Dual fuel towel radiator

[djelectric]

I am hoping someone could give me some reassurance here..

I am fitting a towel radiator in my bathroom. I want to use both the central heating and a heating element to heat the rad.

I don't want the element to be active whilst the central heating is running.

I have carried out some tests on my Danfoss TP9000 thermostat/programmer and there is an available terminal named 'OFF HTG' that when testing with a voltage indicator against a neutral it becomes live when the thermostat is not calling for heat, and becomes dead when it is calling for heat..

This would be perfect for connecting up my element! I have checked the specs of the TP9000 and all terminals are switch current rated at 3A, the element demands 1.7A (400w @ 230v) so I believe the unit is sufficient to drive it (please inform me if I'm wrong here).

The main thing I am concerned about is protecting the element and its provided cable with the 5A fuse recommended by the manufacturer.. Would it be ok to connect in the following sequence:

32A RCBO feeding kitchen ring in CU > 13A SFCU (feeding boiler and controls) > TP9000 > 13A FCU (in zone 2 of bathroom adjacent to rad) > element.

The boiler SFCU has a 13A fuse protecting 2.5mm2 T+E feeding heating systems.. FCU for element will have 5A fuse as stated above..

Anything wrong here?

 

[HandySparks]

Hmm. The boiler SFCU should contain a 3A fuse unless there's a good reason to choose otherwise. It's not just protecting the bit of 2.5mm² cable, it's protecting the boiler and its controls.

 

[Richard Burns]

The electric heating element is thermostatically controlled so if the heating were on and had heated the towel radiator above the element set point the element would be off anyway.
Even if the heating temperature was below the element temperature the element would be n for a very short time as it heated the rail a couple more degrees.

 

[Dave OCD]

Usually the electric element in a 'dual fuel' towelrail is connected via an appropriately fused FCU on a permanent supply, the electrical part is generally switched on as required during the months when the main heating system isn't needed. With your method the element would be powered at all times when heating was off on the programmer/stat unless the main isolator was off. Therefore it would generally be running right through the night which could be quite costly.

 

[HandySparks]

The electric heating element is thermostatically controlled...

Really? I thought that they were usually sized to warm up the radiator to a reasonable surface temperature and be effectively self-limiting due to their low power.

I've never knowingly fitted one with an integral thermostat.

 

[djelectric]

Thanks for your replies!

I missed a little info out.. I plan to use a timer between the fused spur and the TP9000 to set appropriate times for the rail to be heated, i.e. switched off completely at night then back on early morning to warm the towels.

Also gave Tesla UK (element manufacturer) a call earlier today with queries about the element as the instruction manual states 'DO NOT TURN THE ELEMENT ON WHEN THE CENTRAL HEATING IS ON'. The technician informed me it was completely safe to do this, but will be uneconomical as the element will be heating the already heated central heating water.. The built in thermostat is programmed to trip at 75 degrees C, which the central heating wont trigger as it only reaches around 60 degrees..

It is a 'permanently on' element unless controlled by an external timer/thermostat.

I just feel the method I've stated will give the best control..

 

[djelectric]

Will also look into fitting correct fuse for boiler SFCU.. I only noticed it today.. Council fitted stuff you know what its like! Had a periodic inspection done early this year and it went through..

 

[Darkwood]

The flaw in your control is when the room temp is up and the stat clicks the boiler off you then have the element coming on to heat a already hot rail (timer permitting).

Why not fit a pipe mounted stat' or thermocoupler and temp' monitor that will ensure that if the system has hot water circulating then it won't let the element switch on.

 

[Richard Burns]

Really? I thought that they were usually sized to warm up the radiator to a reasonable surface temperature and be effectively self-limiting due to their low power.

I've never knowingly fitted one with an integral thermostat.

You could well be right, I am not really thinking perhaps.
Overall a time switch is easier than messing about.

 

[djelectric]

The flaw in your control is when the room temp is up and the stat clicks the boiler off you then have the element coming on to heat a already hot rail (timer permitting).

Why no fit a pipe stat' that will ensure that if the system has hot water circulating then it won't let the element switch on.

That's a good point.. Could you point me in the direction of such stat please? This would be very helpful.

 

[Darkwood]

There are plenty to pick from, some fixed temp, some adjustable (probably the best for your needs) just tiewrap it to local pipework and use as a control.

https://www.google.co.uk/search?q=p...AL#imgrc=_&usg=__0QwMHgTRhu3ep-LNjCjHXNsywl4=

I let you do the donkey work in picking the correct one for your needs

 

[djelectric]

There are plenty to pick from, some fixed temp, some adjustable (probably the best for your needs) just tiewrap it to local pipework and use as a control.

https://www.google.co.uk/search?q=p...AL#imgrc=_&usg=__0QwMHgTRhu3ep-LNjCjHXNsywl4=

I let you do the donkey work in picking the correct one for your needs

Ah ok, similar to a cylinder thermostat. Thanks mate, very helpful!

 

[djelectric]

So would you deem the rest of the setup is acceptable then? Once I correct the fuse for the boiler.. I was worried about spurring off a spur essentially.. And switching the spur using the 9000 stat.. Cant see anything in the regs on this, just know its not the best of practices..

 

[Darkwood]

I would find out what rated fuse the boiler required, and what load it required, this would be the limiting factor to your original proposal having said that, its not a concern now as you can have a dedicated spur for your rail now and either spur it off the ring and bring in a radial, also your pipe stat' can be located out of sight say in an airing cupboard or anywhere convenient for the piping and a little joint box.

 

[djelectric]

I would find out what rated fuse the boiler required, and what load it required, this would be the limiting factor to your original proposal having said that, its not a concern now as you can have a dedicated spur for your rail now and either spur it off the ring and bring in a radial, also your pipe stat' can be located out of sight say in an airing cupboard or anywhere convenient for the piping and a little joint box.

Right, so I forget tapping into the central heating thermostat altogether and use a dedicated spur, pipe stat and a timer to control it..

Seems easy enough.. And will save me even more energy (beer money).

Thanks mate, really appreciate the advice!