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Thread: Is this true with Evohome?

  1. #1
    Automated Home Legend
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    Default Is this true with Evohome?

    Just seen this post on another forum:


    Quote: There are a number of people who have properties that are unoccupied for long periods in the winter, e.g. holiday homes, people who spend some of the winter abroad, between sales/lettings etc.

    In my case I have a little used detached annex(converted stables) that is fully gas centrally heated with a combi boiler. For the past 25+ years I have set the thermostat to ‘frost Stat’ and when temperature dropped to 5C the boiler fired up, CH came on and all was well.

    Last year I replaced the boiler with a new Veissmann combi boiler; obviously a condensing boiler and again set it to ‘Frost protection’ which the manual tells me kicks in at 5C. Because the property uses so little gas/electricity I rarely check the meter readings. When a few weeks ago I did check, there had been a big jump in electricity consumption – but not gas. When I checked the room temperature was 10C, as naturally was the displayed water temperature, however the internal pump in the boiler was running – but the boiler not firing. I checked over the next couple of days and it was clear that the pump was running 24/7.

    Spoke to my plumber, who assumed a fault but could find nothing wrong. He in turn spoke to the manufacturer who contacted Germany. To cut a long story short it transpires that, on any setting, when the water temperature drops to 8C the pump starts up and it will run continuously until the water temperature reaches 12C. With the boiler not firing this means the pump will run 24/7 all winter. Even now the temperature around 10C internally.

    The factory in Germany added that all(most?) manufacturers now use exactly the same software package, so all newish boilers will be the same.(I hesitate to call it a fault).


    On MSE we get quite a few cases where the old occupants give closing meter readings, and new occupants moving in some weeks later dispute those readings - could this account for some of those cases??


    I now switch off the boiler power unless the forecast is really cold, and put it on every few weeks for an hour or so. Unquote

    Would this be possible with any of the Quick Actions with Evohome? Is the above scenario accurate?

  2. #2
    Automated Home Legend paulockenden's Avatar
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    That's ridiculous. It seems as if they are trying to get water flowing to avoid it freezing in the pipes, but if the TRVs are shut the water won't flow anyway, and the pump will just be fighting the ABV (always assuming one is fitted).

    Oh, and if it's a boiler function I can't see what Evohome could do to stop it.

  3. #3
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    It's not on continuously according to the 100W manual:

    Frost protection

    Frost protection is continuously active. The burner is switched ON when the boiler water temperature reaches 41° F (5° C) and is switched OFF again when the boiler water temperature reaches at least 59° F (15° C) but not more than 68° F (20° C).

    As you say, the pump would be continually fighting against closed TRVs.

  4. #4
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    Boiler frost protection only really protects the boiler from freezing. It's not meant to protect the rest of the system because TRV's may be closed, either manually (a lot of people still don't understand the difference between * and 0 on conventional TRV's) or because you have something like an HR92 that won't open until it's room is below 5 degrees. If it does protect rooms (dependent on whether you have a heating zone valve, what kind of TRV's you have etc) then it's a bonus not an expectation.

    This past winter I fitted a Danfoss frost stat in the boiler closet as my ancient boiler naturally doesn't have any form of frost stat and we were going to be away for several days. I can adjust it between 3 and 10 degrees and have it set to 8 degrees. It's simply wired in parallel with the boiler relay BDR91 so it fires the boiler which in turn runs the pump. I didn't bother to fit a return pipe flow stat because the boiler is in a small enclosed cupboard of its own which means that it doesn't have to run for long before the cupboard would warm up beyond 8 degrees.

    If it triggers then ordinarily the heating zone valve wouldn't be opened (since that's a separate BDR91) but due to my HW/CH wiring arrangement discussed in another thread the heating zone valve will actually open without the CH BDR91 turning on, which would protect any manual TRV radiators, but currently there is only one in the house - all the others are HR92's, so most of the flow would still be through the ABV and thus warms the boiler and not much else.

    If any individual room fell below 5 degrees it would fire the boiler for that room through the ordinary Evohome mechanisms of course.

    By the way I fully understand why the boiler would run the pump at a slightly higher temperature than it fires the boiler - if the water pipe in the boiler drops below 8 degrees while the pump is stopped there's a very good chance that the pipes under the house (or in a loft space) are much colder than 8 degrees and possibly very close to freezing.

    Assuming you did have manual TRV's which were not set to the fully off position, as soon as the pump started running it would warm the exposed pipe work using the latent heat from the radiators - and would tend to equalise any difference in temperatures between interior and exterior pipework and prevent freezing. It also means the boiler can sense the true temperature of all the pipework in the house and thus know that when it reads 5 degrees that all the pipework is really at 5 degrees and its time to fire up.

    I thought about an almost identical scheme myself about a year ago (where the air temperature frost stat alone is enough to run the pump, while both the air temperature and return flow stat are required to fire the boiler) however I abandoned it because there is no way to integrate this with a system using HR92's because they will not open unless their own temperature reading inside the room is below 5 degrees.

    A slightly less power hungry method that they could have used would be to run the pump for 5 minutes out of every 30 or so when the return flow is below 8 degrees and above 5 degrees to keep the temperatures equalised and allow sensing of the remote pipe temperatures, but save a bit of power. Below 5 degrees the pump would run continuously and so would the boiler.
    Last edited by DBMandrake; 26th March 2017 at 08:42 PM.

  5. #5

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    evohome has frost protection at all sensor points @ 5 degrees. (including the evohome unit not just HR92) - your boiler may or may not have something as you describe.

    We have seen issues where incorrect wiring has made the circulating pump be continuous. (nothing to do with evohome, just do to the install process someone wrongly placing a signal wire) I am sure you have checked but just saying.
    getconnected.honeywell.com | I work for Honeywell. Any posts I make are purely to help if I can. Any personal views expressed are my own

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