Viessmann 200 with Honeywell Evohome

Setting a constant temp boiler at 65C means that it will hardly condense. The flow needs to be below 53C for full condensation on natural gas. Weather compensation will keep the flow temp below this for about 95% of the heating season, it is far better than on-off control. The boiler will work like the engine on a car with cruise control - it provides exactly the power needed at all times, no more, no less. It does this by continuous modulation of the burner output and pump speed. Most pre-mix burners are at their most efficient at about 30% of full output, weather comp achieves this for you, Alistair.
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I do agree weather compensation is most efficient but combined with Evohome should be good too. As for the dew point, that depends on combustion.
8% Co2 dew point around 50 degrees C, 10% Co2 aroind 57 degrees C. Most natural gas boilers today vary between this.
A 200 won't work direct with the OpenTherm bridge.

You should only have one zone valve.

Hi Dan (or someone else that knows)

Can you please explain why only one zone valve should be installed? I've seen that mentioned before in threads about Opentherm but suspect it's a general statement about individually zoned systems (like Evohome) and (possibly) modulating boilers.

If there's no zone valve on the CH circuit then when only DHW is on the CH loop will heat as well. Obviously the rads won't heat as all the valves will be shut but the loop will. Or is that the point? ie is there some efficiency gain to having some flow through the primary heating loop even though the CH isn't required?

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erm, yeah. I'm being a bit dumb. I'd always assumed the flow and return are a loop with the rads hanging off it. But that's not the case. Flow and return aren't connected unless a TRV is open
Sorry about that!
Evohome can save 30% of your energy bill. It can make your lounge warm in the evenings without boiling the rest of the house, and it can warm your bathroom towel rails in the summer.

Programmable TRVs can't do that because they can't turn the boiler on and off.

Compare a condensing boiler with/without weather comp and you'll be hard pushed to get a 5% uplift. It's not new and its not clever, boilers have had weather comp for decades (particularly commercials). But in commercial environments 5% is a lot of money and zoning isn't relevant because everything is heated en masse. The lights stay on everywhere too.

Domestic is different, there are normally far more rooms than occupants.

Turn your Viessmann stat down to 65C and the WC versus non WC will achieve almost no savings by comparison.

I liked the post but these 30% claims that relate to something but I am not sure what are a curse. Sure I've done that and a bit better once with upgrades and been disappointed on other occasions when I've had high hopes for a configuration. It's all from what to what and customer interest in using what he has had installed.

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