How to frost protect an Evohome system ?

[DBMandrake]

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!

[MichaelD]

I haven't thought to worry about frost protection of pipes under the floor, but I do have a solution.

You could put a HR92 (or an old HR80) into the place you're concerned about, and set it to 5 degrees. Link it to the EvoHome controller, and it will still switch the boiler on, and keep it on until it reaches 5 degrees. It doesn't need to be connected to a radiator.

I accidentally tried this last night, was using an old, broken HR80 wrapped in clingfilm as an outside temperature sensor. It all worked great, until the outside temperature went below 5 degrees, and that 'outside sensor' switched on the boiler, and it wouldn't turn off until the outside temperature rose. Think I need to switch that HR80 to 'off'. So if you have some old HR80s, this is a great way to use them.

[DBMandrake]

I haven't thought to worry about frost protection of pipes under the floor, but I do have a solution.

You could put a HR92 (or an old HR80) into the place you're concerned about, and set it to 5 degrees. Link it to the EvoHome controller, and it will still switch the boiler on, and keep it on until it reaches 5 degrees. It doesn't need to be connected to a radiator.

I accidentally tried this last night, was using an old, broken HR80 wrapped in clingfilm as an outside temperature sensor. It all worked great, until the outside temperature went below 5 degrees, and that 'outside sensor' switched on the boiler, and it wouldn't turn off until the outside temperature rose. Think I need to switch that HR80 to 'off'. So if you have some old HR80s, this is a great way to use them.

Unfortunately that doesn't solve the problem. I could already wire a frost protect thermostat under the floor in the crawl space to fire the boiler and open the heating zone valve. The issue is that the HR92's controlling the radiators will not open to allow any flow through the radiators until the sensed room temperature of each room drops below 5 degrees, and I think it's pretty likely that inside an insulated room is going to be significantly warmer than a ventilated nearly 1 metre tall crawl space on a windy winter night... so the room might be 8 degrees but the pipes under the floor might be on the verge of freezing if there is no water flowing in them...

Even just running the pump without the boiler would help to prevent freezing as the pipes under the floors would "steal" heat from the radiators which would draw it from the rooms. As long as water continues to flow, the pipes under the floor cant get any colder than the radiators in the room...it would slightly accelerate the cooling of the rooms doing so but only by a tiny amount, and when the room cooled enough it would trigger in-room frost protection.

But without being able to get any flow through a closed HR92 I can't see any good way to do it.

[bruce_miranda]

I solved this in a round about way. I sense the external temperature and in the spaces I am concerned about e.g. loft, garage etc If any of those get dangerously low then I trigger my custom quick action that sets all the zones to 35C. This not just opens all the TRVs but gets the boiler fires and system circulated.

[DBMandrake]

I solved this in a round about way. I sense the external temperature and in the spaces I am concerned about e.g. loft, garage etc If any of those get dangerously low then I trigger my custom quick action that sets all the zones to 35C. This not just opens all the TRVs but gets the boiler fires and system circulated.

I'm assuming that requires a working internet connection or an HGI80 to implement as well ? If the former then I'm not too keen on the potential unreliability of that in adverse conditions...

I've been giving it quite a bit of thought and I can't come up with a clean solution at the moment. I'm going to install a frost stat in the boiler closet to at least protect the boiler from freezing, but that will just fire the boiler and let it circulate around the automatic bypass loop, (since it won't even open the heating zone valve - no point when the HR92's will be closed) it won't do anything for the piping in the loft or crawl space.

I have a big pile of pipe insulation that I bought for last winter that never got installed as I didn't muster up the enthusiasm to be crawling around in the dirt under the house, (hello spiders...) but this winter is looking like it will be quite a bit colder so I might just have to get moving there...

[bruce_miranda]

HGI80, but then that is reliant on my NAS running, which relys on electricity.
I'm glad I did away with the CH zone valve or rather took control of it away from Evohome (your suggestion infact) So now if my boiler fires, the pump and the CH zone valve will open. And while water may not flow through every radiator, it will atleast try to get going through the towel rails.

[DBMandrake]

HGI80, but then that is reliant on my NAS running, which relys on electricity.

Well, your boiler and central heating pump also require electricity. Although a brownout would be more likely to knock out a server temporarily whereas the boiler and pump would recover straight away and with considerably more certainty.

I'm glad I did away with the CH zone valve or rather took control of it away from Evohome (your suggestion infact) So now if my boiler fires, the pump and the CH zone valve will open. And while water may not flow through every radiator, it will atleast try to get going through the towel rails.

I could arrange with a bit of rewiring to open the central heating valve with the frost stat too but figured it probably wasn't worth the effort if HR92's were closed. If any HR92's open due to frost they will trigger the heating zone valve to open anyway.

[MichaelD]

Unfortunately that doesn't solve the problem. I could already wire a frost protect thermostat under the floor in the crawl space to fire the boiler and open the heating zone valve. The issue is that the HR92's controlling the radiators will not open to allow any flow through the radiators until the sensed room temperature of each room drops below 5 degrees ...

But without being able to get any flow through a closed HR92 I can't see any good way to do it.

Do you mean that each (all) rooms have to be below 5 deg for the boiler to fire? I'd have thought that any room below 5 would be enough.

By using a HR80/HR92 under the floor, EvoHome would believe that the underfloor area is a room, and would fire. It would also try to open the valve, but there wouldn't be a valve connected. The HR80 doesn't seem to mind this, I have a spare on my desk that whirrs away happily, but isn't attached to a radiator.

I guess the issue is where the bypass sits when the boiler is running and all the room valves are shut. If you have a bypass, or unmanaged radiator somewhere the other side of the pipes you're concerned about, then you'd be OK

[bruce_miranda]

I think the issue is when the zoned rooms are above the 5C mark but the pipework is sitting outside those zones in an area that might go below the 5C. In that case you want the water to circulate around the pipework but that won't happen if the CH valve is shut because none of the zones are calling for heat. Even if you put an HR80 in these spaces and they call for heat, the water isn't going to flow anywhere besides the bypass because all the HRxxs are shut. The best frost protection is a complete system circulation.

[cvenn]

How about an HRxx connected to a dummy 'null' radiator in those space(s)?