Danfoss HTS3 HW tank thermostat

[TonyW2]

Hi,
(Not a pro, just DIY)
One of these worked OK for 18 years, but seemed to have gone faulty. Replaced it with same, twice in last 2years.

Running on HW only in summer, the domestic water is either scalding or lukewarm. Tried temp settings over 50 & 60. I was thinking that the microswitch was faulty & sticking, but 2 duff new ones is unlikely.
For completeness I have n S plan vented set up, the zone valves operate fine, & the stat is wired across Common & 1 (call for heat).

None is supplied, but does this need heat transfer paste, as I can't see how the bi metal strip can be activated when it is not in contact with the tank & is surrounded by non-conducting & non-convecting air in the insulation covering and its plastic coffin.

 

[Deleted member 120897]

I’d be double checking the motorised valve, as it might be passing, therefore when pump overrun occurs, it’s allowing extra heat into it. Does the cylinder thermostat knock the demand off?

 

[John.g]

Is it a Drayton HTS3?
It shouldn't need any heat transfer paste, presume it is secured up against the bare cylinder with the supplied spring, and located about a third the way up the cylinder and must be 6 ins or so above the heating coil return, did you test the 2 failed ones with a multimeter?

 

[TonyW2]

Thanks for the replies.
ChrisB1982---The zone valve is functioning and when HW "off" the inlet pipe to the coil is cold, even if radiator zone is in use, so it does not allow "passing" of boiler water. Turning stat dial works to cut off demand.
John.g--Well spotted--its a DraytonHTS3 -sited correctly, ( and also tried relocating to top of cylinder so as to eliminate hysteresis effect )Using multimeter across the stat shows switching when the temp setting is altered, but I don't know how I could test it for accuracy.
It just seems to be immune to small chnges in its settings and only acts on/off at 55c point.

 

[John.g]

All these type of cylinder stats have a hysteresis of ~ 8C so minute changes wont switch it but if it does actually switch off at say 55C then it will not switch back in until ~ 47C. If you just turn it up and down then there should be a indicated ~ 8c between clicks which means it is operating correctly, then finally set it to say 60C.
Presume the boiler fires up and circulates hot water through the cylinder coil with a call for HW only?. Also, even though you said it is, ensure its connected between C and terminal 1.

 

[TonyW2]

Yes it does circulate through the coil on HW only.
If the system is completely off and the tank allowed to cool to ambient temp, then setting the stat to 55 results in fairly cool tap supply. If the stat is then increased fractionally to 56 results in scalding hot tap supply.
I can't make sense of it.

 

[John.g]

Just check if the HOT WATER coil motorized valve opens and shuts when you turn the stat setting up and down, with programmer on, if it does this with a (click) difference of 5C to 8C then the stat is operating perfectly as designed. If it doesn't (open/shut the M.valve) then it means that the stat isnt connected in the system, see below.

The are only two logical reasons that I can give, one is that the stat isnt connected in the system and the motorized vale is simply open/closed by the programmer?, the other, (more unlikely) is that there is very slow circulation through the heating coil which may result in very high difference between the hot water outlet temperature and the temperature where the stat is attached to the clinder, if the stat is set to 55C then at cut out the cylinder top may be as high as 70C, would normally expect a difference of 5C to 10C.
When the water has been circulating feel the coil top to and bottom and see if there is a big temperature difference.

 

[John.g]

Are you happy that the stat is in contact with the bared cylinder wall by means of the supplied curtain rail spring cord, attachment shows a (EPH) stat with the spring cord and also a chain link if the stat is to be used as a pipestat.

 

[TonyW2]

Thanks for all the help, but I have made no progress.
The motorised zone valve is responding & working and the coil inlet & outlet both get hot without any massive temp difference in line with the zone valve activity.
The stat is fitted with the curtain wire, (but the bimetal strip is not in direct contact with the tank as it is "inside" the stat case.)
I guess it's time to give up and try a different brand of stat.
Any suggestions?

 

[John.g]

Yes, even though strange that you have that problem with a number of new stats.

The EPH WRP pipe/cylinder stat is excellent, I have installed a few on my relations cylinders and the stated switching hysteresis is 5C + or -3C, I have one in my hand now and it switches at 5C so if one of these doesn't work. its not a stat problem.

I wonder if its possible, after 18 years that the cylinder has become insulated with limescale build up and is insulating the cylinder walls from the water?.