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Thread: S-Plan vs Y-Plan for Evohome ?

  1. #101
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    I use a VR40 to drive an external pump. Boiler pump doesn't have enough head for my system, so I have a LLH and a pump on the secondary side which needs to run when the primary side pump runs. Works well enough. Be aware that it gets installed inside the electrical compartment of the boiler. A GasSafe engineer will be along shortly to tell you that you shouldn't do this yourself because reasons.

  2. #102
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    When relying on the Frost Protection of the devices, you need to also think about pipe runs that drop below the room temp. e.g. ones in the loft or under the floors. The rad stats and boiler will only protect themselves and not the entire system.

  3. #103
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    Quote Originally Posted by bruce_miranda View Post
    When relying on the Frost Protection of the devices, you need to also think about pipe runs that drop below the room temp. e.g. ones in the loft or under the floors. The rad stats and boiler will only protect themselves and not the entire system.
    Indeed, and this is one limitation of an Evohome system - you can frost protect the boiler using a boilers built in frost stat (or 3rd party frost stat as in my case) and Evohome automatically frost protects the rooms themselves as HR92's will open and call for heat if the room gets below 5C. (Unless you set them to the OFF position on the HR92 - so don't do that...)

    But you can't frost protect the pipes because unless the rooms are below 5C the HR92's won't open therefore no flow through the pipes. In contrast a conventional system with manual TRV's will generally have the TRV's set to normal room temperatures when the system is scheduled "off", so the TRV's are open and ready to flow should a frost stat somewhere in the system fire the boiler.

    I haven't worked out an elegant solution to this problem. Possibly the only way would be to have a frost stat under the floor and/or in the loft which then sends some sort of wireless signal to schedule the evohome to run it's normal schedule once every few hours, but not great, especially if it relied on the unreliable internet API! (The only other way would be Domoticz and an HGI80, but would you trust frost protection to a Raspberry Pi ?)

    A specific crawl/loft space wireless frost protection sensor for the evohome system that could put the system into an active frost protection mode where it periodically gives a few minutes of warm water circulation through all zones that aren't already on is certainly a feature that Honeywell could add, but demand is probably low as most people don't realise the risk until it bites them.
    Last edited by DBMandrake; 3rd December 2018 at 09:58 AM.

  4. #104
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    What may also help this situation would be a custom menu which you select when going away and it is configured to bring the heating on at a designated temperature for the whole house once a day for a short time

  5. #105
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    The problem with that idea is the "designated temperature" is going to be room temperature.... If the house is well insulated and the room temperatures have not dropped low enough there won't be a call for heat and the pipes under floor may still freeze.

    Ideally the way it would work is when there is measured risk of freezing around pipes, the system would schedule ALL HR92's to be on, (set set point to HR92's to maximum) but only call for a low flow temperature from the boiler, then shut off again after say 30 minutes.

  6. #106
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    Quote Originally Posted by DBMandrake View Post
    But you can't frost protect the pipes because unless the rooms are below 5C the HR92's won't open therefore no flow through the pipes. In contrast a conventional system with manual TRV's will generally have the TRV's set to normal room temperatures when the system is scheduled "off", so the TRV's are open and ready to flow should a frost stat somewhere in the system fire the boiler.

    I haven't worked out an elegant solution to this problem.
    I think you have. Because if the boiler does it's frost thing, then the pump-controlled zone valve will open, and you'll get defrosted pipes. If you're on a recommended Fig 4 set up, same thing. If an HR92 demands frost protection, the whole shebang needs to circulate, and the pipes will be fine too, irrespective of whether the rest are open or closed. The worst case scenario is the ABV on the heating circuit will bypass (which is why the ABV should be in the longest/furthest away heating loop).

    In any case I feel the issue is moot. Who would have a setback temperature with evohome, even if away from home of less than 8-10C? To allow a building to fall to such a low temp you're going to have many other problems with heat expansion/contraction (e.g. plaster cracking, brick movement), and the only time I can think of - when doing building works - you'd probably have drained the system.
    Last edited by jvallis; 3rd December 2018 at 12:38 PM.

  7. #107
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    Quote Originally Posted by jvallis View Post
    I think you have. Because if the boiler does it's frost thing, then the pump-controlled zone valve will open, and you'll get defrosted pipes.
    No, because if all HR92's are closed there will be no flow through the pipes beyond the automatic bypass valve whether the zone valve is open or not. Water can't flow if there isn't a complete loop.

    Also in a standard S-plan configuration the heating zone valve will NOT open if the internal frost protect in a boiler triggers. However in my modified S-Plan configuration which tries to keep the heating zone valve open most of the time it would.
    If an HR92 demands frost protection, the whole shebang needs to circulate, and the pipes will be fine too, irrespective of whether the rest are open or closed.
    Which pipes ? Only the branches that are actually flowing will heat up, so depending on the topology many of the pipes will remain cold. For example in my system there is a 22mm manifold from the boiler that runs under the floor to a central point and there is then a star configuration from the end of the manifold to each individual radiator. Upstairs is handled by a completely different branch and doesn't go via the main underfloor manifold.

    So a single HR92 being on would ensure the 22mm mainfold was protected, but all the branches to the other radiators would remain cold, and some of them are many metres long. The only way to fully frost protect pipes is for all TRV's to flow in frost protect mode.
    The worst case scenario is the ABV on the heating circuit will bypass (which is why the ABV should be in the longest/furthest away heating loop).
    How many installations have their ABV anywhere other than the boiler closet, realistically ? In my installation it would need to be in the middle of the house under the floor in a very inaccessible position. No thanks. Not to mention that the ABV has to be installed before a heating zone valve, which is often in the boiler closet as well.
    In any case I feel the issue is moot. Who would have a setback temperature with evohome, even if away from home of less than 8-10C? To allow a building to fall to such a low temp you're going to have many other problems with heat expansion/contraction (e.g. plaster cracking, brick movement), and the only time I can think of - when doing building works - you'd probably have drained the system.
    You're confusing room temperature with under floor temperature.

    We have a raised floor with about a 1 metre ventilated crawl space under the house with pipes running willy nilly in the crawl space. On a cold windy winter night it can get very cold under there - much colder than inside even unheated rooms.

  8. #108
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    Quote Originally Posted by DBMandrake View Post
    You're confusing room temperature with under floor temperature.

    We have a raised floor with about a 1 metre ventilated crawl space under the house with pipes running willy nilly in the crawl space. On a cold windy winter night it can get very cold under there - much colder than inside even unheated rooms.
    If I were you, I'd get those pipes lagged. Also for efficiency, so that you can further keep your flow temps lower.

  9. #109
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    Quote Originally Posted by jvallis View Post
    If I were you, I'd get those pipes lagged. Also for efficiency, so that you can further keep your flow temps lower.
    I've lagged quite a few of them a couple of winters ago, but some of them are REALLY difficult and unpleasant to get to as it means crawling through very tight holes in the brick support walls under the floors and.... well... I'm not as skinny as I once was and nearly got stuck in one last time... As they are long runs of 8mm microbore some of them are also quite twisted and wriggly so don't lend themselves well to standard foam pipe lagging without spending a lot of time in difficult locations trying to straighten the kinks out a bit. I intend to grit my teeth and do some more next summer though.

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