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Thread: Evohome without the hot water kit

  1. #1

    Default Evohome without the hot water kit

    Grateful for any thoughts/advice from those who might understand these things. I have a modern 5 bed house with a Classic FF280 boiler, 18 radiators (plus one bypass) and an OSO 250 Direct Unvented Cylinder. We have also installed a wood burner in a family room that tends to be our room of choice in the winter.

    The thought of having automated TRVs is attractive for a number of reasons (ie; a warm bedroom just before bed without having to heat all of the house).

    I am still accumulating quotes but one installer has said that it is unlikely that he will be able to fit the Honeywell hot water sensor to the OSO tank. I have asked OSO for advice. If this is the case, then I am just wasting my money by going for an Evohome system just to control 18 TRVs in 12 zones? Should I be looking at other home automation systems?

  2. #2


    You need a more experienced supplier/installer who knows how to deal with this :-)

    We use our own sensor elements which means we have much more flexibility in fitting options but if there is still no sensible way to fit a sensor on a pre-insulated cylinder (which can sometimes be the case) then we also have the means to control hot water by time only from Evo and have the temperature controlled by the existing cylinder stat.

    So hot water should not be a problem with Evo :-)

  3. #3


    Thanks. CH/HW timing is currently managed by a Tempus 7 (2/5 day controller). I imagine what one of the installers has in mind is using this to continue to control HW.

  4. #4


    Keeping the HW on a standard timeclock rather than under Evo control is fine, you'll just need to decide what to do if you then want to use Evo's away/holiday settings.
    Sensible Heat

  5. #5


    Things move on. I have now spoken to OSO (the manufacturer of my unvented tank) and their advice is that the HW sensor can be safely inserted under the insulation at the top of the front access patch. Problem solved...... ah no. Two Honeywell-approved installers have provided quotes stating that the two existing inline motorised valves can be used. The third is adamant that all radiators including the small by-pass radiator have to be TRV'd and the heating in-line valve needs to be replaced with a Honeywell automatic bypass. The difference in the cost of the 3 solutions is about 400 - broken down by extra costs for a wall mount for the controller, an extra TRV, a bypass valve and system inhibitor. I am not sure that the savings or the convenience justify a cost of 1900! I was wavering with the 2 quotes of 1500. Am I missing something?

  6. #6


    Are you not wanting to use either of the first two installers who do not want to install the automatic bypass? Unless you have had recent works carried out that change the plumbing then presumably the system has worked fine up to now without the automatic bypass valve fitted?

    I'm not sure what good the"bypass" radiator serves when it connected after the heating two-port zone valve - when the valve shuts due to no heating demand then it is no longer open to the boiler so cannot provide a path for water to flow during the pump run on (should your boiler require this).
    Sensible Heat

  7. #7


    I am not a plumber so my knowledge of how CH systems work is pretty sketchy. However, as regards my present system, I imagine that the hall thermostat controls the in-line valve and, in the unlikely event that all the 18 TRVs are closed and the CH system is both on at the control panel and on at the thermostat, then the HW has somewhere to go. In truth, I have never known a situation where all the TRVs were closed at the same time; i.e., never ever an occasion when just the bypass radiator was hot. I just had a helpful conversation with an Evohome tech (in Holland). Their view is that the specialist installer is right in that the Evohome system is designed to have all radiators TRVd. For safety, a bypass has to be fitted in case the Evohome controller doesn't close down the boiler when 'heat' is not required to one or more radiators. I could stick with my present configuration but it would mean that when there was any demand for heat then the bypass radiator would heat up. A small issue when heating periods are programmed but not that efficient when it is a individual radiator demand led system.

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