Results 1 to 10 of 23

Thread: How to frost protect an Evohome system ?

Threaded View

Previous Post Previous Post   Next Post Next Post
  1. #1
    Automated Home Legend
    Join Date
    Sep 2014
    Location
    Scotland
    Posts
    2,361

    Default How to frost protect an Evohome system ?

    This might sound like a silly question when we all know that Evohome schedules zones to 5 degrees when "off" and that if any zone drops below 5 degrees the boiler will still come on even in heating off mode and that particular radiator will open and warm up to keep the room above 5 degrees.

    But there are two aspects of frost protection that this does not solve - frost protecting the boiler, and frost protecting underfloor pipes in a crawl space that are potentially a lot colder than inside a room where an HR92 is measuring and may suffer from wind chill effects if the crawl space is ventilated.

    Most boilers these days have built in frost protection that measures the air temperature inside the boiler casing and if it drops below 5 degrees the boiler will come on and keep running until the return flow pipe reaches about 30 degrees then shut off again. My boiler is ancient and has no such protection, however it is quite easy to retrofit a frost protect kit consisting of a a 5 degree air temperature sensor mounted in the boiler cupboard (usually electronic to make sure it is accurate, such as a Drayton RTS3) connected in series with a pipe stat attached to the return flow.

    The idea being that you only want the boiler to fire if there is a risk of freezing - eg air temperature below 5 degrees, but you don't want it to keep running until the room its in warms up above 5 degrees (which might never happen if it was in a garage) its sufficient for it to run until the return flow reaches about 30 degrees - enough to warm up the boiler and pipes but not enough to put unwanted heat into the house or run unnecessarily.

    With such a system you could add an additional air temp sensor in your crawl space (in parallel with the one in the boiler closet) so that if the crawl space dipped below 5 degrees the boiler would also run until the return flow reached 30 degrees.

    Right away we have a problem with the HR92's, which will prevent this traditional scheme from working...

    When the house is scheduled off with a conventional system, as the rooms cool the TRV's will all open well before the rooms get down to a temperature where freezing is a concern, but because the timer keeps the boiler off no heat is put out. Thus when a frost protect system kicks in (through detecting <5 degrees either in the crawl space or in the boiler closet) and fires the boiler by bypassing the timer all the TRV's in the house will already be wide open thus the flow of warm water will pass through all pipes and radiators warming them all up to 30 degrees (or whatever the return pipe stat is set to) which is enough to prevent freezing but without wasting too much heat or warming the house up. This will protect both boiler and underfloor pipes.

    But with HR92's unless they each individually detect their room has dropped below 5 degrees the valve will remain firmly shut so any frost protect system will only be able to protect the boiler and not crawl space piping as none of the pipes will be able to flow. Has anyone thought about a solution to this problem ? Or do you all just have well insulated pipes ? (I don't! )

    With an S-Plan system you have the additional complication that something has to open the heating zone valve when frost protect kicks in otherwise the water wouldn't be able to flow through the radiators and pipes anyway... with a traditional S-Plan two relay config you could wire the (external) frost protect system to open the heating zone valve which would in turn fire the boiler, but on my three BDR relay config I have a separate relay firing the boiler which complicates matters considerably. As now I need yet another relay - a DPST as most frost stats have only one switched live output which would have to independently run the heating zone valve and boiler. Not that it matters anyway because even if I open the zone valve the HR92's will be closed!

    At the moment I've resigned myself to the idea that with HR92's frost protecting the crawl space piping electronically is not feasible as I would not be able to get them flowing, so adding insulation is my only option... I can frost protect the boiler which I will do, but all that will do is warm up the boiler and boiler closet, it won't help with the underfloor pipes.

    Does anyone else worry about frost protection of pipes under the floor in particular or is it just me!
    Last edited by DBMandrake; 21st November 2016 at 01:58 PM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •