Heating control advice

[worms]

A question for those more technically competent than myself:

The programmer for my kerosene Rayburn 480k is a bit temperamental, has been for a few years but seems to be getting worse. Easily replaced but the Rayburn part comes in at £250-£350. That seems a bit steep for what appears to me to be a fairly standard 2-channel 7 day programmer which you can get for as low as £19!

The body is actually a Honeywell ST6400C with a different face which (I think) makes it technically a ST6400C1037. The 6400C retails in the £40-£50 range. The new equivalent from Honeywell seems to be a ST9400C which I believe can even fit directly on to the old 6400C base plate. I can get one of these online for under £60.

Before I do this, is there something that I am missing? I will probably need to insert a blanking plate since the non-Rayburn face looks smaller than the existing one, but I can live with that for a £300 saving!

It all seems so obvious that I fear I am missing some vital reason why I should fork out the extra money!

 

[chuckey]

First of all I must confess I know nothing about your actual bit of kit. In general programmer are a time switch that switch the thermostat into circuit. Because of the nature of the control valves and motors used in boilers, when the switches open there is a big spark that make the switch contacts dirty, and often the first sign is that the boiler does not fire when the timer says it should. So if inside the controller you can unclip the plastic covers from the relays (electrically controlled switches) you can clean their contacts with some very fine emery paper (200 grade or better) and then meths/petrol/WD-40 to get the dust out. The other standard problem is that if the clock is a mechanical one, these, when old, suffer from not re-starting/running bckwards after a mains dip because the motor is actually k*******d.
Your actual thermostat can(will) suffer the same same contact damage as well and can be cleaned in the same way. My 30+ year old wall thermostat needs its contacts to be cleaned about every 10 years or so!!
So the next time the boiler does not fire when it should, rattle the thermostat about, to make sure it is not the one causing the problem.
If you are not able to get the existing controller working, substituting it with any other will be OK, providing it offers the facilities that you require, multiple times/days/night dip/CH/DHW etc.
Frank

 

[worms]

Thanks. The problem is definitely with the controller rather than thermostats etc. It is a digital display and the display switches on and off for no reason and at times I can only start up cooker or heating but not both (whole display switches off when I start the second one). One thought is that since this is most problematic after a mains switch-off, it might be related to the internal battery back-up and might be solved by replacing the battery - I think Penners raised a similar issue recently.

 

[MdB]

As before, if it is the battery and you are finding it difficult to get hold of, give us a shout and I should be able to come up with something.

 

[worms]

MdB, that's very kind of you. Since the programmer has been acting up for a couple of years (albeit to a lesser degree than now), I have decided that I ought really to replace it. I bought a new ST6400C on ebay tonight for £23.99 (including postage), which is pretty good since this morning I was looking at Rayburn ones for £350! The only difference seems to be that the new one will say "Honeywell" rather than "Rayburn" and the switches will be labelled "Hot Water" and "Heating" rather than "Cooker" and "Heating".

Once I've got the new one safely working, I will take the old one to bits and see about replacing the battery and cleaning all the terminals so that I can keep it as an emergency spare. Chances are that if I took the risk on trying to fix it up now, something would go wrong, I would be without heat and cooker and end up buying something locally at several times the price!

 

[Penners]

it might be related to the internal battery back-up and might be solved by replacing the battery - I think Penners raised a similar issue recently.

That's right. I have a Honeywell ST799 that was playing up, due to the on-board battery failing. Unfortunately, in my case, the battery was soldered into the circuit board and Honeywell told me it was not replaceable. So, for the sake of a tuppenny-ha'penny hearing aid battery, I had to buy an entire new controller.

But I hadn't had the good sense to post here soon enough for MdB to make me his kind offer before I spent the money!

 

[worms]

During my quick background reading on this I came across a thread on DIYNot (or some such forum) that covered getting the soldered battery out of the ST799 and how to file off the lugs on replacement batteries. In my case, since the cooker occasional comes on at random and then shuts off again and the clock retains its settings after being switched off for several days, I'm not convinced that the battery is the problem. But then these battery problems do seem to manifest themselves in bizarre ways, so who knows?

 

[chuckey]

When the cooker comes on, does the programmer show a change of programme? Its just that I have remembered my Sister-in-laws DHW cylinder timer. They live in a property with absolute rubbish mains, their kitchen flourecent light was always flashing and their 30mA RCD tripped off randomly even though the wiring was > 200M ohms @500v. Anyway this thing kept on forgetting its correct set times, so I replaced it. I put it on test in my shed at a very different location, and it kept its settings for more then a week (compared to two days oop north). I came to the hypothesis, that the battery was old, its internal resistance had risen, so allowing noise and rubbish coming in on the mains not being filtered by the battery, and so it was getting into the timer bit and causinng havoc.
Frank

 

[worms]

Sounds plausible and while our mains isn't too bad, the lights do dim a bit when anything major is switched on. A couple of years back the 30mA RCD tripped from time to time when the cooker started up on timer - never found out why and the problem hasn't recurred recently. Most obvious problem with the controller in the past was that when you manually switched the cooker off, but still on timer or slid the switch across to completely off, the whole thing would swtich on again after a couple of seconds and you had to press the off several times until it decided to agree! In the last few weeks the display has occasionally been going blank when both cooker and heating are manually switched on at the same time and won't allow both at once for a while after a mains switch off.

It will be interesting to see if the new programmer solves the problem...fingers crossed!

 

[chuckey]

I would put money on the battery being old and high resistance, or the main smoothing capacitor being old and low in value. The act of switching the cooker off pruduces a high voltage spike back into the mains and into the timer works (see above). If the timer was wrong in itself, it would go wrong when you are not doing anything. If you take the power of the timer it should keep time for more then three days, if the battery is poor it will be shorter.
The job of replacing a battery in something like this is trivial (once you have the battery), the most difficult part is to re-align the ribbon cable with the scale if it is an LCD type. Shame it this time of year (you will be using your heater?) or I would change the battery for you.
Frank

 

[Lime]

Is it just me or has anyone else notice this phenomenon?
That a digital timer can be set wrong by 10 minutes or an hour and remain exactly like that for years.
Whereas no sooner is the timer reset to exactly the correct time something always occurs within a very short time to put it wrong again?