Wiring the Central heating

[diy_dude]

I plan to use a Danfoss TP9 programmable thermostat with DHW output. I dont want to change the plumbing, so there is only 1 x 2port valve which turns the flow to the radiators on or off. The boiler water circulates through the hot water cylinder whenever the boiler is on.

Does this wiring diagram look OK?

 

[Nixt]

Then your cylinder stat has no control over hot water temp when your heating is running.

 

[diy_dude]

Thats true, however this kind of self regulates due to the temp of the water coming from the boiler. Well, thats what it did over the winter!!

Given this caveat, will the wiring I suggest work?

 

[Nixt]

No it doesn't. For the water to be hot enough for the central heating it means that the dhw in the cylinder will reach to high temperatures and be wasteful. Try Danfoss wiring diagram and fit 2 zone valves.

 

[diy_dude]

OK, point taken.

Given that its spring, I hope to run without CH for much of the time but we still need hot water.

If I wire up as I have shown, will I be able to have regular hot water and occasional CH until it gets colder later in the year (and I have enough money to employ a plumber to install a second zone valve)?

 

[chrishutt]

If the TP9 is a programmable room stat, why do you show an additional room stat?

 

[Nixt]

Sorry no expert only go by manufacturers diagrams myself

 

[diy_dude]

The actual room stat is a remote unit from the TP9 controller unit, they are wired together using 1mm2 T+E cable

 

[chrishutt]

The remote sensor is not a separate switch as shown. The actual switch is within the main TP9 unit.

Anyway, the only problem with your proposed wiring is the lack of control of DHW temperature when CH is on, as mentioned. The live feed to the motorised valve auxiliary switch is usually taken from the permanent live, but I can't see any harm in taking it from the CH switched live as you've shown.

 

[stem]

The live feed to the motorised valve auxiliary switch is usually taken from the permanent live, but I can't see any harm in taking it from the CH switched live as you've shown.

Chrishutt is right about using the permanent live. But there is a problem with taking it from the CH switched live as your drawing shows.

When the hot water switches on, and if the heating is also on, the live from the hot water to the boiler will be joined at the boiler "on" connection through the auxiliary switch in the central heating valve to the live terminal on the valve motor thus keeping the valve open and the heating on when the CH switched live is removed. Effectively by-passing your programmer and room thermostat settings.

That is why the live feed to the auxiliary switch is always from the permanent live.

 

[nickso]

The live feed to the motorised valve auxiliary switch is usually taken from the permanent live, but I can't see any harm in taking it from the CH switched live as you've shown.

Chrishutt is right about using the permanent live. But there is a problem with taking it from the switched live as your drawing shows.

When the hot water switches on, and if the heating is also on, the live from the hot water to the boiler will be joined at the boiler "on" connection through the auxiliary switch in the central heating valve to the live terminal on the valve motor thus keeping the valve open and the heating on when the CH switched live is removed. Effectively by-passing your programmer and room thermostat settings.

That is why the live feed to the auxiliary switch is always from the permanent live.

the man speaks the truth.......more commonly known as backfeed.

real bummer to find if you are tracing a fault so i would do as he says and take it from the perm live.

you would be far better off spending the time and money to get 2 valves installed and save money and hassle in the long term.

 

[diy_dude]

Thanks for pointing this out, I now see exactly what you mean about backfed.

Problem is that I do not have a perm. live in my connection box. If I used the HW output rather than the CH one, would this work?

eg.

 

[stem]

Problem is that I do not have a perm. live in my connection box. If I used the HW output rather than the CH one, would this work?

No, because when HW switches off you will loose the live to the boiler via the auxiliary switch. So the radiators will go cold too.

Permanent live required from boiler, Danfoss TP9 or FCU

 

[diy_dude]

Stem: With only one valve, the TP9 is set to 'linked'. The HW will never switch off while the CH output is on.

I know there is a good argument for getting another valve fitted, however I cannot afford that right now.

 

[stem]

As long as the HW output stays on all the time when the CH is on that will work. You will need to change it if you upgrade to a second valve in the future and have the HW & CH channels switched separately.

When you next have your boiler changed, the engineer will insist that you have thermostatic control on the hot water system under new energy saving regs. (unless you opt for a combi)