DIY Doctor UK DIY Forums

Hey All,

Trying to find out if this is something I can fix or just needs replacing.

I had a house renovation about 8 years ago and one of the things they did was add in a Horstmann T50 RF thermostat up to my boiler.

It's worked fine up till a few weeks ago where it doesn't seem to be turning on the heating consistently. The signal isn't getting through.

I've had to walk the thermostat upstairs till it was nearly touching the receiver box next to the boiler and even then it sometimes doesn't pick up a signal. Most times if it does, it only stays on for a minute or so before turning off again.

Replaced the batteries to no avail so it's not short on power. Is this something that I can replace just the thermostat or is the receiver at fault? Do I just scrap the lot and buy a new one? If so, is that easy for a novice to wire in? I can wire up a plug fair enough and from what I've seen online, the receiver box is not a lot different so long as I keep the same wires going to the same ports?

Any help would be appreciated, thanks!

I had the same problem with a Horstmann thermostat can't remember model number. I found you had to turn down and up or up and down once taken closer for it to work. I was lucky there was a Honeywell in parallel so it would run without it.

Only cure seems to be replacement, non RF Horstmann seem good, just the RF version. On tablet at moment when I get to PC will look up the wiring diagram and say how to replace.

Now looked at pictures and yes same thermostat as I had, and it seems same fault, also known as HRFS1 there are 5 connections on the base, the NC is not used and some times there is a link between L and Com so could be 3 core and earth or 4 core and earth and the earth is not used it just has to be there.

It is still made so you could replace like for like but likely you will not want same again as seems common fault, I would say hard wired programmable thermostat does not need to be expensive, £37 will get you a Flomasta 22199SX Wired Digital Programmable Room Thermostat which I have used and is ample, but once you look at wireless to get one which will fail safe the price jumps to around the £150.

With oil the wall thermostat often controls room temperature, with gas normally the TRV controls room temperature and the wall thermostat is only there to stop cycling.

This may seem daft, but it is down to how the boiler works, with a modulating boiler to gain the latent heat the return water temperature controls the boiler output, the TRV controls this as they close the by-pass valve opens and so return water temperature goes up and boiler turns down, if the boiler can't turn down any more it starts to cycle, and each restart is at minimum output, however it will never completely stop so the wall thermostat is there to switch it off in warm weather.

If the thermostat has anti hysteresis software which as the room approaches the target temperature starts to switch boiler off/on to stop over shooting, this causes the boiler to restart at maximum output which does not allow use of latent heat, so makes the boiler less efficient, so the mid range thermostats are OK for oil which does not modulate but not for most gas boilers, with gas main control is with the TRV heads.

So there are some smart systems where the TRV head tells the wall thermostat when it is required, looking at £100's to set that up, TRV heads at £40 plus, but there are also stand alone TRV heads eQ-3 for example which can be bought for under a tenner, but the eQ-3 can't turn the boiler on, so in one room the TRV and wall thermostat have to have a similar schedule manually set may not be the same temperature, in hall ways with stairs the radiator is low so TRV is low and the wall thermostat is a lot higher, so my house 2 degs C different in settings, but they change at same times.

The reason for saying this now, is unless there is a cooker where the receiver is, the thermostat may work OK in that room once you set up with electronic programmable TRV heads.

In real terms with just a simple switch on central heating once the TRV heads are set up, you could manually turn off boiler in warm weather, but we forget, I use Nest so it auto turns off boiler when I go out, but would not say Nest is the best, think reading about them likely Drayton Wiser is the best then closely followed by Honeywell EvoHome but these are expensive, in theory Hive should work well, however poor reports seems looses RF links between wall thermostat and TRV heads.

I some times think it is like opening the page blind folded and sticking in a pin, I got Nest because it would work with two wires, not for geofencing or occupancy detection, but now I have it, I use it.

In theory I should be able to say hay google turn living room temperature to 22 degs C, in practice likely it will turn off lights instead, yesterday I had the google mini playing music, so said hay google turn off, and it replied turning off 5 switches and turned off my lights, but asked to turn them on again says it does not know how to do that yet. Just like a naughty boy, but giving it a smack does not good.

So may be simple is best?

Thank you Ericmark for that very comprehensive reply that I understood about 33% of. I'm not an electrician of any sort but I understood enough that RF replacement will be expensive, wall mounted one will be cheap to buy but expensive to fit because I am guessing that means running wires down from the combiboiler to my living room.

For the moment it seems to be obeying the method of "Turn it down, then turn it back up again while it's in the bathroom" so that it knows the signal hs been sent fresh and it's within a couple of meters of the receiver.

I'll bear in mind what sort of prices wiring in a proper wall one will be for sometime down the line.

If you fit programmable TRV heads it can't over heat, the eQ-3 are only £10 think about it.