Replacing CH timer (horstmann 425 tiarra)

[Simon Dyer]

Hi, I wish to replace the above timer with a 7day one that seperates the hot water and heating cycles. The heating system is a primatic one...but runs very well Do different models have different connections ? or can I just buy the one I like !

 

[Simon Dyer]

Anyone ?

 

[brumylad]

The main issue here is to ensure a safe method of separating the hot water and central heating cycles with a primatic hot water cylinder. The whole idea is that the expansion of the hot water in the radiator / boiler pipework is taken up by an air pocket in the primatic hot water cylinder.

The hot water and CH is usually made separate by fitting a pump and dividing the flow of hot water coming from the boiler using a motorised valve. I really would not advise that this is done on your system without changing the HW cylinder and other changes to accomodate expansion.

It might be possible to introduce a motorised valve in the primary circuit (pipes going from the boiler to the HW cylinder). This is called a C plan. Contact the manufacturer of the cylinder and ask their advice.

I'm afraid its not just a matter of changing the box on the wall. Sorry for bearing the bad news.

 

[Simon Dyer]

Thanks for the info. Will wait untill I upgrade system to seperate the cycles. Ideally I would still like to replace the timer as I would like different times for the weekends etc. This is the one I have http://www.horstmann.co.uk/pages/central2.html , thx[/url]

 

[brumylad]

425 Tiara
Twin circuit hot water and central heating programmer. Offers 16 programmes for fully pumped systems incorporating motorised or mid-position valves and 10 programmes for gravity H.W./pumped heating systems.

Its difficult to understand why this programmer has been used if your CH and HW is not already separate. I presume your system has a pump but no motorised valves?

Wiring in a new programmer should be fairly simple if this is the case.

 

[Simon Dyer]

It seems to have a valve on the top pipe that goes into the cylinder. I have 6 cables going to the timer.
1 power supply
2 boiler
3 pump
4 room stat
5 + 6 go to a junction box that connects to the motorised valve and hot water thermostat

I can have hot water but no heating but not heating without hot water. I presume I am just being a moron and thats the way it goes

 

[brumylad]

Hmm..., sounds a bit strange for a primatic system. I always thought that pumped primary circuit (flow to hot water cylinder) was not recommended.

Is it a 2 port valve or a 3 port valve (3 pipes or 2 pipes coming of the silver box thing)

Any chance of a picture?

 

[Simon Dyer]

Its on one 22mm pipe that comes out of the side of the h/w tank. Is it a vavle to stop h/w heating ? On the boiler I have 4 pipes, 1 gas and 3 water.

 

[brumylad]

Sounds like a 2 port motorised valve, yes a valve to stop the HW from getting hot. Sounds like you have gravity circulation for the energy supply circuit to the hot water which can be stopped with the valve. And a pumped circulation for the energy to the radiators which is controlled by the pump being on or off.

This sounds to me like a C-plan.

Check out this site for a wiring diagram for your system. (about 3/4 down the page)
http://www.gasman.fsbusiness.co.uk/wiring.htm

 

[Simon Dyer]

Yep that looks like it. So back to original ? what timer can I use ...and another, why do i have 3 not 4 pipes to boiler ? Many thx for your replys

 

[ChrisR]

Here's another version, pipes and wires: http://content.honeywell.com/uk/homes/Catalogue/Sundial Plans/3.1 C Plan.pdf

I think you can use any old timer you like. Or any new one come to that. There are lots of options, with a smallish number of variations, so if you decide what features you want we'll probably know of one which will do them. You can have things like one or two hour boost during an OFF period or addition to an ON time, 7 day control or 5+2 day, 2 or 3 on times per day, holiday timer, single button daylight saving adjustment... I keep a number of popular programmers available at below shop prices. If this gets deleted go up a level and click on my name

 

[namsag]

someone think its danfoss do a digital straight replacement using existing backplate with no need to alter wiring.

 

[Simon Dyer]

Any ideas why I only have the three pipes ?

 

[ChrisR]

I expect the COMMON pipes are combined near the cylinder, hence 3.

I think you'll find nobody does a straight swap replacement for the Tiara with 7 day control. Even Horstmann's later programmers, apart from having different wiring use pins instead of blades for the contact between the backplate and body. But it isn't a big deal to rewire a programmer, usually even the backplate wall screws are usually in the same places. There are lots to choose from. I have several.
Go here , click on my name and email me.

 

[namsag]

Danfoss FP 975 is a straight digital replacement using existing back plate and wiring with blades instead of pins for contacts for the hortsman 425 525 and 527 range.