Probably a daft question (they usually are)

[larkim]

But why do external weather sensors help indoor boilers to deliver a better performance? Surely the temperature outside is irrelevant - its the temperature inside that counts?

Matt

[The Natural Philosopher]

not quite. A falling outside temp indicates that indoor temps will be dropping fast and sometimes getting the boiler on ahead of time means that the slow heating bits of it are ready.

In system analysis terms its a feed-forward term you can use to counteract system lag in the feedback loop..

[Roger Mills]

Probably because they *anticipate* that it's going to get cold inside, and react in time to stop it happening.

Some systems control the temperature of the circulating water, based on some function of the outside temperature - and are designed to put enough heat into the building to balance the heat losses.

[Andrew Gabriel]

The heat loss by the house is (roughly) proportional to indoor temperature - outdoor temperature. Given this (and some other configuration data which it can learn if necessary), the boiler can work out how much heat to generate to compensate for the heat being lost. This is normally done by modulating the burner to the right heat output, which will automatically set the radiators to exactly the right temperature to transfer the right amount of heat into the house (providing the radiators are sized sufficiently big). In this mode, the radiators will always be running at the lowest temperature required to transfer the right amount of heat into the house, and that's the most efficient way of operating the boiler.

The internal thermostat will ideally almost, but never quite, click off. However, you probably also have something more intelligent than a plain on/off internal stat. The boiler will use the difference between actual internal temperature and the current setpoint temperature to boost the heat output calculated above when heating the house up from cold or from a lower setpoint temperature, when the output required will be temporarily more than just the heat lost to outdoors, in order to increase the temperature.

But to repeat, the key point is to have the radiators running as cool as possible, whilst generating exactly the right power output required at any point in time. Boilers are more efficient the cooler you can run the water in them. If you have an internal therostat which is clicking off and on cyclicly, that's because the radiators are too hot and transfering more heat into the house than it needs, so it has to reduce that by periodically turning them off. This means the boiler isn't running as cool as it could do, and therefore isn't running as efficiently as it could do.

There are times when even an efficient system will click off and on, such as if the heat output required is less than the lowest modulation level in the boiler. Also if the radiators are not large enough to deliver the required power at low tempertures, although in this case increasing the radiator sizes will improve system efficiency. (This is why condensing boilers benefit from larger radiators.)

And no, it's definitely not a daft question.

[Dave Liquorice]

Thought that was more down to the radiator specs being based on a flow temperature approaching 80C and delta T of 10 degrees. Giving a return temperature of around 70C which is too hot for a condensing boiler to work in condensing mode.

IIRC the return temp to a condensing bolier should be below 60C which means the flow is only 70C and thus as the radiator to room delta T is now 50C instead of 60 so won't deliver it's rated output to the room. To compensate you fit larger radiators.

The only daft question is the one that isn't asked.

[Andrew Gabriel]

I think that's the same argument - you are increasing the radiator size so that the radiator can give off the same power whilst running at a lower temperature.

There is a step in the efficiency curve at 55C which is the dewpoint for the moisture in the flue gas, so it's worth ensuring you can get the return temp below that to recover the latent heat so the boiler is running in condensing mode, but any reduction in return temperature improves performance, whether you are above or below the dewpoint.

My system runs at 45C flow 35C return for outside temp down to somewhere between -1C and -3C, which is about as efficient as I can get it.

[Andy Wade]

The step is in the *gradient* of the efficiency curve, not in the efficiency /per se/.

See

[David WE Roberts]

[Rick]

For a standard domestic install they are useless ... a sales gimmick ....

Where would you put one single sensor in shade on N side ... so boiler would over heat, or in sun on hot side, and boiler would under compensate.

Quality room stats in multi zone set up with an efficient boiler is what is needed for an average house.

[The Natural Philosopher]

I wouldn't say so

Here with underfloor heating with a time constant measured in DAYS they would be a huge help.

[Steve Walker]

No, they can have a purpose. If you get up at a certain time and set the heating to come on before that, some days it will get hot too soon and some too late, depending upon the room starting temperature and the rate of heat loss (proportional to internal/external temperature difference). An outside temperature sensor can allow the system to predict how much earlier or later it needs to turn on to be hot at exactly the time you require.

SteveW

[Steve Walker]

That's a little oversimplified, because of the house's thermal mass, but it's still valid, just not the full picture.

SteveW

[Andrew Gabriel]

A system can either learn your house's thermal mass over a few operating cycles, or have appropriate data programmed in.

[PeterC]

It did seem that the 90% quoted for modern condensing boilers is achievable only if a) it's on full load - not modulating b) the return temperature is about 40C and, from b), c) most of the walls are covered in rads!