Heating controller woes

[Huge]

The heating controller (a Grasslin Towerchron DP72) on my (elderly) oil fired central heating system finally gave up the ghost before Christmas, and kept "losing its marbles". Since the LCD display was intermittent anyway, I decided to replace it, which I did with a Horstmann Centaurplus C27.

I fitted the replacement yesterday, and I have a problem. So far as I can see, the two controllers are plug compatible, and the system I have is pumped heating and gravity hot water (there are 4 pipes coming out of the boiler, and one of them has the heating pump installed in it - I assume the other 2 pipes are for the gravity hot water) with room and tank 'stats. So, I just unplugged the old controller and plugged the new one in its place - having checked that the wiring looked appropriate.

But ... when the new controller requests heating, the pump starts but the boiler does not fire. It works fine when hot water is requested - the boiler fires and the pump does not start. I can get heating by manually switching on both heating and hot water, so we won't freeze, but what's going on? And how can I get the controller to fire the boiler when heating is requested but not hot water?

Also ... there is a three port valve fitted to the hot water tank. It has never worked since the day we moved in (the head has been removed and the valve set in what looks like the mid position). If we do indeed have pumped heating and gravity hot water, why would we need a 3 port valve? I thought these were used for pumped heating and hot water, using a single pump and the valev to switch between the two circuits? And what are the implications of it not working? I was quite happy with how the system worked before I replaced the controller, so I never bothered doing anything about the defunct valve.

(Oh, happy days; the boiler died completely a few weeks ago, and the mender said we needed a new one because the failed component (ignition electrodes) were unavailable. I got some from the 'net for £15. Then my computer's gone wrong (won't reliably power up) so I've had to buy a new one. Then the Disco gearbox went into "failsafe" mode and has had to have some (as yet unbilled for) work. Sigh. What bleeding next?)

[Dave Liquorice]

Not checked but is that controller compatible with gravity HW and pumped CH? There may be a small switch or hand bag jumper that needs to be set for correct operation under those circumstances. The symptoms you describe fit with the switch/jumper being set for pumped HW & CH.

[ARWadsworth]

Two simple thing spring to mind. You need to remove a link on the C27 to make it work for gravity hot water and the second thing is you must press the reset button to make the C27 know that you have removed the link.

Adam

[Huge]

OK, the "book" says that's for fully pumped systems, but I shall give it a try.

Aaaeeiiii!!!! That means programming it again... :o(

[ARWadsworth]

You cannot have "heating only" with a gravity HW set up.

Most odd. Is there a cylinder stat?

Adam

[Huge]

Well, quite, but running the pump without firing the boiler is singularly pointless.

Yes.

[ARWadsworth]

The book I have says "for GRAVITY HOT WATER remove the link".

I am not 100% sure that you will need to reset. But guess who installed and set his brothers programmer up (altered it to 3 time periods per day which did need a reset) and had to do it all again because he had not pressed the reset button. Go on, have a guess.

Adam

[ARWadsworth]

That would not happen with a gravity set up. The programmer is set so that HW ON fires the boiler (allows gravity HW), the CH ON fires the pump and CH ON always calls for HW to be on. Gravity HW does not have a cylinder stat.

controller,

You probably do have a Y plan system and some tight arsed git has done a bodge job when the 3 port valve failed, ie they have turned it into some sort of half brained idea for free instead of fixing the problem.

Adam

[Huge]

Well, on the suck it and see principle, I tried it, and it now works as expected. Hurrah! Car fixed, boiler fixed, new computer arrived this morning (rudest postie in the world...) Things are looking up.

Ummmm. Would his initials be AW?

[Huge]

Precisely the conclusion I have reached.

Well, we're having a new boiler next summer, so it can all get sorted out at the same time. I expect there's 25kg of scale in the hot water tank, too.

[Huge]

Yep. I think as ARW suggested, I have a Y-plan system being run as if it was a gravity hot water.

[Roger Mills]

Well, you *can* if you have a suitable 2-port valve and tank stat to turn it into a C-Plan system - but I doubt whether that's what the OP's got (although the 3-port valve is puzzling!)

With a 'normal' gravity system, the programmer allows you to *ask* for heating only but, by sleight of hand, also turns on the boiler - and hence the HW - when you do so. According to

you have to remove a blue link from the back of the programmer in order to make it behave like this.