Efficient operation

[dawoodseed]

The boiler I'm planning to get is a Vaillant ecoTEC system boiler, in order for it to be in condensing mode most of the time I'm oversizing radiators a little and using a quick recovery cylinder. Will this make a difference? One of my relatives has a Vaillant ecoMAX system boiler and on the inside of the control flap it advises to set the boiler flow temp to maximum if it is connected to a thermostat, I thought condensing boilers should be set as low as possible. Shall I tell him to ignore this advise?

[405 TD Estate]

While far from an expert I believe condensing boilers work more efficiently at lower temps though of course your lowest temp is governed by the coldest room or maybe water temp if not seperate and not enough for a shower or bath. I think that is why they should be set as low as possible. If you set the boiler to a lower temp with a thermostat the water may not be hot enough to heat the room. if you set the Temp to Max there will be enough heat but you will waste gas. I would set the boiler temp as low as i could get away with, and even consider changing it from spring/Autumn (little heat needed) and winter (lots of heat needed) would keep things as efficient as possible.

[Roger Mills]

I don't profess to be an expert on condensing boilers, but my understanding is that many of them will modulate the heat input as necessary by turning down the flame, depending on the temperature of the return water. This being the case, the flow temperature will presumably be lower when there is less heat demand (e.g. in milder weather). If this is so, I'm not exactly sure what the role is of the boiler stat - but it may perhaps govern the maximum flow temperature, in which case it would only come into operation occasionally.

[Bob Eager]

My understanding is that it's the flow temperature that's regulated by the stat. This doesn't need to be as high all the time....and keeping it lower increases the condensing effect.

But I'm not an expert either!

[dawoodseed]

can get away with. (Hot enough to keep DHW at a decent temperature)

[Roger Mills]

It's certainly true that the flow temperature of a non-condensing non-modulating boiler is controlled by the stat - but I'm not sure whether that applies to modulating condensing boilers.

What's your understanding of how modulation works?

I'm sure Andy Hall will be along soon, and put us all right!

[Bob Eager]

That it modulates between 'nothing', and the stat setting...but I still await others!

[John Rumm]

Some - in the coldest weather. The boiler will do a reasonably good job of keeping the return temp down where it can anyway though by modulating the heat input.

Depends on the type of thermostat. If the boiler is the type that supports a external stat with proportional control, then that makes some sense. It would in effect leave the internal boiler flow temp control set at maximum and hence "out of the picture" and the actual flow temp would be controlled based on the actual heating demand.

If however the external stat is the more typical "on / off" style device, then the advise in the manual does not seem correct.

[Harry Bloomfield]

Bob Eager expressed precisely :

My understanding is that the flame size is modulated or adjusted depending upon the difference between the stat set temperature and the actual temperature of the water exiting the boiler. The closer the two temperatures, the less heat input is needed.

[Ed Sirett]

Sometimes you have to imagine what the instruction writers are driving at. From their POV if you add another control elsewhere the it is confusing to some users to have more than control! Therefore they are suggesting that it should be set to max. That way the external control can always heat the house even in the worst weather.

The smarter user can override the instructions when they understand all the implications. If you're one of those you will know what to do.

[Bob Eager]

That's what I was trying to say - in a kind of inverse way!

[Mark]

I can't see how else it could work in the latter case. I always believed that condensing boilers modulate down when the return temperature rises too high (unless a weather compensator is fitted).

Please correct me if I am wrong ;-)

M

[John Rumm]

That is my understanding also... The most logical control setup for a condenser is treat the internal water temp stat as a maximum limit, and then modulate for optimal (or at least better) return temperature.

For a non condensing, modulating boiler, maintaining the flow temp at or near the stat temperature would make as much sense.

This does open up the question of how the boilers are not true "designed from the ground up" condensers will behave, since they are based on a re-engineering of an original design.

[dawoodseed]

Thank you for all useful information.