It must be time for frost stat questions

[mike42]

So my heat only boiler has two low-voltage terminals for the frost stat. If left open the boiler will turn on so if there is no frost stat present a link needs to be put in to join the terminals. But I want to put in a frost stat .....

I've been scratching my head wondering how am I going to connect up a regular Drayton frost and pipe stat. I think I've figured it out but is this the normal way of doing it ? It seems complicated even though I have managed to do it without extra relays etc.

Please see attached I hope its self-explanatory. It should say "Boiler Connections" and I've only shown the 2 relevant ones. For the boiler to turn on, I think both stats need to be calling for heat which is the same for regular S-Plan systems I think...

Note that my existing Drayton Frost Stat doesn't give access to the "satisfied" terminal so I plan to buy another which does. The pipestat does give access to all 3 terminals (Common, Satisfied, Call) so I think I can reuse that.

 

[stem]

The frost stat and pipe stat are wired in series, both have to 'call for heat' [i.e. be on] for the boiler to operate. Starting from a cold heating system, they work like this:

1. The temperature where the frost stat is located falls below the setpoint, so 'calls for heat'. [on] Because the heating system is cold the pipe stat is also 'calling for heat'. [on]

2. The heating system starts up and hot water circulates through the heating system.

3. The water in the pipe where the pipe stat is located reaches the set point and switches the heating off

4. After a time the pipe where the pipe stat is located cools and switches the heating back on. Steps 3 and 4 repeat until:

5. The area where the frost stat is located is above its set point and switches the heating off.

 

[ericmark]

I must admit is seems odd, the whole idea of a frost stat is to run when otherwise the heating would not run, so I would have expected to find a parallel connections, but I note many boilers have a separate connection for the frost stat, never really considered why.

Maybe just for ease of connecting. So I have looked at a random set of instructions

2-stage frost-protection system – the frost-protection system protects the appliance against the risk of frost damage. The first stage enables activation of the pump should the temperature of the appliance fall to 7°C. The second stage becomes active when the temperature has dropped to 3°C. Should the second stage become active, the appliance will function on minimum power until it reaches 30°C.

seems I was wrong, however after reading this, I would say reading the installation manual for your boiler is the way forward, the system used very likely changes boiler to boiler, one can't assume, you must actually read the instructions.

 

[fixitflav]

so I would have expected to find a parallel connections

So would I. I don't think the description you posted covers it either, it only protects the boiler, not other parts of the CH system, or water supply pipework and tanks. It also needs a froststat (subject to air temperature), in the coldest part of the house.

 

[mike42]

I've got mine working now. What Stem describes is I think for a traditional boiler expecting a "switched live" input.

My boiler has two low voltage terminals that when OPEN (not connected to each other) the boiler fires. Therefore, the wiring is the complete opposite. The boiler is an Intergas HRE OV.

My first attempt in post 1 that I can't delete, was my first attempt but in the end I had to put the frost and pipe stat in parallel and also ensure that both stats go OPEN when they are calling for heat. Finally I put my programmer/timer in series also going OPEN when request heat. This all has the combined effect of the boiler calling for heat when either of the two conditions hold:

1/ The timer/programmer call for HW OR
2/ BOTH the frost and pipe stats call for heat

During a frezing spell, this has the effect of turning on the HW for a short period until the pipes heat up.

If anyone cares, I'll upload a diagram.

 

[stem]

What Stem describes is I think for a traditional boiler expecting a "switched live" input.

Not exactly. It's a simple on/off switch, it will switch whatever is applied to it (within reason) so 230V or 24v it doesn't know what's connected to it.

However, I missed in your post that the boiler fires when the circuit is left 'open' and not when it's 'closed' as would be usual. So, you are correct, the contacts would be in parallel in this instance, such that if either of them close when the frost stat, or pipe stat reach their set temperature the frost protection is switched off.

I can't say I've ever seen such an arrangement before, but hey ho, every day's a school day.

Sorry for any confusion.

 

[mike42]

Not exactly. It's a simple on/off switch, it will switch whatever is applied to it (within reason) so 230V or 24v it doesn't know what's connected to it.

However, I missed in your post that the boiler fires when the circuit is left 'open' and not when it's 'closed' as would be usual. So, you are correct, the contacts would be in parallel in this instance, such that if either of them close when the frost stat, or pipe stat reach their set temperature the frost protection is switched off.

I can't say I've ever seen such an arrangement before, but hey ho, every day's a school day.

Sorry for any confusion.

No probs made up for the fact that some of your older posts had helped me. Perhaps I'm getting too old and confused these days but did you post something about Opentherm ? That's what I've finally got working on my old S-Plan.

Btw my previous boiler just had _one_ terminal which needed a SL connection.