work when the normal heating schedule was off.

That’s how I thought it would work; however the switched live from the frost stat is wired to the live to the boiler and then another 2 live which run to the wiring centre in the airing cupboard. One is connected to the pump live and the other is (I think) wired to the valve.

When the frost stat switches, the boiler and pump run but the valve doesn’t open. I’m presuming that’s because the Tado is set to “OFF” overnight.

I disconnected the switched live from the frost stat last night and will re-fit when I get the pipe stat too

 

[Bon]

Hi guys, just as an update. I have now purchased a new Honeywell “frost protection kit” from BES which contains a T4360A1009 frost thermostat and an L641B1004 low-limit pipe thermostat - purchased from here

BES.co.uk | Next Day Delivery On 15,000 + Plumbing Supplies

Order Frost Protection Kits from our gas and plumbing supplies range online at BES. Low prices. Free next day delivery available. 15k+ Products. P/N: 15164

www.bes.co.uk

Now - the online instructions/guide imply that a permanent live is daisy-chained through the frost stat and pipe stat and fed to the switched live of the heating system to over-ride the controls

However, the instructions that came with the kit shows a permanent live and neutral to the frost stat, with the switched live in series with the pipe-stat and then to the switched live of the heating system.

The existing (frost stat only) installation had a 3-core flex with the neutral connected to a terminal block off the mains but not actually connected to the frost stat wiring terminals (only live and switched live were connected at the stat)

I’m just wondering
Why it would be different across the 2 bits of literature
If whoever installed it originally did it wrong

It will be installed to the guide that’s come with the new kit, it just bugs me when I don’t “get” things

 

[stem]

Generally mechanical dial thermostats are less accurate than digital thermostats, so they have what is know as an accelerator heater inside. The idea being that when the thermostat switches 'on' the heater also comes 'on' adding a tiny amount of heat to the thermostat to help it switch off a tad earlier than it would without it, thus reducing the temperature differential between it switching 'on' and 'off'. This heater requires a neutral supply to operate. Normally this is for room thermostats, so the occupants don't notice the difference in room temperature as the thermostat turns the heating 'on' and 'off'.

With a frost thermostat, this differential is not so important, so the accelerator heater is not essential (If it is in fact actually fitted. I've never looked) hence it not being shown in the diagram you posted above.

I guess the other diagram you refer to with the N is this one:

There could be several reasons for this, it may be a generic Honeywell thermostat drawing; or it shows terminal 2 as a convenient parking terminal for an unused N wire; or the frost thermostat does have an accelerator heater fitted and if you wish to use it, you can. I don't know. However, as you say:

The existing (frost stat only) installation had a 3-core flex with the neutral connected to a terminal block off the mains but not actually connected to the frost stat wiring terminals

Then I would use terminal 2 in place for the neutral and get rid of the terminal block, which I think you said you were going to do.

When you have the thermostat open, if you can see the internals, I would be interested to know if terminal 2 does actually have anything connected to it, such as an accelerator heater, then you will know the answer, and if you post back so will I.

 

[Bon]

Hi @stem

Took a while but please see attached pics

I can’t see terminal 2 (2nd from right on pics) goes anywhere

 

[stem]

OK. Thanks for trying. I expect that it does go somewhere. If not they are usually left blank like this example...