Well actually it was spotted, and considered, and maybe by others too, but your layout always has at least 1/3 of the radiators operating without TRVs (unless the diagram misrepresents the actual number of rads) so it should have the necessary bypass capacity built in. What I'd probably do is omit the TRV on the towel rail because it's unlikely to be needed and also save the cost of the bypass valve.
Bypass valves are clearly essential if all the rads have TRVs but that's obviously not considered good practice. Equally if the house is so large that the number of rads with TRVs is more than 3 times the number of primary rads ( those in the rooms with zone thermostats and no TRV) then you would need one. On your layout, and if the house is as small as the no of rads suggests, you'd need the bypass valve to increase the efficiency of the boiler a hell of a lot to pay for itself in any reasonable time scale.
Errrr but haven't we just put the towel rail on the DHW? In which case it will make no difference to the heating circuit and you will need a bypass if all rads have TRV's
A dual fuel towel rad my be an option. You can get small electric elements that fit into some towel rads. These allow you to heat the rail when there is no demand for the rads.
You could just take the TRV of the towel rad. Many of these don't have enough output to make the TRV that important, and it is rather too easy to confuse the poor things by covering them with towels (something of an occupational hazard for a towel rad!)
The problem being that these rads may be zoned off, leaving the only alternative path as the towel rail, that currently has a TRV. This would most likely only be a problem as the heating demand goes off, the zone valves close, but the boiler continues to run the pump on overrun for a while.
Pushing the DHW through the towel rad would be a baddie, the rail will be steel so it will rot in no time with potable water, there are towel rads that you can put DHW through but they'll be steep and it sounds like you've got the bits already.
I've just thought of a way to force a CH demand when the DHW is in use: Strap a pipe stat on the Combi H/W outlet and connect such that a pair of contacts are made on temperature rise. Use the contacts to switch a live feed to the CH demand. These contacts will be in parallel with the CH demands from the 2 zone valves and will give a CH demand when there is H/W flow. The only drawback could be if the boiler disables the CH loop completely when DHW demand is on (to force all available heat to H/W). The workaround for that is to lag the pipe near the pipe stat such that it retains enough heat to leave this CH demand on for a while after the DHW goes off. Won't heat the rail while having a shower but would give you a nice warm rail to look forward too after finishing a bath.
Does that make sense?
Also be aware that firing a high kW boiler to feed a towel rail will result in a lot of cycling which is not best practice but it's not really for extended periods.
Put it at any convenient point between flow and return relatively near the boiler. Although others have recommended 22mm pipe, I have found 15mm adequate for this short loop and have actually fitted a gate valve to throttle full flow through the valve if I choose to. You shouldn't need to do this but I found the bypass valve I got was popping off rather than opening smoothly and this sorted the problem. Here's one at BES:
formatting link
I have item code 12161 which now seems a bit steep, 17884 seems a bit more reasonable. Also shown on that page is a cylinder stat item code 9777 which shows dual use as a pipe stat. Using a local outfit would save you the postage.
Which is why my suggestion, which you snipped said "What I'd probably do is omit the TRV on the towel rail because it's unlikely to be needed and also save the cost of the bypass valve."
Let me try and make that even clearer for you and the OP.
"What I'd probably do is omit the TRV on the towel rail because it's unlikely to be needed (and then you'll also always have some bypass capacity even if both heating zones are zoned off) and (now you can) also save the cost of the bypass valve."
That's a saving of at least £50 plus fitting time.
Is it just me (maybe tetchy after pointlessly debating which side of a rad the TRV must go on with Drivel) or is it like having teeth pulled in here sometimes?
Whilst manufacturers claim the TRV bodies are bidirectional (and they are much better made than a generation ago) sometimes you can have rogue units which hammer or sing when presented with a reverse flow (i.e. vertically installed on return or horizontally installed on flow).
If you want the TRVs on the return put them in horizontally, i.e. so the control head is lying horizontally.
There is at least one make which are truly reversible.
That should be OK. Many combi boilers will have a built in smart bypass. In addition you have the towel warmer. I'd be tempted to put a TRV on the towel warmer and rely on or install a sprung bypass valve elsewhere. This is from my own experience at home. The bathroom gets over heated in winter. Our bathroom is tiny (1.4m x 1.75m) and it gets too hot. So sometime I will be modifying the system.
The one way valves are unhelpful and will at best do nothing and at worst cause noises and problems.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.