Funny hysteresis

[PaulAH]

Last year I fitted a Danfoss RET B-LS room thermostat. Since then it seems to have deteriorated. The hysteresis is so small that it switches itself off before the boiler has lit. Worse, it sometimes turns off a few seconds after the boiler has fired, followed by a continual round of short-cycling.

This isn't right, is it? What degree of overlap might you reasonably expect?

Also several segments on LCD display have faded, making it impossible to read. (Yes, I have replaced the batteries!)

I emailed Danfoss sales but have not had a reply. Don't want to slag them off completely - the rest of the control equipment is good.

It's just over a year since the stat was purchased so I guess it's out of warrantee. But this is not the sort of performance you would expect from an expensive bit of kit (v expensive compared with basic stats). Any suggestions?

 

[meldrew's_mate]

Typical hysteresis from a mechanical stat is 1 - 2 degC, most singin' an' dancin' electronis types try to emmulate this to avoid cycling. You may have a problem with the internal temperature measuring circuit, usually based around a thermistor (inexpensive component).

If you play a hairdryer on it does it respond appropriately?
Can you hear it click the relay in and out?

Without a display you are a bit limited, try checking the connections are correct and well made throughout your system, don't discount other sources of your most unusual symptoms.

You could always replace it with a mechanical type to help isolate the problem

 

[PaulAH]

Thanks for the suggestions MM, but I honestly think this one is a turkey. Being battery powered, it operates off the wall, completely disconnected from the system - and is just as stupid. If the thermistor is faulty it should be their problem, not mine.

What I'm after is a like-for-like replacement from Danfoss or even, as you say, a basic mechanical model. We have two of these in the system and they work a treat.

 

[Agile]

Most electronic stats have an actual hysteresis of +/- 0.5 degrees.

If yours is switching off very quickly then either its faulty or there is a source of heating in the vicinity. You could try monitoring the temperature close to the sensing NTC.

One small point to check is that the connection terminals are tight.

Since its so new I suggest you return it to the makers with a proper letter explaining that as a professional heating engineer you do not expect this problem to arise and suggest that they replace it to give you confidence to specify their products in the future!

Use these words and I can almost guarantee they will replace it for you!

Tony Glazier

 

[PEDANTICVINDICTIVEMAN]

Tell danfoss about your hystericalnes and they should hopefully replace free of charge, or better still get really hysterical and try to get them to come to it to fix their peice of shiitte and stop wasting yoiur time.

 

[oilman]

Ring Danfoss and find the name of their area rep. He will be able to get a replacement, 'cos he can. The warranty can be argued to be irrelevant, as for it to work for only a year is unreasonable. You could also ask if they have so little reliability, how do they expect to sell in the market etc. etc.

I've had this sort of problem with Drayton, and from what I hear, Honeywell can be problems.

You could tell them they have a great opportunity to investigate design problems by getting a faulty field tested unit, (this is really valuable information from their point of view).

There may be configuration settings for oil boilers, there is something similar on the Honeywell CM67.

 

[PaulAH]

Thanks for your suggestions, guys. Nah, I'm sure it has nothing to do with loose connections or wrong settings. This model is effectively a standalone battery operated switch. It shows the same odd behaviour whether on or off the wall.

Nearest rad is about 10ft away in a large conservatory, so it can't be a heat surge triggering it (nor the sun - happens at night too). I've placed a digital thermometer next to it, resolution 0.1deg. It doesn't even register a 0.1 deg change before the thing swiches off.

Just found the technical spec. It gives the "switching differential" as +/- 1degC.

Thanks again
Paul

 

[CH4]

The problem could be caused by a bad/intermittent contact within the thermostat. Sliding the DIP switches back and forth a few times (to clean them) or cleaning/tightening the battery holder contacts might help.

Are you using the thermostat in "Chrono-Proportional" or "On/Off" mode? In Chrono-Proportional mode (DIP switch 2 set to the right), the cycle rate is supposed to be adjustable - using DIP switch 3.

 

[PaulAH]

Thanks CH4 - the DIP switches are really the last possibility. Hard to imagine how they explain some of the LCD segments fading but I'll give it a go...

HOWEVER, sent a polite email to Danfoss sales and got a gusher back today from the supervisor, no less, apologising profusely for the failure and offering to exchange the stat. Well done Sandra Smith (for it was she).

Will probably ask her for a different model as I never liked that one anyway. The control dial was so stiff it ripped your flesh.