Heating valve actuator oddity

[Harry Bloomfield]

Following on from problems with the Danfoss HSA 3 valve actuator earlier in the week, my heating system is now running fine using the compatible Sunvic SD 2701, which I had in as a spare.

I have just got around to pulling the 2 year old Danfoss HSA 3 unit apart, to see if it might be repairable for use as a spare and this is what I found -

Internally and externally it was like new, no obvious signs of any wear and tear at all. The motor also checks out fine. The only problem I could find was no continuity between the centre (N/O) contact and the common of both of the switches, when the button is operated. The N/C contacts are fine.

It just seems a little odd that both of the micro switches have suffered the same failure mode simultaneously. The items fitted are SAIA rated at 250v 6amp.

So its off to Maplin for some replacement microswitches.

[Doctor Drivel]

Get the highest Amp rating you can.

[Andrew Gabriel]

Physical size is going to be more important.

[Doctor Drivel]

To clarify. The highest amp rating that fits in. They you are.

[geoff]

In message , Harry Bloomfield writes

The HSA really suffers from s**te microswitches

easy to replace

[Roger Mills]

Three-port valves are horribly unreliable and are probably the single cause of the greatest number of CH woes reported here. Maybe time to replace it with 2 x 2-port valves and convert to S-Plan.

[Harry Bloomfield]

on 23/12/2009, geoff supposed :

That might explain it.

You might think so, but I am struggling to get the replacement microswitches to actually be switched by the cams. Other than possibly rubbish switches, the HSA seems to be more robustly built than the Sunvic.

The cam doesn't seem to protrude out towards the switches as far as it should, which might explain the early demise of the original switches.

[Harry Bloomfield]

Roger Mills presented the following explanation :

On that point I certainly agree. The actuators barely make it out of the guarantee period before they fail. Mine average 18 months to 2 years, which is why I now keep a spare set up ready to swap out.

[<me9]

I had nowt but trouble with 3 port (Danfoss) valves, Micro switches failing regularly.

That's what I did, I haven't had a problem in more years than I previously had problems in months.

[<me9]

I tried several different makes of microswitches in HSAs It didn't make much difference, they still failed in months. the 2 port ones that I replaced with (also Danfoss) must have done about 10 years by now.

[Harry Bloomfield]

Harry Bloomfield formulated on Tuesday :

In the interests of experiment, I have now got to the problem.

So from the top.....

The N/O contacts on both microswitches had burnt out. I picked up 2x replacements plus 2x extra spares from Maplin yesterday, then fitted them in place of the failed ones. Under the plastic half round plate which holds the microswitchs, is a brass cam plate with has at its centre a plastic hub which has the return spring wrapped around it. After fitting new switches, the N/O ones were still not making reliably.

I drilled a small hole in the half round plate, one directly above the operating button of each switch, so I could see what was happening. What I found was that the brass cam ring is just barely low enough to catch the switch buttons - they are not able to reliably push them in. Usually the cams are riding over the edge of the buttons. To hit the buttons square on, the brass cam plate needs to be about 1.5mm lower. The new microswitches are physically identical to the old ones, so I strongly suspect that the N/O contact were burnt out as a direct result of the poor alignment between the cam and microswitch operating buttons.

My next trick will be to skim 1.5mm off the bottom of the hub and make a 1.5mm thick washer to get the cam to properly align with the switches. My lathe is tied up with another project at the moment, so this will have to wait a while.

You would though think, that manufacturers might spot these simple problems in the early design stages.