How do others control a C/H towel rail zone?

[Dave]

I've built a 3 zone C/H system: Z1=upstairs, z2=downstairs and z3=towel rails (that heat shower and bathroom). Z1 and Z2 are controlled by timer thermostats. Z3 is controlled by a dedicated timer and is logically OR-ed with Z1 so that it also runs when Z1 demands heat.

When only Z3 timer is on but there's no demand from the other zones (during the summer, for example), once the TRVs on the towel rails have shut off the boiler keeps going (via the auto bypass) to keep the primary temperature at the boiler set point. This is a waste, a little bit noisy and presumably is slightly excessive for the ABV. It's a system boiler so I don't have separate control of the pump. How can I can improve this? I'd wondered about a flow switch in series with the timer to detect when both Z3 TRVs were off, but this would need a timer so that Z3 demand was only inhibited for 10-20 minutes before the pump runs again to see whether the TRVs have opened or not. Is there a better way? Pressure change maybe?

Dave

[Rick]

My towel rails are on the HW circuit ... this is 22mm up to last HW take off then a 15mm pipe back to the thermal store, a very slow speed circulator is on the 15mm pipe ... so I get instant hot water at all times .......... the towel rails are plumbed in series with the HW loop.

This way I get HW whenever the pump (built in timer) is running, and this is good in summer when heating is off.

[Set Square]

Why not do away with at least one of the TRVs in Z3, and use a programmable room stat?

How often do you want this zone on by itself? If not very often, instead of having a separate zone, you could feed it from a point between the pump and the zone valves. It would then be or'd with both CH and HW so that, in the summer, the towels would be warmed whenever the HW was being heated. [You could probably effectively do this without any plumbing changes - by jamming its zone valve open and disconnecting the volt-free contacts in that zone valve from the boiler/pump control logic.

[Andrew Gabriel]

I have same 3 zones (although z3 is actually the radiator underneath an unheated towel rail in my case). I simply have this radiator connected directly across the boiler output, so it operates whenever either upstairs or downstairs zones call for heat. It also has a TRV.

[john.stumbles]

I did one for a client with teh towel rail coming off before the mid-position valve (Y-plan system) so it's heated when either HW or CH is on. Not perfect but a good near-zero-cost solution. In your case I'd be inclined to fit a room thermostat near one towel rail (or even a cylinder/pipe stat onto one) and use that to control that zone excatly as the you (presumably) have done for the others (and remove or disable the TRV from that rad, natch).

[PeTe33]

That's just what I am doing myself, I have also made provition for one of them electrical element insert things to heat the towel rail when neither zone is calling for HW i.e. Summer.

[Nick Atty]

And, of course, when you run a nice full bath, the tank thermostat kicks in, the HW starts, and you come out of the bath to nice warm towels.

[Dave]

Thanks for the various responses. I chose to have a separately controlled zone because the airing cupboard and workshop (with boiler and HWC) are on it too - forgot to mention that. In the absence of any other ideas it looks like I need to find either a flow switch or a pressure switch. Intuitively it feels like a flow switch would be better for detection but less reliable in the longer term. Any recommendations for either?

Dave

[Set Square]

I'm not sure why you rejected the idea of a programmable room stat. Simply remove the TRV on whichever radiator is the slowest to heat its space, and put the stat there.

If you use a flow switch, you'll need all manner of additional complications - like having to use a timer to by-pass it every few minutes in case the TRVs have opened again.

[Dave]

The original reason was because there isn't an obvious place to put a stat when you want to control 4 totally separate areas with different thermal characteristics. The reason not to change now would be the pain of wiring it.

When the flow switch detects a decrease in flow (presumably they're adjustable?) I could trigger a 555 timer to switch out the demand from the Z3 timer for about 15 minutes.

Dave