Frost & Pipe Thermostat

[Teamclarkjones]

Hello wise ones - can you offer any advice please?
We've recently moved into a house with a gravity fed system and boiler located in the garage. We've found that over the last few nights the central heating is coming on in the middle of the night, notwithstanding that the timer on the controls is set so that the heating should shut off at 10pm and stay off until around 6am.
I believe this might be because we have a Honeywell frost stat in the garage but no pipe stat, so when the temp in the garage dips below 5deg, the boiler is triggered and then stays on until the house reaches the requisite temp.
Assuming this is right I think we need to get a pipe stat fitted Asap as I'm currently waking up in the night ringing wet of sweat which is v unpleasant!
I wonder though if there is a temporary fix that might help us in the meantime?
We have a thee way valve fitted to the hot water tank which enables us to have HW only, CH only or both HW & CH. Overnight, the lever sits in the CH position (as this is the last thing to have been on before the timer switches off at 10pm).
If we were to set the system so that the lever was sitting in the HW position overnight (eg by setting the water to come on for ten mins after the heating shuts off), would the radiators still get hot if and when the frost stat kicked in? Or would this heat the water in the HW tank only? I could live with this - presumably it would be cheaper than heating the whole house and would certainly avoid the night-sweats!!
If that isn't the case and we need to get a pipe stat fitted, can anyone give me a rough idea of how much it might cost to get this installed (we're in Birmingham), and whether a plumber or electrician is best suited to the job?
Thanks in advance

 

[stem]

I believe this might be because we have a Honeywell frost stat in the garage but no pipe stat, so when the temp in the garage dips below 5deg, the boiler is triggered and then stays on until the house reaches the requisite temp.

Sounds a logical assumption

Assuming this is right I think we need to get a pipe stat fitted Asap

OK

We have a thee way valve fitted.......If we were to set the system so that the lever was sitting in the HW position overnight would the radiators still get hot if and when the frost stat kicked in? Or would this heat the water in the HW tank only?

No, moving the lever will only serve to fix open the port to the central heating as its primary purpose is to open all ports when the system is being filled with water.

I have two suggestions. One, as you suggest is to add a pipe thermostat wired in series with the frost stat. The pipe stat should be fitted on the return pipe to the boiler and set at 20 degrees so when the water going back to the boiler reaches that temperature the heating is turned off. If the return pipe is close to the frost stat, it is a simple wiring job and will take less than an hour, longer if there's a significant cable to run. This would be my preferred solution.

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• stem
• 22 Dec 2010
• 3

A second suggestion that I have seen which works reasonably well, is to move the frost stat so that it is one to two inches above the pipe, with a small section of insulation removed from the top half of the pipe where it passes below the thermostat. The heat from the pipe will then switch off the frost stat when the pipe gets warm.

 

[Teamclarkjones]

No, moving the lever will only serve to fix open the port to the central heating as its primary purpose is to open all ports when the system is being filled with water.

Sorry, I'm undoubtedly being really thick but I can't get my head round this bit ... If there are three options (HWonly CH only or both) and it is set to CH only, as currently, this explains why the radiators are getting hot. If it is set to HW only, wouldnt this heat only the water in the tank? How would setting it to HW only open the port to the central heating? Isn't this already open, hence the hot radiators at 3am?
Sorry to be dense ... This is all very new to me!!
(Thanks for the reply though!!)

 

[stem]

OK. I'll try to explain. The motorised valve is wound open by a motor.

1 For hot water only, the valve is unpowered. Its mechanism is held back at rest by a spring. At this point, the central heating port is closed and the hot water port is open.

2. When heating and hot water are required, the motor is powered and winds the valve to a central position, where it is held by a microswitch that detects it is in the middle. At this point both ports are open.

3. When only central heating is required, the motor winds the valve to the far end of its travel and closes off the hot water port. The frost stat will likely be wired to by-pass the programmer and put the valve in this position.

The operating lever has two positions. The default position is AUTO 'unlatched' and this allows the motor to take control of the valves position. If the lever is moved across to the MAN position and latched, the valve is held in position 2 as described above with both ports open.

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• stem
• 1 Apr 2010
• 8

Having said the above, if the valve is latched in MAN and is told to go to the central heating position only by the controls, it will do so and likely release the lever from its latched position as it winds past anyway.

 

[Teamclarkjones]

Hmm. Feel I might have confused matters by referring to a "lever"...
I don't propose altering the valve from "auto" to manual. It would be left on auto but, as the pin on the valve that moves between the three settings (HW /CH / central (ie both)) "sits" in the last known location until told to move, I'm suggesting setting the timer so that overnight the last setting was auto but HW only. (At the moment the last setting was auto CH only).
Based on your post above, wouldn't this leave the valve to the CH closed as and when the frost stat kicked in, thus heating the tank but stopping the rads getting hot?
Thanks for your patience!!!!!

 

[stem]

Sorry I'm not so good explaining with words. I like pictures better!

the pin on the valve that moves between the three settings (HW /CH / central (ie both)) "sits" in the last known location until told to move

I'm not sure what "pin" you are referring to. Most of the valves I've seen look like the one in the photo, not seen any pins anywhere.

Anyway you are correct in that the valve does indeed remain electrically held in its last used position. This is because there are certain control signals present at the valve even when the heating and hot water are set to be off.

However, when the frost thermostat turns 'on' it will by-pass the programmers control of the central heating on and off periods and turn just the heating on. This will then drive the valve fully across to central heating only position, regardless of where it was before.

 

[Teamclarkjones]

I see. That's a shame. For info, here's a pic of the valve - you can see the black pin / lever I'm referring to, this moves to indicate the current setting.
Thanks again for your help!
_

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• Teamclarkjones
• 22 Jan 2015
• 1

 

[stem]

OK thanks, I think I've only ever seen one of those. Is it called an iflow or something like that?

The most common makes are Honeywell, Danfoss, Drayton and Horstmann they all work on the same principles and are electrically interchangeable. So I would imagine yours will be too.

With your system, the bottom line is that the frost stat just overrides the programmer, so it would achieve exactly the same thing as you going up to the programmer and turning the central heating to 'on'

 

[MrTherm]

It's a "three port" valve and not a "mid position" valve.

 

[stem]

By that do you mean it's one that doesn't have a central position, it's hot water only or central heating only? I didn't think they existed anymore.

AFAIK the term '3-Port' also applies to those that do have a mid position. The Honeywell shown in my photo is also called a 3-port valve.

 

[Teamclarkjones]

This definitely has a mid position, ("M") which is indicated when both the CH & HW are on. We have three options - CH only, HW only or CH & HW.

 

[stem]

That would make sense. Perhaps MrTherm will clarify his post.

 

[Rampage77]

That photo is a Drayton mid position valve.

I would wire a pipe stat into the system as instructed by previous poster.

 

[Teamclarkjones]

Thanks all. I've had a sparky out today who has looked at the frost stat and said that because there's a frost stat installed, fitting a pipe stat won't make any difference - the rads in the house will still come on whenever the garage temp dips below 5deg and stay on until it rises above 5deg. Fitting a pipe stat apparently won't change this. He's basically said I should live with it, as the rads in the house coming on is better than frozen pipes.
My understanding is that this is, well, just wrong. Frost stats should ideally be accompanied by a pipe stat, set to c20deg, which will trip the boiler off when the pipes reach that temp, notwithstanding the temp of the air in the garage. Should I pick another electrician out of the yellow pages and start again??

 

[stem]

What tosh! Your electrician is wrong and you are correct. The thermostat in the garage will turn on the boiler. That bit he has right; but when the water in the pipes and radiators gets warm. The pipe thermostat will turn it off again. OK when the pipes cool, the boiler will come back on and repeat the cycle, but the water in the system will only be lukewarm and should not make a noticeable difference to the temperature of the house. The purpose of the set up is to just keep warm water circulating in the system to stop any exposed parts freezing up.

For it to work, the pipe thermostat contacts need to be wired in series with the frost stat contacts as per my earlier diagram.

...and whether a plumber or electrician is best suited to the job?

I would have chosen an electrician too because the work involves cables not pipes, but I think in this case you chose the wrong one. A plumber would probably be more likely to understand what you are trying to achieve.