2-zone central heating

All,

I'll be having my aged boiler replaced soon from an old gravity HW/pumped CH arrangement to a combi. I was considering 2-zone (upstairs/downstairs) thermostatic controls but am not convinced this is necessarily a good idea. I will have TRVs on all rads except one in the hallway, where the thermostat will be be.

I can see the attraction of not heating upstairs rooms during the day when they are empty, but am concerned that this may cause condensation problems in upstairs rooms.

Any advice and opinions gratefully receieved.

Just shutting down part of the house during part of the day shouldn't be a problem.

However, fit a quality combi with weather compensation, and you've done the job properly. This will save you another 15% over and above the savings just by fitting a condensing boiler. It is simpler to fit too.

I would certainly zone upstairs and down if you can afford too. Why heat the areas your not using? I have individual control of each of the rooms at my home.

As Mystery says VIESSMANN are very good. Have a look into WC as mentioned.

Thanks for your replies. These weather-compensation boilers seem to be highly recommended right now, so will definitely have a look.

So you don't think condensation will be a problem? I guess if the upstairs rooms are heated somewhat during the day then it should be OK.

On the zoning for upstairs: is it better to put the stat in a well used bedroom and remove the TRV in that room?
Alternatively I could have a rad on the landing and then have the upstairs stat on the landing as well. I think I prefer the stat in the room since the landing is small and will get convected heat from the hall rad, which will be on the downstairs zone.

I'll be doing the zoning myself after the boiler has been installed, maybe some considerable time after(!). I take it that an S-plan with two 2-port valves and 2 programmable room stats are my best option, with a bypass valve (if boiler requires such) before the motorised valves?

If you don't heat upstairs rooms during the day, they will still get heated from downstairs, either by convection or transfer through the walls and downstairs ceiling. How much will depend on whether you keep the doors closed or open. If you leave them open all the time the downstairs rads will heat upstairs by convection; and as heat rises this will be to the detriment of downstairs. The downstairs rads are effectively being asked to do twice as much work. If you close the doors, there will still be a heat loss through the downstairs ceiling and the walls on to the landing, but not so much.

The heat loss per m² through internal walls and ground floor ceiling in a modern house is about four times greater than through the external wall per degree difference. So, assuming a 22C diff (21C to -1C), you will get the same heat loss per m² of ceiling when the upstairs room temperature drops to 15.5C and the downstairs stays at 21C.

@D_Hailsham

Thanks: This seems to suggest that, unless my ground-floor ceilings are well insulated (they are not), and that Mrs echoes and our lad close doors (which they don't) then during the day, I wouldn't actually be saving much energy since the temperature gradient between d/s and u/s will encourage heat loss via upstairs and just make my d/s zone work harder. And I've sized my d/s rads assuming a heated room above... Hmmm. I think properly insulating the loft should be a much higher priority.

echoes said:

I think properly insulating the loft should be a much higher priority.

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Agreed. Use the Boiler Size Calculator to find out how much difference insulation will make on boiler size. (NB: a detached has two walls, front to back; a semi has one; a mid terrace has none.)

You may be able to get free or discounted loft and cavity insulation. Check out Energy Savings Trust Grant Search

Mr Hailsham i do tend to agree to a point, however in my own e periance, at my own home i have found that we saved money by zoning upstairs from downstairs (before we went to UFH that is.) Our floors arnt insulated and the rads were sized on heated rooms above.

I cant remember the e act figure but anything over 50m2 has to be zoned now anyway.

The more economic situation would be to insulate the loft and then have each individual room on set back programmers. Have a little look at heatmisers website.

HTH Gareth

EDIT it's anything over 150m2

Thanks again gents - I will have a look at that site, and hopefully get round to implementing it before next winter!

looneyfitter said:

Our floors aren't insulated and the rads were sized on heated rooms above.

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It's unusual for floors and internal walls to be insulated, except the ground floor.

The more economic situation would be to insulate the loft and then have each individual room on set back programmers.

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I agree about insulation. Not sure about set back programmers - sound like overkill to me.

EDIT it's anything over 150m2

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Good to see you picked that error up. 50m² was a bit stingy!

After the loft has been insulated (thanks for the link to the energy saving trust BTW - from £95 apparently! Seems alarmingly cheap...), I think I'll get a programmable stat in the hallway for the downstairs zone (lounge no good due to open fire), and another stat in the main bedroom for the upstairs zone with TRVs in all other rooms. Those heatmiser ones look the business at a reasonable price.

According to the Veissmann installation instructions, the weather compensation module isn't compatible with external controls though, so may forgo that. I'll discuss it with the boiler installer - thanks to you all again for getting me a bit more clued-up.

echoes said:

According to the Veissmann installation instructions, the weather compensation module isn't compatible with external controls though, so may forgo that.

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Where did you read that? I can't find anything on the Viessmann website. Can you post the link?

Installation manual

On page 4:

The boiler is either supplied with a constant temperature controller (Vitotronic 100)
or a weather-compensated controller (Vitotronic 200).
The Vitotronic 100 (HO2A) provides a 7-day electronic timing for hot water and central
heating. A separate room thermostat may be fitted.
The Vitotronic 200 is a weather compensated controller and includes a 7-day timing
function, outdoor sensor as well as the option to control three separate heating circuits
with independent time and weather-compensated control. Additional remote controllers
are available. A standard room thermostat can not be connected.

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Looks like the WC controller can do sophisticated independent zoning (up to 3) but from my reading so far, it seems that you need to use Veissmann remote accessories instead of a 'standard' room stat., and I can see why.
TBH, it looks like the dog's danglies, but I just have this automatic revulsion to vendor lock-in... perhaps I need to get over that!

I'll go and swot up on the technical guide now and Vitotronic 200 instructions...


EDIT: Would I be right in thinking that the weather compensation controllers function by actually analysing the temperatures in the house and outside, and adjusting boiler flow temps accordingly to achieve condensing mode most often and reduce cycling? If so I can see that a simple 'call/satisfied' signal provided by a standard room stat would be insufficient.

Thanks for the link. I had been looking at the Viessman 100 model.

You are quire correct, the 200 model uses proprietary controls, so you are tied into them if you want weather compensation.

EDIT: Would I be right in thinking that the weather compensation controllers function by actually analysing the temperatures in the house and outside, and adjusting boiler flow temps accordingly to achieve condensing mode most often and reduce cycling?

Click to expand...

"Pure" weather compensation just looks at the outside temperature. It works on the basis that heat loss is more or less proportional to difference between outside and inside temperature, and radiator output is approximately proportional to the mean water temperature. So you need a high flow temperature for low outside temperatures, and low flow temp for high outside temps and an inverse linear relationship between the two. More sophisticated systems have a curve relationship, rather than linear.

The other thing that has to be considered is changes in the required internal temperature. If the system has been set up by the installer for a 20C internal temperature and the home owner decides to change it to 22C. The "curve" will have to be changed. On simple systems this would mean an installer visit, but on more sophisticated ones the owner will just have to dial in the new indoor temperature and the curve will be automatically recalculated.

Have you read the Viessman Guide to Weather Compensation

As for achieving condensing mode more often, this will be true when the weather is warmer. But if you want to maximize the time when the boiler is condensing, the system has to be designed from scratch to work with lower temperatures as condensing does not occur until the return temperature is below about 55C. This means that radiator outputs will be lower than given in manufacturers' literature, so larger rads will be required.

If you are seriously considering weather compensation, take a look at the Remeha Avanta boilers. They all have WC facilities but you are not limited to Remeha controls as they adhere to the Opentherm standard, which means that any Opentherm control can be used.

As for zoning I doubt if you will see any benefit - unless you have a very large house; thermostatic valves will probably be all that is required.

Thanks for the reply - this has been an unexpectedly educational topic so thanks for enlightening me. I'll give that WC guide a going over for curiosity's sake.

It's the ever-increasing dilemma of being an engineer: I like to know and use the technology, but I also want to be able to easily maintain it, and fix it easily when it goes wrong. When the service costs get more costly than the energy saving it's no good. It's the same with my newish car - the fuel savings tend to get spent on the mechanic, repairing the fuel saving devices.

I'll save the zoning project for a rainy weekend and just stick with my TRV's for now, since there's no shortage of other jobs on the list...

Cheers