Replace Drayton RTS1 with Honeywell CM927

Postby eddiebove » Fri Jun 09, 2017 1:24 pm

I am looking for some help with a DIY project I'm planning. Any advice would be much appreciated!

I want to replace an old Drayton RTS1 thermostat with a modern wireless unit. The current thermostat is located next to the internal garage (ground floor of three storey house) and means we cannot accurately control the heating in the rest of the house. I figured that replacing this with a new wireless thermostat would mean we could have the wireless unit in the living room (first floor), which would be far better.

I figured the easiest way to do this is to install a wireless unit receiver in the location of the current thermostat, and then place the wireless thermostat unit where we needed. I would rather not change the current timer unit as this would be a bigger job that I may not be able to do myself.

The current thermostat is a Drayton RTS 1 (three wires in N, L, 3 - blue, red, yellow respectively)
The current timer for heating/hot water is a Drayton LP241
The boiler is an Ideal Icos H24
All units are from ~ 2004 when the property was built.

The replacement unit I'm looking at is a Honeywell CM927 with it's receiver unit, the BDR91.

CM927: ... tal/CM927/
BDR91: ... Relay-Box/

Can someone please advise me if I can just replace the Drayton RTS1 with the wireless receiver unit BDR91 with the existing wiring? The wiring diagrams on page 4 of the BDR91 user manual are a bit confusing (I'm a confident DIY person but relatively new to wiring diagrams). If someone could also let me know how to wire in the BDR91 that would be perfect!

I realise that to utilise the timer functionality of the new CM927 I would need to set the heat to "always on" on the Drayton LP241. I'd like to keep that unit to manage the hot water.

If anyone would like to suggest alternative replacements I'm very open to ideas!
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Postby ericmark » Fri Jun 09, 2017 9:47 pm

The Ideal Icos H24 it seems is a condensate boiler, this means the flame height varies to match the return water temperature, So using thermostatic radiator valves (TRV) of the bi-direction type the radiator temperature is gradually adjusted to suit the room so the hysteresis is kept small. This can be reduced further by using electronic heads, these have two sensors and measure both air and water temperature so they compensate when the radiator is hot. They also allow you to switch off rooms during part of the day, so no heat in bedrooms during day for example.

However the wall thermostat is still required, but it's job is not to control the house temperature, it's job is to switch off the boiler during the summer months so the boiler does not cycle.

So the wall thermostat needs to be in a room where the temperature can be cool, you want the wall thermostat set to something like 19°C any warmer and in summer it is too hot, However for a living room 19°C is too cool, so we would normally place it in the hall, however the hall will normally have a large radiator so that when the front door is opened the room re-heats fast, this means it will switch off the heating before the house is all warm.

However fitting a TRV on the hall radiator you can set it so hall heats fast until 18°C but takes a long time to get that extra degree to turn off the wall thermostat at 19°C. But tradition is to not fit a TRV to the room with the wall thermostat in, it should be fitted in the coldest room.

The correct method is to use something like the EvoHome, with that system every radiator connects to a central hub, which in turn tells the boiler when to start and stop, however it is expensive.

There are some other systems not as good like "Nest" where the wall thermostat tells all the eTRV's what temperature is required, but this does not work two way, as a result rather a flop.

Using the self contained eTRV is in some ways a lot better, only change the heads in the rooms where you need the extra control, and you can program every room independent works very like the EvoHome but without the central control unit, not as good, but reasonable and cheaper.

However to set up these controls you must close doors, if they are left open top of house always really hot and bottom of house cold, to compensate you need a heat recovery unit which pumps the heat back down stairs.

So job one is set the lock shield valves, these adjust the flow in each radiator and ensure on switch on all rooms heat even, once rooms are hot the lock shield valve does not do much, then you set the TRV's start set at number 2 and slowly increase until room at temperature required, however the boiler clearly can't heat rooms if it's not running, If the wall thermostat switches off two soon then adjust the lock shield valve in that room to slow that room heating up, If like in my mothers house you want a fast warm up in that room then fit a TRV in that room to slow it down as it gets near to shut off temperature.

To answer your original question, I got a cheap programmable thermostat for mothers house, it was actually free standing so could be moved around the house, so in living room in day and bed room at night. To start with it worked well, then it would miss the wireless signal, either the house was super hot or super cold, on doing some research I found the more expensive types have a one hour shut down, if the signal is not received every hour it auto closes down, then even more expensive how two way coms, normally they also let you read temperature on your phone.

After error one getting cheap programmable thermostat I moved to error two, I bought two eTRV's for two main rooms which connected to a central hub, there was no need to get the type which connects to central hub, the stand alone are a lot cheaper and less to go wrong, OK the ones I have I can change set temperature with a smart phone or PC, but why would I want to?

What finally got mothers system working well was adding a TRV to hall radiator, it took some setting to start with, but once set it has worked well, there are two stops on the thermostat the low one has been set to 12°C this is for frost protection and the high one to 19°C the TRV on hall radiator was set to match this, so hall heats from ambient to 18°C fast but slows down getting the last bit so heart of winter boiler does not turn off/on it just varies flame height, as summer comes then the wall thermostat starts to turn off the boiler until heart of summer when I turn the thermostat to 12°C there is a programmer, this switches off heating at 3 am and back on at 6 am which is the same time as the eTRV's switch from night to day temperature.

Clearly you can't set until winter arrives, but look at the Pegler Terrier i-temp i30 heads for the TRV and also the Evohome the latter is clearly the best but expensive the former not as good as it does not turn boiler on/off but with the wall thermostat set it will work much better than standard wax TRV's however for room with wall thermostat you want the TRV to have a high hysteresis worse then the wall thermostat, so don't use eTRV heads in room with room thermostat.

There are exceptions, my house is open plan and the TRV stops upper rooms over heating but still controlled by room thermostat down stairs, the boiler is not condensation type so it will not vary flame height, and the Myson radiator circulates the air.
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