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Thread: New unvented cylinder - nowhere for Evohome Probe :(

  1. #1
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    Default New unvented cylinder - nowhere for Evohome Probe :(

    Hi all,

    My parents have been using Evohome for over a year now, with great success. But now they are having their hot water system converted from gravity feed to mains pressure. The unvented cylinder is a Warmflow IN250UV (250L unvented indirect), here is a schematic of the tank:

    Warmflow.jpg

    I have both the strap on sensor and the insertion sensor, but there are no available wells. Where could we attach the temp sensor?

    The plumber doing the install has never used Evohome, but he seems to think that as the secondary return isn't being used, instead of blanking it off he can get a 22mm "probe pocket" that would allow the probe to be inserted there, something like this:

    http://www.sensorshop.co.uk/Pocket-6...h=thermopocket

    150mm2-500x500.jpg

    Does that sound feasible? More to the point, would it be safe?

    Thanks in advance!

    Gareth
    Last edited by garmcqui; 26th October 2016 at 09:45 AM.

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    You need to investigate whether the Primary Coil Thermostat can be removed/replaced. If not then there is no easy way to use the Evohome hot water control.

    Looking at the manual, the thermostat is a straight-forward on/off type known as single-pole double-throw where the pole looks to be terminal B and terminals and 2 and 3 are the change-over contacts, operating like this:

    B-3 connected when demand has been met
    B-2 connected when demand has not been met

    Perhaps call the manufacturer the manual does not say if the thermostat can be removed, but I'm sure it can for replacement following failure.

  3. #3
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    Thanks for replying,

    So you suggest adding the Evohome insertion probe to the same insertion pocket as the existing thermostat? I am assuming the existing thermostat must remain in place?

    Would the plumber's suggestion of converting the unused secondary return to a pocket to house the probe not work?

  4. #4
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    My unvented cylinder manufacturer, Oso, said that it was imperative under G3 regulations that the tank's temp stat and top limiter were not tampered with; consequently, the HW kit should be installed in series with these two safety devices. Again on the manufacturer's recommendation, my installer accessed the side of the tank via the electrical connection panel. The probe was pushed up the tank under the foam installation. If this is done at a slight angle and paste is used then the probe tank contact is good and there is little chance of the probe dropping.

    My HW kit is set at 57 to 62C with the tank stat slightly higher. The top limiter is pre-set at 85 +/-3C.

    FWiW, as no one has said otherwise, if the HW kit fails (e.g. loss of comms to the HW sensor box) - and the HW demand is ON - then the tank will keep on heating. If you remove the tank stat then the top limiter is the sole safety device.

    https://www.youtube.com/watch?v=61uDmQF5I2E
    Last edited by HenGus; 26th October 2016 at 12:41 PM.

  5. #5
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    That video is dramatic, but surely that's only likely to happen if using an immersion heater. When heating using a boiler you're going to be limited by the maximum flow temp of the boiler itself, and that shouldn't be enough to cause a tank explosion.

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    Quote Originally Posted by HenGus View Post
    FWiW, as no one has said otherwise, if the HW kit fails (e.g. loss of comms to the HW sensor box) - and the HW demand is ON - then the tank will keep on heating. If you remove the tank stat then the top limiter is the sole safety device.
    Absolutely agreed. Do not rely entirely on the Evohome hot water kit as the only way to limit the temperature on an unvented cylinder. It would be unsafe and no doubt contravene safety regulations.

    I have recently installed the Evohome hot water kit on an indirect heated vented cylinder, and as discussed in another thread in the few days I've been using it I've had one occasion already where the sensor failed to report that the tank had reached my set temperature of 50 degrees and continued heating for at least 10 more minutes. In fact it had only stopped heating because I was monitoring the operation of the system at the time and intervened to stop it, discovering that the tank was already up to 58 degrees. On a vented tank there is no explosion danger to the tank but there was still a scalding risk for users of hot water had it continued much further.

    So as HenGus suggests the original stats, both the high limit and normal stat should not be removed or bypassed in any way. Connect the BDR91 in series with the normal stat and set the normal cylinder stat to about 5 degrees higher than your desired hot water temperature and configure your desired hot water temperature in the Evohome configuration.

    Under normal conditions the Evohome will cut off the supply to the hot water zone valve when the target temperature is met however in the unlikely event that it does not cut it off in a timely fashion (due to a wireless comms failure for example) then the normal cylinder stat will cut it off 5 degrees higher.

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    Quote Originally Posted by garmcqui View Post
    The plumber doing the install has never used Evohome, but he seems to think that as the secondary return isn't being used, instead of blanking it off he can get a 22mm "probe pocket" that would allow the probe to be inserted there, something like this:

    http://www.sensorshop.co.uk/Pocket-6...h=thermopocket

    150mm2-500x500.jpg
    6mm is too small, needs to be: 6.62mm according to my digital caliper, if you use that one, then perhaps drill out the hole to 7mm to give a small amount of clearance. Like Paul says, the probability of the tank overheating is negligible or no more than any other tank design because the boiler water temperature would be the limiting factor and that can't get hot enough to create a damaging problem IMO.

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    It's true that on an indirect cylinder that it can't heat any hotter than the boiler flow temperature (which shouldn't really be any higher than 80, even for hot water reheat IMHO) so there won't be damage to the cylinder, however 80 degrees is 3rd degree burn temperatures...

    So for safety of users of the hot water (especially children) there should really be some kind of safety thermostat set to 60-65 at most that does not rely on a wireless connection.

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    Agreed and it is the installers responsibility to ensure a safe system exists when they hand it over, unless it's a DIY install but even then it is prudent to ensure safety is observed in all modifications and at all times.

  10. #10
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    Thanks for all your replies.

    The new cylinder is in and working perfectly...

    IMG_3258.jpg IMG_3255.jpg IMG_3256.jpg

    The Evohome insertion probe will have to go into the thermostat pocket on the top left of the cylinder, unfortunately this means it will be measuring the warmest part but hey ho!

    Another question - as you can see there is a zone valve (came with the cylinder) attached to the feed for the heating coil, which is so the cylinder thermostat can shut off the supply for safety. The problem is, there is currently another zone valve on that feed which is controlled by a BDR91 and shut off when DHW temp is reached, but it is next to the oil boiler at the other end of the house (~50m away).

    As we are using Evohome to control the DHW temperature, the cylinder thermostat is going to be left at 65C (max temp), so the installer says we can either:

    a) not use the zone valve in the loft, leave it manually opened and run a cable the ~50m to connect the cylinder thermostat to the zone valve already in use, so it is in series with the Evohome BDR91.

    b) connect the tank thermostat to the zone valve in the loft, and let evohome control the other zone valve at the boiler as normal.

    I'm leaning towards option B, as it's less hastle. Either way, the safety mechanism of the thermostat is intact, as either zone valve will isolate the feed to the coil if overheating occurs.

    What do we think?

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