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Thread: Evohome cylinder strap on sensor incompatible ?

  1. #1
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    Default Evohome cylinder strap on sensor incompatible ?

    Hi All,

    I decided to go ahead with a gravity circulation (plus immerser) to S-Plan conversion earlier than I originally planned as discussed in other threads.

    I have the hot water kit and have been doing some preliminary testing before the real conversion work begins later this week, however I seem to have hit a stumbling block. The cylinder is a vented, 95 litre combined indirect loop and immerser system, currently connected in a gravity loop configuration - eg very slow to reheat and requires the boiler temperature set very high to get any gravity circulation....so we tend to use the immerser a lot, especially in summer.

    Just so I could play around with the evohome hot water configuration I went ahead and installed the strap on sensor on the cylinder - I cut back a section of insulation about 1/4 of the way up as documented, cleaned the copper until it was nice and shiny then fitted the sensor and strap. The sensor is bound and the relay is connected to a test power lead so I could finish the binding process.

    I thought a good test would be to turn on the immerser and see what the reading went up to - well, it went up to 34 degrees and didn't go any higher, even though after an hour turned on the hot water coming out of the tap was easily 50+ degrees.

    Anyone have any ideas ? Is it possible that the cylinder has a double skin, and therefore the outside copper layer is not in direct contact with the hot water, preventing me from getting a meaningful reading ? When I remove the sensor and touch the copper it feels cool (less than body temperature) to the touch, so 34 degrees seems plausible given how cool it feels.

    Or could it be that the location of the immerser element near the top prevents much heat transfer to the casing of the cylinder low down where the sensor is installed, and that heating via the indirect loop which I believe is nearer the bottom would heat the bottom of the cylinder much more ? (Grasping at straws here)

    If it is double skinned I guess that means an insertion sensor would be necessary - there is an insertion hole through the top for the thermostat for the electrical element that could potentially be used, however (a) this would disable the immerser not leaving it available as a backup (not a big deal if the new configuration performs much better) and (b) the insertion sensor provided with the hot water kit looks bigger diameter than the existing thermostat.

    I'd have to pull the existing thermostat out and measure it to be sure of that, I have replaced it in the past however I don't recall exactly what size it was. If the insertion hole is too small for the sensor I'm kind of screwed even though I've ordered all the hardware to do the S-Plan conversion ?

    Another question - is it normal for the hot water sensor reading on the controller to only update once every 8-10 minutes ? This seems very infrequent if you had a quick re-heat cylinder - it seems that overshooting the set temperature would be a problem if it could continue to heat for another 10 minutes after the set temperature had been reached ?

    This is not a quick re-heat cylinder but it seems strange to me that HR92's would report their temperature every 4 minutes but the hot water sensor only every 8 minutes.
    Last edited by DBMandrake; 17th October 2016 at 10:45 PM.

  2. #2
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    The HW sensor is slow to report to the controller; that said, I have no boiler overrun set and I have never seen the cylinder temp rise above the set target temperature.
    Last edited by HenGus; 18th October 2016 at 10:09 AM.

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    Automated Home Legend paulockenden's Avatar
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    I think you are seeing stratification in your tank.

    You'd think the water inside would all be at much the same temperature (perhaps with a slight gradient), but in fact a region of hot water will sit on top of a region of cooler water. There's a small layer known as the 'mixing layer' between the two.

    And it's actually a GOOD thing. It means that as you draw water from the tank, the hot water output (from the top) remains hot, rather than getting diluted with the incoming cold water.



    So if your immersion (the term 'immerser' always confuses me!) sits at the top of tank, above the sensor, then the hot layer probably doesn't stretch down far enough.

    Does that make sense?

    P.
    Last edited by paulockenden; 18th October 2016 at 09:45 AM.

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    Which is why in many cases, atleast in an unvented cylinder, the temperature sensor is placed in about the middle. And some big unvented cylinders have the ability to put in two immersion heaters, one towards the top and the other towards the bottom.

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    Automated Home Legend paulockenden's Avatar
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    Some people (especially those trying to maximise the use of solar panels) also install a small mixer pump between the hot output and cold feed. If you power it on for a few minutes it breaks the stratification, and mixes the hot and cold regions of the tank.

    If you google 'destratification pump' you should find a few examples.

    P.

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    Quote Originally Posted by paulockenden View Post
    I think you are seeing stratification in your tank.

    You'd think the water inside would all be at much the same temperature (perhaps with a slight gradient), but in fact a region of hot water will sit on top of a region of cooler water. There's a small layer known as the 'mixing layer' between the two.

    And it's actually a GOOD thing. It means that as you draw water from the tank, the hot water output (from the top) remains hot, rather than getting diluted with the incoming cold water.



    So if your immersion (the term 'immerser' always confuses me!) sits at the top of tank, above the sensor, then the hot layer probably doesn't stretch down far enough.

    Does that make sense?

    P.
    Hi Paul,

    I think you're spot on. I had completely forgotten about the effects of convection and the stratification that can occur if you don't have convection. In a conventional electric only cylinder (more common overseas) the element is always at the bottom like a kettle so you get convection that heats the entire contents of the cylinder, as the heated water rises to the top displacing the cold water to the bottom where it too gets heated, until everything is hot.

    The immersion heater in this cylinder is mounted from the top and I hadn't realised just how short they are until I googled some images of the elements - you're quite right, the end of the immersion heater would be well above the half way point on the cylinder and therefore quite a bit higher than where the strap on sensor is mounted. (Honeywell recommend the bottom of the strap on sensor is about 1/4 to 1/3 of the way up the cylinder, so its relatively low down)

    So when you switch on the immersion heater at the top there is no convection between the bottom and top of the cylinder, thus the top 1/3 or so gets hot and the rest at the bottom stays more or less cold - makes perfect sense in hindsight.

    It also explains why we have not been able to fill a bath successfully ever since we started using the immersion heater as the primary heat source for the cylinder - only the top 1/3 or so of the cylinder will be hot thus the effective hot water capacity is greatly reduced.

    So not only should the conversion to S-Plan bring lower cost, greater efficiency and speed in heating the water, it should (assuming the indirect coils are near the bottom, which I can only assume they are) increase our effective stored hot water capacity to 2x to 3x what we are getting with the immersion heater due to the entire tank heating up.

    Last night as a test I turned the boiler flow up to 75 (normally 65 at this time of year) to try to encourage the gravity circulation loop to heat the cylinder to see what reading I could get on the theory that it would heat the whole cylinder including the stat area - when I got up this morning the hot water was more or less cold and the indirect pipes to the cylinder were both cold - for some reason gravity circulation sometimes struggles to "kick off".

    Once it starts circulating it will keep circulating with the feed pipe getting too hot to touch and the bottom pipe staying cold for a long time, however I have noticed that its a bit hit and miss whether circulation will actually start even with a high boiler temperature... probably because there is too much hydraulic resistance in the indirect pipes which are only 22mm instead of 28mm and there are a couple of right angle bends at T joints that won't be helping either...

    Converting the gravity loop to pumped S plan should be night and day for hot water performance even with 22mm piping and an older cylinder.
    Last edited by DBMandrake; 18th October 2016 at 03:19 PM.

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    Automated Home Legend paulockenden's Avatar
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    Probably worth buying a longer immersion heater too!

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    Quote Originally Posted by paulockenden View Post
    Probably worth buying a longer immersion heater too!
    It's all money though isn't it

    There isn't actually any room for a longer cylinder in the boiler room as the cylinder is just mounted above the boiler and not in the loft.. and depending on the design inside the cylinder it may not be possible to fit a longer than usual immersion element. The whole point of the exercise is I don't want to be using the immersion heater at all on a regular basis - it's purely there to be a backup in an emergency, and it has only been "abused" as the primary heat source due to inadequacy of the gravity loop.

    The irony is the gravity loop worked much better before I installed Evohome because the wall stat only turned off the pump and left the boiler on, and there was no pump overrun. It still took a long time to heat the water though (over an hour) due to the constant cycling of the boiler in those conditions as the gravity loop couldn't transfer heat fast enough.

    With Evohome and TPI the boiler is on so little during periods of partial demand and with a much lower average flow temperature such that there simply isn't enough boiler output for the gravity loop to "piggyback" on other than during the depths of winter, and pump overrun tends to sap any latent heat that the gravity loop could make use of. So I decided it was time to do an S-Plan conversion (something I can do for myself) which should solve many problems in one swoop including getting a good quantity of quicker heating hot water year round, independently of central heating demand and being able to schedule and monitor it.
    Last edited by DBMandrake; 18th October 2016 at 03:48 PM.

  9. #9
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    If you're doing this vented, or unvented?

    If the former, be sure you put the link to your supply tank and the vent pipe on the correct sides of the pump. Else you can end up with horrible siphon effects, sucking air into your heating loop. You'll find lots of info about this (particularly from people who suffer from this problem) if you Google it.

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    Quote Originally Posted by paulockenden View Post
    If you're doing this vented, or unvented?

    If the former, be sure you put the link to your supply tank and the vent pipe on the correct sides of the pump. Else you can end up with horrible siphon effects, sucking air into your heating loop. You'll find lots of info about this (particularly from people who suffer from this problem) if you Google it.
    Can you elaborate ?

    The current configuration is the rear left hand side bottom and top tappings on the boiler go up to the header tank and vent, those vertical runs to header and vent have T joints and from the T joint there are short horizontal links to the indirect loop on the cylinder.

    The front right top and bottom tappings on the boiler go to the pump and return from the radiator manifolds as well as the automatic bypass valve. My intention is to cap the side of the T joints that currently feed the indirect loop from the header and vent risers, but they will be otherwise untouched.

    The indirect loop of the cylinder will then be fed from the output of the pump via a zone valve and the return will join the return pipe near the ABV return.

    Before you made this comment I was considering just leaving the return of the cylinder indirect loop where it is - into the T joint on the header tank riser, as both this and the central heating return go into different taps on the boiler but both are bottom taps so I would have though this would be OK ? Are you saying this may interfere with the level in the header ? It's a bit awkward to join into the central heating return pipe instead but I will if I have to.

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