Boiler Sizing Too Big?

[chris watkins]

A boiler sized to match heat loss will slowly heat the house to temperature, might take 3 hours. A bigger boiler heats the house quickly, allowing temperatures to fall when the house is empty but getting up there quickly when required (mobile phone control for example). This save energy.

"A boiler sized to match heat loss will slowly heat the house to temperature" No it won't you know, as heat is leaving as fast as it is being put in, so it would not heat up but just stay the same.

Installing big boilers to heat things up quickly does not save energy it wastes it, why do you think all modern boilers modulate ?? i.e. match there out put to only what is required?

 

[gpbeck]

"A boiler sized to match heat loss will slowly heat the house to temperature" No it won't you know, as heat is leaving as fast as it is being put in, so it would not heat up but just stay the same.

Installing big boilers to heat things up quickly does not save energy it wastes it, why do you think all modern boilers modulate ?? i.e. match there out put to only what is required?

Yes it will you know.

Heat loss is proportional to temperature drop across the walls.

At 6 am the house might be 12 degrees, 0 outside so gradient 12. Boiler is sized to heat gradient of 20, so initially is 66% oversize. So house heats up ever more slowly until it gets to 20, when it is balanced out.

Of course the boiler will modulate. It's not always freezing out there. But high insulation and rapid warming for occupied times is the future.

 

[oz-plumber]

Yes it will you know.

Heat loss is proportional to temperature drop across the walls.

At 6 am the house might be 12 degrees, 0 outside so gradient 12. Boiler is sized to heat gradient of 20, so initially is 66% oversize. So house heats up slowly until it gets to 20, when it is balanced out.

Of course the boiler will modulate. It's not always freezing out there. But high insulation and rapid warming for occupied times is the future.

I'm lost - on many angles.
Decipher?

 

[gpbeck]

Trying again. This idea that a boiler sized to heat loss at 20 room temperature won't heat up the house is plain wrong because heat losses are less when the house is cooler. So the boiler heating exceeds heat losses and the house warms. It is a decelerating curve and a powerful boiler makes it get there quicker.

This is why small boilers attract customer complaints, particularly in a poorly insulated home. Still cold when they leave for work!

 

[gpbeck]

One of the many problems with education nowadays is that facts are simplified and then presented And accepted as truth.

 

[oz-plumber]

Who sizes heating systems to heat up from temperature 0 C to 20 C in 3 hours.

It doesn't matter what temperatures you use but I'm yet to find anyone who will calculate a system heat up time of 3 hours.
Most would calculate a system heat up time of an hour.

 

[gpbeck]

I'm not suggesting sizing a boiler like that! Would be crazy. Just pointing out what would happen if OP did.

 

[gpbeck]

And in my post that confused you the house heated from 12 to 20 in 3 hours. Very poor, but I know a house that does :-(

 

[chris watkins]

One of the many problems with education nowadays is that facts are simplified and then presented And accepted as truth.

Tell me about it gpbeck, like post 25, sweeping statements lead to confusion & miss understanding. Which could lead others to think it is OK to install oversized boilers for the odd times that a dwelling would get that cold. :wink5:

 

[doitmyself]

A boiler sized to match heat loss will slowly heat the house to temperature

No it won't you know, as heat is leaving as fast as it is being put in, so it would not heat up but just stay the same.

But a boiler is sized to raise the temperature from, say, -1C to 21C, taking into account the rate of heat loss, plus 10-20% extra for a quicker warm up.

If we ignore the time it takes for the boiler to reach working temperature, initially the boiler will be producing heat much faster than it is disappearing through the walls. Now, rad output depends on delta-T between rad and the air. (It also depend on flow and return temperatures, but we will assume they are constant. )So when the room is at 0C a rad can give out about 50% more heat than when the room is at 20C; but this will depend on the output of the boiler . As the room temperature rises, the rad output reduces, until equilibrium is reached at the required room temperature.

When this happens the room stat turns the boiler off and the temperature start to drop. If you left the boiler off until the house has cooled down to -1C, you would need the full output of the boiler to raise the temperature to 21C again. But this doesn't happen as the thermostat turns the boiler back on when the temperature has dropped by a degree, or even less. This means the boiler only has to provide enough heat to raise the temperature by 1 degree, not 22. So the boiler only has to provide about 5% of its output to raise the temp by 1C.

 

[Dotty]

One of the many problems with education nowadays is that facts are simplified and then presented And accepted as truth.

Did you actually examine this statement before you posted it? ALL your arguments are now invalid.

 

[brum]

i would still want to check myself as i would imagine if you install on a custs say so and its not quite right you as the professional trained installer would probably end up liable in todays world

If I install a heating system for a customer I ALWAYS get it designed by a professional ,cos if it don't work there is comeback.Saying that they will cover themselves and maybe oversize..brum