Honeywell Thermostat faulty

Discussion in 'Plumbers' Talk' started by pal123, Feb 3, 2021.

  1. pal123

    pal123 Member

    The Honeywell Indicator Room Thermostat T6360 T6360b1028 (Ebay: https://www.ebay.co.uk/p/1804523840?iid=160931970480) is no longer setting the temperature for the boiler. The boiler is a Worcester Combi 241 Junior.

    I am assumingthat the thermostat is faulty. I do have, lying around, a spare salus digistat rt500bc (https://www.plumbarena.co.uk/product/salus-rt500bc-programmable-room-thermostat/). Could I either 1) Replace the existing thermostat or 2) Replace the existing thermostat with the Salus digistat?
     
  2. MGW

    MGW Screwfix Select

    Both are on/off thermostats and the replacement has volt free contacts, so yes you can swap, but question is if there is a neutral going to the old one? The old one should have a neutral as it has a anti-hysteresis resistor, but it is common not to connect the neutral, which is not necessary bad.

    There are three reasons for a thermostat, one frost protection, two anti cycling, three set room temperature. Years ago we used the wall thermostat to set room temperature, but then the condensate boiler arrived, this type of boiler needs the return water cool, or it will not work efficiently. So two ways to control is either the modulating wall thermostat connected to boiler ebus, or the TRV head on the radiator.

    The TRV head however has a problem, it can't turn the boiler off, so it would cycle on/off all summer unless either manually turned off or you have an on/off wall thermostat, so in winter wall thermostat does nothing it is on all the time, but on a warm day it switches off stopping the boiler from switching on/off (cycling) all the time, and the large droop between on and off temperatures is good.

    However one the TRV fitted as standard is often a bit hit and miss, and two owners often don't realise they should be using the TRV as prime temperature control. No wonder when they are marked *123456 that does not make it easy to set temperature required, the eQ-3 and terrier i30 are battery powered electronic programmable replacement heads for the majority of TRV bases, they come with a selection of fittings. When I got the bluetooth eQ-3 they were £15 each, non bluetooth were £10 each, but it seems the price has jumped, likely due to leaving common market.

    The problem is as you move to the electronic wall thermostat, they tend to have built in anti-hysteresis software (the sine wave of temperature caused by using on/off control) this software is great with an oil boiler that only turns off/on, but with modern gas they should not be turned off/on but allowed to modulate (turn down) as the home warms up so they can gain the latent heat from the flue gases. And mid price range wireless tend to use a mark/space ratio (switches off/on slowly reducing on time) to reduce the hysteresis, and this stops the boiler from gaining the latent heat from flue gases.

    The higher price range either uses algorithms to work out when to switch off, or uses linked TRV heads, former method used by Nest for example and latter method used by Hive for example. However as to if these high end systems actually save enough energy to be worth fitting is in question, the connection to ebus is better but Worcester Bosch are like Apple computers, they don't allow use of third party controls. So with Worcester Bosch no real option have to use their own special thermostat or control by return water temperature, and Hive it seems is ideally suited for Worcester Bosch in the way the wall thermostat links to TRV heads.

    Anyway the fact remains that many mechanical wall thermostat don't have the neutral connected, and for new thermostat you will need a neutral, so it will need some testing first.
     
  3. pal123

    pal123 Member

    Detailed reply thanks. The boiler uses mains gas and is a condensate boiler. All the boilers have manual (not smart) TRV heads. Are you sure Worcester Bosch don't allow 3rd party controls? So the Salus Digistat is not compatible with the Worcester Bosch boiler?

    Why would a like for like replacement hermostat (in case of 1) need a neutral? Yes (2-Salus Digistat or a compatible type) would need a neutral
     
  4. pal123

    pal123 Member

    No worries a boilerman has replaced the thermostat like forlike. Thanks anyway
     

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