Danfoss TP9 Central Heating Controller

[raden]

Yup, my error there

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[Tree]

Umm..I'm new to all this, as you might have guessed.

I'm cheered by Michael's interpretation of how the TP9 works, indeed so much so that I've ordered one. Either it does what I want it to, or I'll have to add a relay as per Set Square's suggestion.

Will a 'changeover' cylinder stat not prevent the HW from getting too hot? From what I understand, it's common to wire so that when it's below temp its connected to the HW line on the controller, and when it's above temp it's connected to the CH line - so that the boiler will fire even when the cylinder is up to temperature if required for CH.

There is also currently (only!) an adjustable boiler thermostat, so the HW/CH water temp can never go above what the boiler stst is set to, I think.

Thanks for all the replies,

Rich.

[Set Square]

In HW-only mode the cylinder stat will turn the boiler off when the HW is hot enough - which is fine.

Problem is that when the CH is on, the boiler needs to run - so this goes on heating the HW even if it's above the cylinder stat setting. You need extra hardware - such as a zone valve in the HW circuit - if you are to stop this happening.

As you say, if you set the boiler stat to a temperature suitable for the HW, the HW won't overheat - but the chances are that the radiators then won't get hot enough. HW shouldn't be more than 60 degC, but the flow temp to the rads should be around 80 degC.

[Michael Chare]

If the OP does not have a CH zone valve now, he will be no worse of fitting a TP9 programmer. Arguably he would be better off since the TP9 will only run the boiler when the pump is on rather than all the time that CH is required. (except when HW is required anyway.)

Fitting a HW zone valve (boiler interlock to be precise) is a requirement nowadays under L1. But in practical terms the HW zone valve in my own C/H system failed some time ago (in the open position). This has not been a problem.

[Set Square]

I'm not so sure! He may be better off with a conventional programmer and a changeover stat on the cylinder. That would ensure that in CH-only mode, the boiler ran when the room stat demanded heat. It would also ensure that in HW-only mode, the boiler would shut down when to HW got up to temperature. I don't think it would do this with a TP9.

Not sure whether you could use a TP9 *plus* a changeover cylinder stat - that needs more thought, and my brain has gone to bed!

[Tree]

Good. I've ordered one which should be here by the weekend, and I'll find out if it does what it should or not!

Also ordered a tank stat.

Thanks for all the help,

Rich.