Horstmann C27 heating controller wiring

[IanJones]

I have just installed a C27 controller to my system, which consists of

boiler, pump, tank stat and room stat. I have wired as best I can t wiring diagram. The diagram shows a link between the 2 stats but I hav no wires left. Does any one know if I should fit a bridging wire and i so where.

The system is gravity fed.

At present the stat on the tank starts and stops the boiler, and th room one activates the pump only.

The controller I had previously started the boiler from the room stat.

very confused.

anyone got a photo of the wiring of the same controller?

Thanks Ia

-- IanJones

[Set Square]

Do you have a motorised valve in your gravity HW circuit? If so, you have a C-Plan system. Have a look at the C-Plan wiring diagram in

particular attention to the wiring of the secondary contacts in the valve (orange, grey and white wires). It is essential that you get this bit right.

If you *don't* have a motorised valve, I don't see what function is served by having tank stat - and, in any case, you would probably need an additional relay somewhere to be able to make use of it.

Essentially, when HW only is calling for heat, you need to run just the boiler, and turn it off when the demand is satisfied. When CH is required, you need to run both the boiler *and* the pump. In this condition - unless you have a valve in the HW circuit, the HW will continue to get hotter - even when it's above its stat setting.

Perhaps you can provide a few more details about your physical system.

[Dave Liquorice]

You need to bring the "HW satisfied" aka HW HOT contact from the tank stat to the CH stat somewhere but I forget where, it is very confusing but logical when you follow it through. Works the logic and produces this, I think it's correct (ASCII art fixed pitch font required...)

Tank Stat HW ON ---------o Cold Comon o----------------- BOILER +---o Hot | +-------------+ Room Stat | Cold o--+-------------- PUMP CH ON ---------o Common Hot o-- no connection

Live wiring only, earths and neutrals ommited for clarity.

You answer you own question, thats what the tank stat does. B-)

This is true but is the way a lot of older systems are installed.

[Malcolm]

If you visit the screwfix web site and enter 94777 in the quote number, it brings up the Horstman control pack that features this controller. If you download the pdf instruction manual below the main image this gives the wiring for various systems (2 or 3 port valves, pumped or vented systems etc)

I just installed one of these kits (purchased 9 months ago !) and it was simple to follow with the existing cables. It also shows you how to add links if you omit any room stat etc.

Malcolm

Dave Liquorice wrote:

[IanJones]

There does not appear to be a motorised valve.

So are you suggesting that I omit the connection to the tank stat an connect the boiler direct to the controller?

If I do this it means that the tank temp is not being taken int consideration so the water in the tank could exceed required hot wate temp, or will it stay at temp set on room stat?

Thanks Ia

-- IanJones

[Dave Liquorice]

I'm not, I'm not sure about Set Square... B-)

Have you not seen my previous post?

Without a tank stat the hot water will always rise towards the temperature of the primary water, as set by boiler stat, probably around 80C .

It will still do this with a tank stat when the heating is also calling for heat. When the heating isn't calling for heat the tank stat will shut the system down when it is satisfied.

[Set Square]

OK - I've just drawn a circuit diagram on the back of an envelope which I

*think* will work.

Tank stat must be the changeover type - with common (COM), nornally open (NO) and normally closed (NC) contacts. The room stat can be a simple on/off device.

Make the following connections: Programmer HW output to tank stat NC Tank stat COM to boiler switched live (SL) Programmer CH output to room stat input Room stat output to pump *and* (this is the clever bit!) to tank stat NO

So the boiler will run whenever there is either (or both) a HW or CH demand and the pump will run whenever there is a CH demand.

The only problem is that the boiler will run when there is a CH demand

*even* if the HW demand is satisfied - so the HW will get *too* hot. The simplest way to fix this is to install a zone valve in the HW circuit and convert it to a C-Plan system.

[Malcolm]

The simplest way (can't be bothered to troll through all the posts) is to install a three way valve, room stat and tank stat. This way you can set different timings for the HW and CH and both can be switched so you have one, t'other or both on at the same time. - also does your boiler have pump over-run ?

If you get stuck, drop me an e-mail and I'll take some photo's of the wiring as I've just installed a Horstman instalation kit - it might help ?

Malcolm

Set Square wrote:

[Dave Liquorice]

You describe what I drew in my other post...

Not forgetting to make provision for any pump overrun if the boiler requires it. This being an old and gravity system the boiler probably doesn't need it but worth checking.

[Set Square]

I don't see how it can *possibly* require it - otherwise it wouldn't work in HW-only mode without the pump running - which it clearly does.

[Set Square]

It may not actually be that simple - because the gravity HW circuit and pumped CH circuit may well be separate, right back to the boiler. Converting this to a fully pumped Y-Plan system could involve quite a lot of plumbing - whereas inserting a zone valve in the HW circuit and adding a tank stat involes very little plumbing.

Don't forget that a C-Plan system also gives you independent timing and temperature control of HW and CH. The only downside is that hot water recovery is slower because it relies on gravity circulation. But I'm not aware that this is a problem at the moment.

The boiler can't possibly require pump over-run - otherwise it wouldn't work in HW-only mode, in which the pump doesn't run at all.

[Malcolm]

Agreed that no two installations will be the same, maybe I'm just lucky in that everything apart from the boiler and room stat is in the upstairs airing cupboard, thus the three way valve is fitted 18" away from the pump. I'm not an engineer so have no idea what you mean about C and Y plans, I was simply upgrading the system I have at home with new components, and actually getting the heating system to work (something that the cowboy plumbers who installed the system failed to do a few years back !) and thought I might be able to add something to the discussion.

Malcolm

Set Square wrote:

[Set Square]

You don't need to be. Have a look at

where the various plans are explained.

Basically, C-Plan is gravity HW and pumped CH with a zone valve in the HW circuit and a cylinder stat - wired in a way which provides boiler interlock.

Y-Plan is fully pumped with a 3-port mid-position valve to direct the flow to the HW circuit or CH circuit or both at the same time.

S-Plan achieves a similar result to Y-Plan but uses two (or more) 2-port valves.

Etc.

[Dave Liquorice]

But has a open gravity loop... bung in a valve in the HW circuit and you lose that when the HW is satisfied.

[Set Square]

Well, sort of! With a C-Plan system in HW-only mode, the boiler is fired via the volt-free contacts in the valve. These will open and stop the boiler as soon as the valve *starts* to close - so you've still got a flow path for as long as it takes the valve to close completely. The CH circuit is also open (though normally not subject to too much gravity circulation, one hopes!)

[Dave Liquorice]

My two port spring return honeywell valves close in about 5s or less... Hardly enough time to cool the heat exchanger sufficiently under gravity only flow.

True but as you say shouldn't have any gravity circulation. If you do the you are wasting energy all year when the HW is calling but there is no demand for heating. If the CH circuit does gravity flow it ought to have a non-return valve fitted to kill such circulation.

[Set Square]

My perception is that, in reality, older boilers which hold quite a lot of water can take care of themselves without overheating with *no* flow after they've stopped firing. I used to have an old Baxi Bermuda as part of a fully pumped W-Plan system. That had no pump over-run, so that the pump stopped dead when the room stat turned the burner off - but that never came to any harm.

[Dave Liquorice]

You can't assume with 100% accuracy that boiler doesn't require pump over run just because it hasn't got it wired ATM. You need to check the boiler spec to be sure.

[Set Square]

I suspect that we're getting at cross purposes needlessly. I took (perhaps wrongly) some of your earlier remarks to imply that even if pump over-run wasn't needed, *some* - possibly gravity - flow was still necessary to prevent overheating. Apologies if you didn't say or mean that.

[I'm not sure how you'd implement pump over-run on a C-Plan system anyway - it might present a bit of a challenge!]

[Dave Liquorice]

Oh quite possibly but it's a fun bit of gentle banter. B-)

No I probably wasn't being precise enough.

Hum, I would assume that if the boiler needed pump over run it would have a stat ouput that was live until it had cooled enough. Take that to the valve live and Robert is your mothers brother. If it doesn't have a stat you'd have to fit a pipe stat close to the boiler, probably best on the flow, and use that instead.