Zoning and thermostats

[TheOldFellow]

I have an "L" shaped bungalow. It sits with bottom of the "L" facing approx. South. Diagram:

modern condensing oil boiler is in the garage at the top of the "L". The bottom of the "L" (I call it the East Wing) has a lot of big double glazed argon/K-glass windows, so it gets a lot of solar gain from any sunshine, this is also the main living area with wet UFH over

100mm KS. The current thermostat (Honeywell CM67) is here too. The UFH currently does not have a thermostat, just a hot water sensor - it pumps when there is heat in the supply. Trouble is, as the West Wing is cold. As the day progresses the East Wing stays warm due to the hot slab, while the West Wing gets colder. I need to get the boiler on but not put more heat into the East Wing. I think the simple answer is probably to re-site the Thermostat into the West Wing (A), where my office is, and put a thermostat on the UFH (U). But there is a complication in that between the office and the garage there is a Bedroom/ensuite which was built as an extension, and it gets very cold. Everything is well insulated, cavity and in most cases extra internal 25-60mm KS. All the rads have modern TRVs. Is it best to fit a second Thermostat (B) wired in parallel with the first (A) in the End Bedroom to force the boiler to stay on if that calls it? Or do I really need zone valves? The boiler has a bypass already, of course. I think I have made up my mind, but I know you guys have good ideas about this kind of thing, so I'd appreciate your thoughts. R.

[David Hansen]

On Mon, 19 Oct 2009 10:40:46 +0100 someone who may be TheOldFellow wrote this:-

If you want to do it properly you need these, together with a suitable pipework layout. How easy modifying the existing pipework is depends.

If the bedroom/ensuite needs to be heated "all the time" then put thermostatic valves on it and connect it to pipework which is "live" whenever either thermostat is calling for heat.

[TheOldFellow]

One of the great benefits of a bungalow, like mine, is that all the plumbing is in the loft. It's a bit like wiring, you find the feeds and you can reconfigure at will. I'm changing all the rads in the West Wing over time anyway.

Not sure what zone valves gives me? If the boiler is called, and the rads have TRVs, then it will all 'work out'. Or did I miss something.

R.

[David Hansen]

On Mon, 19 Oct 2009 18:43:44 +0100 someone who may be TheOldFellow wrote this:-

An "ideal" system would have programmable thermostats in every main room so that the heating for that room can be tailored. That might be say the living room, kitchen and bedrooms. Core areas, hall and toilet/bathrooms have thermostatic valves.

If a room with a thermostat also has a thermostatic valve then it is most unlikely to work properly,unless the thermostatic valve is defeated in which case why have it.

[TheOldFellow]

Ok, I see the point now. My difficulty is that if I put in zone valves I would link the room thermostat to the zone valve, but I also need to call the boiler. I suppose there are a couple of solutions to that:

1) Leave the boiler called by a timeswitch and let it use its temperature regulation and the bypass to manage it's own destiny. 2) Install some relays that call the boiler when any zone valve opens, unless zone valves typically have 'boiler-calling contacts'. (Note to self, find out about zone valves!)

It just seems to be adding complexity. I think I'll leave the TRV's there and just add an extra thermostat in parallel.

Thanks, David, for taking the time to help. Appreciated. R.

[Andrew Gabriel]

You run into the problem that the zones are all much smaller than the minimum boiler modulation, so it can't properly modulate down to drive just a few radiators if you are using low flow/return temperatures for efficient boiler operation.

What I've had to do is some intelligent interworking between zones. When the first zone calls for heat, the boiler fires up and quickly gets the radiators up to temp. It can't keep them there because the zone only has 3-4kW output, but the boiler can't drop below 7kW. (It will cycle on and off, and with low flow/return temperatures for efficient boiler operation, the hysteresis is too large.) This effect would be much worse with individual rooms as zones where outputs would be much smaller than my zones and at times of low heating requirement, they tend to stay out of sync. So what I do at the point where the first zone's water is up to temperature is to look to see if another zone is close to demanding heat, and bring that one on slightly early. What I tend to find is that these zones usually cease their heat demand very close together. I also have some code to bunch the end of call for heat when a zone is almost satisfied, but the boiler has had to cut off the burner, but that's not yet deployed on the system.

This artificial bunching of zone demands helps the boiler to work efficiently. I don't know of any commercial system which does this (this one is my own design and runs on a PC).

Of course, a roomstat in conjunction with TRVs relies on generating bunching of demand. If you go to separate zones, you'll need to find some way of mitigating the loss of that effect, as I have.

If I was doing another system from scratch, I would put TRV on all rads. Then you simply remove (or fully open) the head in the room where you have the thermostat, but you also have the flexibility to change the room with the thermostat, which with RF links, is really easy, and with wired ones, I would make a connection available in multiple rooms within each zone.

As a safety measure if your system design uses a radiator as the bypass loop, have one fewer TRV heads available than there are TRV valves, so there's always one open.

[The Natural Philosopher]

They do.

parallel the lot up with lotst of 4 core, cos strictly they should all be on the switched fused spur that runs the timer and the boiler.

[David Hansen]

On Tue, 20 Oct 2009 09:41:38 +0000 (UTC) someone who may be [e-mail] (Andrew Gabriel) wrote this:-

Thermal store. Only a relatively small one is needed to match up boiler and radiator characteristics.

The problem would be at least as bad with all thermostatic valves [1]. With them all largely closed and just putting a little water through each radiator then most of the water will be going round the bypass and the boiler cycling on and off. With motorised valves there is at least one radiator fully open to the boiler, plus any towel rails and radiators on thermostatic valves which happen to be open.

[1] I know this is not permitted from new these days, but you then get back to the problem of where to put the one thermostat which is why thermostatic valves were introduced in the first place. It is far better to have several thermostats, which can react to things like solar gain and (if programmable) cover different occupancies.

[David Hansen]

On Tue, 20 Oct 2009 10:35:33 +0100 someone who may be TheOldFellow wrote this:-

A recipe for short cycling.

They all have auxiliary contacts which can be used to control the boiler. The output of any number of these contacts can be connected together and any one calling for heat will activate the boiler.

If the thermostats are programmable it also makes sense to have one master time clock which enables or disables everything else. The output from this goes to the thermostats and the auxiliary contacts. Each thermostat then controls its individual valve.

This sounds complicated, but it can be done with one three core and earth cable from the master time clock to a junction box beside each valve. One core is the neutral, one the live from the time clock and one the "return" wire taking the boiler signal back to the boiler. With more than one valve the cable continues from one junction box to the next. One three core and earth joins the thermostat to the junction box (only two core and earth is necessary, but the extra core allows the neutral to be taken to the thermostat position just in case it is needed in future).

[TheOldFellow]

I'm wondering if I can use the relatively large thermal mass of my UFH slab to create a bit of demand smoothing.

But this has been a very interesting and thought provoking thread.

My 3 years old Mistral Oil boiler doesn't modulate, it's an on/off burner. I'm told modulating oil burners are rare below 90,000 BTUs, not so gas of course, but then the control is easier.

R.

[Doctor Drivel]

Yep. Have two anti-cycle stats. Have the zones with TRVs on all rads with a Smart pump on each zone.