How to frost protect an Evohome system ?

[Little Tinker]

I was told that typically (as in most of UK for a few cold nights with reheat during day) it was the localised cooling effect of cold draughts blowing past that are most likely to lead to frozen pipes in loft or under floor than simply falling temperature. SOme areas will be more susceptible of course. It'd be unusual to to have totally unlagged pipes under floor, so I wouldn't overly worry about it. If you can actually crawl around underneath then make sure everything has at least some covering.

If I was worried and the house was occupied I'd set the HR92 downstairs to 8-10C, and expect that would fire boiler and circulation before harm was done. Or maybe a short blast to 20C at 3am on all the end of pipe run rads. If house was going to be unoccupied I'd remove the heads and open manually, so that circulation could pass by when boiler and pump fired.

Even though you have three BDR can you not still connect frost stat to CH valve "as normal" and use its orange wire as an alternate / parallel boiler and pump trigger?

If it is one particular area of crawl space troubling you rather than lots of branches on the pipes then you could maybe put a zone valve across the far end flow and return there (ie probably just under the furthest radiator). Bring power to it over 3+E cable via a nearby frost stat, connecting core 3 to the orange wire on that valve to run back and trigger the CH valve (which in turn can trigger boiler and pump).

[Little Tinker]

I'm wondering if the following might also be a contender. I think it's mentioned up thread that firing boiler may not be necessary as long as water can be circulated. A variation on the zone valve at far end approach would be...

1. Take boiler control BDR to boiler, and to white wire (NC of auxiliary switch) on remote valve. Bring grey wire (common on switch) back to pump and CH valve. So far everything still works pretty much as is when far valve is closed, except CH valve probably now opens whenever pump is running. I forget whether that is a bad thing or not as the 3 relay scenarios for what is wanted open with what continue to confuse me a bit . If it really matters then I suspect some similar NC / NO wiring could be done on the CH valve so that boiler can fire pump without opening CH valve, but opening CH valve would also fire pump.
2. Use frost stat in suitable location to control that valve on the blue / brown wires. Connect permanent 240V to orange wire (NO on switch). When weather is cold valve opens, bringing 240V to pump and CH valve via auxiliary switch to circulate cold water around opened circuit.

If the rooms happened to call for heat while this valve was open they might not get much unless it had a flow restrictor such as a partially closed gate valve to help balance flow.

The magic white wire is found on 28mm (and 1") valves as it is used in C-Plan wiring which usually has larger pipework for the gravity HW loop, and IIRC the replacement head units which are universal and hence should fit a 22mm valve. I think you could also solder a wire to the unused contact on the switch on old 22mm valves, but I wouldn't be surprised if the new valves have only the minimum amount of required componentry and hence this not being available.

If you wanted to fire the boiler as well then taking the BDR signal down to the remote valve and bringing the return to boiler / pump / CH should achieve that, and you'd then probably want to take the frost stat output via the pipe sensor that you mentioned so that you'd just be taking the chill out of the pipes before turning boiler off.

[Rameses]

Use a dts91 and just sense the boiler as a zone

[bruce_miranda]

Use a dts91 and just sense the boiler as a zone

I dont think that is a solution for the problem asked. The issue here is when there is pipework that might be outside a zone that might freeze but because all the zones are shut, there won't be any flow even if the boiler fired up.
I think there should be a frost protection mode that fires the boiler and makes the water flow through the entire system for any amount of time before returning back to a previous setting.

[Rameses]

I dont think that is a solution for the problem asked. The issue here is when there is pipework that might be outside a zone that might freeze but because all the zones are shut, there won't be any flow even if the boiler fired up.
I think there should be a frost protection mode that fires the boiler and makes the water flow through the entire system for any amount of time before returning back to a previous setting.

Ok use IFTTT command "IF outside temp drops below xxx then enable xxxx (can be a zone or quick action)" this will keep water flow.

[DBMandrake]

I dont think that is a solution for the problem asked. The issue here is when there is pipework that might be outside a zone that might freeze but because all the zones are shut, there won't be any flow even if the boiler fired up.
I think there should be a frost protection mode that fires the boiler and makes the water flow through the entire system for any amount of time before returning back to a previous setting.

No it doesn't solve the problem at all. (Also I'm not sure how you bind a remote temperature sensor to a zone without an actuator ? I've been told before that this cannot be done in a multi-zone configuration, specifically by the Honeywell guys and Richard as well)

My solution is low tech - I'm insulating all the pipes under the house. In answer to someones question earlier in the thread, no, most of the pipes are not currently lagged. The main 22mm manifold from the boiler in one corner of the house to a central underfloor location has that old fashioned fabric weave lagging but all the pipes from the manifold to the radiators are completely un-insulated, and some of the runs are quite long like 5 metres worth of 8mm microbore...one new run of 15mm PVC that an engineer installed for the living room when we first moved in turned out to be literally just lying in the dirt on the ground...

I managed to finish about half of it last weekend before I was exhausted and sore all over from crawling around in the gravel and dirt in the crawl space. It's pretty tight in a few places, I managed to get stuck in a hole through an underfloor brick wall at one point but managed to free myself and discovered that I'm not claustrophobic as I first thought I might be, as I kept my cool and didn't panic. So by the time I'm finished every non gas pipe under the floor and in the loft will be insulated. I've even decided to insulate the main cold water supply pipe which runs in mid air about 10 metres through the crawl space from one side of the house to the other - if I'm going to do all these pipes I might as well be thorough then I can have peace of mind in the winter.

There is one air brick on each side of the house that vents the crawl space although on the night that I was doing the work it was relatively warm (10 degrees) and still in the crawl space. But I could see on a very windy cold night that it could get quite cold under there due to the vents so the insulation should prevent any wind chill effects on the pipes.

For the boiler I have a Drayton RTS3 that I'm going to install in the boiler closet to just fire the boiler to protect it. I'm not sure if I'll bother with the return pipe stat or not - as the boiler is in a small closet as soon as it is running it warms the closet right up so that should be enough to switch the 5 degree frost stat off again in a reasonable amount of time. Even on the coldest -6 degree nights we've had, the boiler closet has not gone below 18 degrees with the boiler running intermittently to maintain the bedroom temperature, and typically the room is about 25 degrees when the boiler is busy. If it was in a large room or garage then it would need a return flow stat to switch it off otherwise it would run continuously in a cold room as the waste heat from the boiler wouldn't warm a large room.

[bruce_miranda]

Ok use IFTTT command "IF outside temp drops below xxx then enable xxxx (can be a zone or quick action)" this will keep water flow.

Which relys on an internet connection, so again not an acceptable solution.

[paulockenden]

Just schedule a downstairs loo or similar for 10 minutes at 15 degrees at around 3am. On a normal night it probably won't even kick in, but if it's really cold this should be enough to keep things moving.

It tends to be outdoor pipes (e.g. garden taps) that freeze overnight. Indoor pipes (even in lofts and crawl spaces) will typically freeze only when a house has been left unheated across a number of very cold days. That's why people return from holidays to find burst pipes, but it almost never happens with a lived-in house.

[DBMandrake]

Just schedule a downstairs loo or similar for 10 minutes at 15 degrees at around 3am. On a normal night it probably won't even kick in, but if it's really cold this should be enough to keep things moving.

Again, this seems to be missing the point. If only one room comes on most of the pipework under the floor will still not flow and remain cold.

It tends to be outdoor pipes (e.g. garden taps) that freeze overnight. Indoor pipes (even in lofts and crawl spaces) will typically freeze only when a house has been left unheated across a number of very cold days. That's why people return from holidays to find burst pipes, but it almost never happens with a lived-in house.

Perhaps so, but I think the best (albeit most awkward and uncomfortable) solution is to simply insulate all the pipes. At least I should see some minor energy savings from that as well, and I only need to do it once.

[paulockenden]

Again, this seems to be missing the point. If only one room comes on most of the pipework under the floor will still not flow and remain cold.

What if you schedule the rad at the far end of the pipe run?