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Old May 20th 19, 08:54 AM posted to uk.d-i-y
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On Saturday, 18 May 2019 13:18:59 UTC+1, Jeff Layman wrote:
On 18/05/19 10:33, Harry Bloomfield wrote:

Our new Vailant can inteligently decide just how much heat it needs to
produce to meet its desired room temperatures, taking into
consideration the outdoor temperature and indoor. It doesn't overshoot,
rather it learns what is needed and can modulate right down to match
the demand. The pipes don't creak and moan anymore.

Well, I really am willing to be educated, but I really don't understand
what it is supposed to be doing to "intelligently" decide something.
Firstly, I assume that as with almost any device, it will have a "sweet
spot" where it is working at its highest efficiency. Its specs may state
"85%" efficient, but in practice that might mean from a low of 82% to a
high of 88%. Now it should be operating as near to 88% all the time as
it can, but obviously there are constraints - it won't be so efficient
starting from cold, for example.

Max efficiency is at minimum power output. So there's always a conflict between providing enough power quickly enough & maximising efficiency.

As far as I am concerned, when the room thermostat calls for heat, the
boiler should go flat out to heat the room as quickly as possible. In

that would use more gas due to reduced effieincy & cause overshoot, which of course uses more gas.

any case, it's the room thermostat which tells the boiler what to do,
and it's the thermostat's hysteresis which is the arbiter of how
comfortable or uncomfortable you will be. I've switched ours to the
lowest hysteresis possible, which is +/- 0.5 deg C. That may mean the
boiler in on/off more frequently, but I'm a lot more comfortable with
the room at a steady 20.5 - 21.5 deg C, than I would be at 19 - 23 deg C.

And does it really matter most of the time what the outside temperature

When it's mild out the primary temp can be kept lower and burner power reduced to improve efficiency.

It will almost always be much lower than the room temperature, and
so maximum heat will be required to warm the room up.

The previous boiler either ran and produced full heat, or none, though
it did modulate, it only modulated based on its internal temperature.

Having said all the above, I see from the manual of my W-B Greenstar that :
"The gas supply to the burner is controlled according to the level of
demand for heat. The boiler operates with a low flame if the demand for
heat reduces. The technical term for this process is modulating control.
Modulating control reduces temperature fluctuations and provides an even
distribution of heat throughout the home. This means that the boiler may
stay on for relatively long periods of time but will use less gas than a
boiler that continually switches on and off."

Out of interest, does your boiler's spec state that it is most efficient
when it is ticking over (or whatever it is doing to decide what heat it
needs to supply)?

So exactly how does the boiler do what it's supposed to do? You set the
boiler hot water temperature to the rads at, for example, 60 deg C, and
you set the room temperature thermostat to 20 deg C, and the hysteresis
to 1 deg C. How exactly does it (in the case of my W-B Greenstar)
"operate with a low flame if the demand for heat reduces"?

the same way it always operates, except at reduced burner output & reduced fan speed

How often
does the "demand for heat" reduce?

Most heating season days don't need full power output.