Frost stat goes out of calibration

[cornishpasty]

Hello everyone.

I have got an ongoing problem with my Honeywell FrostStat on my central heating. This is the third new thermostat to go out of calibration. It starts out bang on when fitted but gradually gets increasingly inaccurate until when it is set to 1 degree it is actually clicking on and off at around 7 degrees. I have no pipe stat on my system, so this means my heating is on all night, when there is no chance of my pipes freezing!
I was under the impression that these failed pretty rarely, so I am wondering what could have caused 3 in a row to do the same thing? Has anyone ever heard of something like this before?

Cheers.

 

[WHPES]

Have you spoken to Honeywell? They're usually pretty good tech support.

 

[H20]

Can't say I have ever come across such a problem - maybe there is a problem with the stat's diaphragms.

Have you tried a different manufacturer's frost stat?

 

[street elbow]

Its been a while sense I fitted a frost stat, but 7 degrees is about the correct setting for a frost stat, are you sure it is not pre set @ 7 deg.

Also 1 degree is far too low for a frost stat setting, pipes burst at 4 degrees as the temperature rises. So pipes could still burst with a frost stat set at 1 degree.

For the amount it will cost to install a pipe stat, it would be well the expense.

 

[moony]

yeah install a low limit pipe stat - this will at least stop it being on all night.

 

[SciencenotPlumb]

I was having this problem with my Honeywell stat too, boiler in loft of bungalow. There is a pipestat on my system so this meant my system jerked on in the middle of the night for five minutes every hour or so waking us up because we wondered what was happening. I placed a radio link thermometer in the loft and it read 13C !!!.

OK, the problem was that one of the pillars holding the dial had cracked probably due to strain of mounting stat on the rough wall flexing the casing. This lifted the capsule, and effectively changed the calibration to . . . 13C

I replaced with a Drayton one. These are electronic, not capsule. Same thing!!!! Test showed me that the Drayton was actually out of calibration by 7 degrees.(I have calibrated refrigerators with which I can test these things - Note that I am a Scientist, not a Plumber) Drayton Helpline was mystified by this and clearly didn't believe me so I checked and double checked and I'll be speaking to their QA people firmly today.

What did I do? drilled the broken pillar on the Honeywell, fixed it with a long nut and bolt, and recalibrated it on re-assembly. You can do this by taking the dial off and screwing it up until its right. It helps to have those calibrated refrigerators of course. Set it to a cautious 7 degrese last night (This is the cold November snap of 2010) and sure enough on it comes right on calibration. Minus 1 outside, Plus 6 degrees in the loft and stat comes on at 7 degrees. Confident now, I shall set it to 3 degrees and sleep soundly.

Your problem though. Honeywell Capsule types - watch for mounting strains and loose mounting pillars. That said, I'd say these units are deliberately factory made to be pessimistic by about 7 degrees. If you think about it, the manufacturers cannot risk being blamed for a freeze up when one of their units is on watch. Also, with respect to you plumbers, when fitting one (certainly at a time when its not cold) you'd probably have no way of knowing whether it was properly calibrated or not, you would rely correctly on the manufacturer being right. When you install one of these it may not be at the right temperature, they take an hour or more to stabilise, and that could fool you into thinking its OK.

The electronic Drayton is certainly set too high and I'll report back here after I've taken them to task today.