Insistent central heating

8 Nov 2018
Reaction score
United Kingdom

Hello. First time poster, happy to be torn to shreds if appropriate!

I have recently moved house and found that the room thermostat was wildly inaccurate. It was a Horstmann HRT-2.

I replaced this with a Drayton RTS-8. When I switched the heating power back on, the 3A central heating fuse blew immediately. I replaced the fuse, put the old 'stat back and everything appeared normal.

When the CH timer (Drayton Lifestyle LP522, set to turn off at 21:00) turned off, the boiler continued to fire and the room stat "calling for heat" LED remained lit. Turning the 'stat temperature down turned off the LED and shut the boiler down.

My understanding is that what should happen is (if stat isn't satisfied) - timer on, stat on, zone valve switch on, boiler on OR timer off, stat off, zone valve switch off, boiler off

I have timer off, stat on, switch on, boiler on. Removing power to the timer removes power to the stat.

The timer CH light is off so timer thinks it isn't calling for heat yet the 'stat is receiving power.

My theory is that when I replaced the thermostat I fried the timers switched live output switch and it is stuck as permanently on.

Can anyone suggest whether I'm in the right ball park? If so, I'll be buying a new timer unit and will move on to why the new stat blew the heating system fuse.
Last edited:
Sponsored Links
My theory is that when I replaced the thermostat I fried the timers switched live output switch and it is stuck as permanently on.
That's what usually happens. The short circuit current flowing through the programmers switching contacts before the fuse blows, effectively welds them together creating the situation you describe. You have been rather unlucky though. Every time I have come across this situation there has been a 13A fuse fitted instead of a 3A fuse.
Thanks Stem, appreciate it! At least the fuse did its job eventually. I am going to test the output from the controller this evening with a multimeter and if confirmed will be visiting screwfix tomorrow!
As I am a big fan of solved problems, I thought I would post the outcome of this.I had indeed toasted the programmer output switch. It was showing 240v even when indicating CH off.

I have replaced the unit and all is well. Thanks again to Stem for your wisdom.

Next to try and understand how I managed to blow the heating fuse by replacing the thermostat! Sounds like a new thread to me....
Sponsored Links
No need for a new thread. If your HRT2 is as below. It is wired as shown....


....and the RTS8 as below.


So from the above diagrams, it can be ascertained that:

The wire in HRT2 (1) is the 'live' and that goes to RTS8 (L)
The wire in HRT2 (3) is the 'switched live' and that goes to RTS8 (3)
The wire in HRT2 (4) is the 'neutral' and that goes to RTS8 (4)
Thank you stem and flameport. I assumed I had wired it incorrectly, despite having checked the wiring diagrams before commencing the work. Having checked against the diagrams that stem has kindly provided, I can see that I did wire it correctly.

So the possibilities are

1) incorrect wiring in the original thermostat - which works but with unusual temperature detection (clicks off at 21C thermometer mounted next to it says 19C, then won't click back on until desired temp changed to 25C) - the anticipator may be set incorrectly or the neutral connection may be at fault
2) faulty RTS8 thermostat
3) gremlins

I'm happy checking voltages safely, any advice on where to go next would be gratefully received. Even if the answer is "stop fussing with the thermostat you pedant".

You can check the wiring to confirm the Live, Switched Live and Neutral if you or someone you know, has and knows how to use a multimeter safely. Remove all three wires from the thermostat and isolate them. With the power 'on' they can be checked against other known voltages such as an earth, or nearby socket. From that you will be able to identify the Live and Neutral, and by elimination the Switched Live.

It is unlikely that the wiring was wrong originally, because if you have swapped the wires over as per the diagrams above, then you will have swapped like for like [two live switching wires that are electrically joined together when heat is required, and a neutral for the accelerator] in which case it would have blown the fuse before. So, is there a possibility that:

A stray whisker was poking out and created a short circuit?
That the wires were trapped and crushed together behind the thermostat?
Or you made a mistake when noting where the wires were connected originally?
Was anything else changed?

It's possible that the stat is faulty, and has an internal short circuit, but pretty unlikely I would have thought.
Hi Stem,

All of those are indeed possible, the most likely is a mistake made by me!

I'll isolated and identify the wires, check for shorts in the stat and make sure there are no loose connections, stray wires or damaged insulation causing a short.

Stem, you're a legend.
Just in case anyone should stumble across this thread - I installed a Drayton RST-2 thermostat and all is well now. The RTS-8 had a built-in short between Switched Live and Neutral which was frying everything it touched when calling for heat. Thanks Stem and Flameport for your advice.
The neutral on a room stat simply connects to a small resistor called a heat anticipator , very low current doubt if this was blowing fuses, think you had wired it wrong
I replaced a thermostat in old house as it seemed wildly inaccurate. The new one was wireless and could be placed on a table, so I tried moving it around, I found it was not thermostat that was wildly inaccurate but down to location.

When one reads the instructions for a thermostat it shows it needs to be in the path of the thermals produced by the radiator, and it also must not be fitted on a cold wall, the use of double glazing has also change where radiators are placed so under the window is no longer where they are placed so there is a problem finding a wall suitable for mounting a thermostat.

So I tried different positions and on a tea trolley close to radiator seemed to work well, so I thought what about about 6 inches from floor just to the side of the radiator, it would be in the thermals caused by radiator, had to be set about one degree higher but it worked well, i.e. in the same position as a thermostatic radiator valve (TRV) would be fitted.

Then the new thermostat failed, it lost its RF link, so I tried using electronic TRV heads, they really work well. Had to set the lock shield valve so radiators don't heat up too fast or the TRV can't adjust in time and it over shoots specially if fitted to return, but once done they kept the temperature within half a degree of target temperature.

I realised in that home what I wanted is a wall thermostat with a large difference between on and off temperatures, whole idea of the wall thermostat is to stop boiler cycling in the summer, during the winter months it should not switch off, temperature is controlled by the TRV's!

Well moved house and new house had a wiring problem, only two wires from house to flat under house and wanted to control CH and DHW with two wires so fitted Nest, this is a completely different system, unlike old house boiler does not modulate so the TRV control is not as good, problem with moving from gas to oil, so no longer can the boiler just tick over at min output and keep house warm without any hysteresis.

So in this house the wall thermostat should tell the TRV heads what temperature is set so they all sing from same song sheet, great theory, but it doesn't work, the wifi fails to tell the TRV's to change setting, can't blame Nest as simply not sure if it's Nest or the Energenie TRV heads. But this hitec system simply does not work. Seems I will need to remove link between TRV head and Nest and simply program them to same temperature schedule.

Wish I has gas and a modulating boiler, but that's a big expense having a LPG tank installed, but with gas using TRV heads to control temperature worked very well, the wall thermostat in the hall was set just a little higher than the TRV on hall radiator and so any warm day and heating switched off, but on cold days the boiler modulated and worked A1 with TRV control.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links