Power vs BTU: Is There a Direct Relationship?

[techniker]

Hi
Air conditioners power should vary with thermostat and fan/Airflow rate levels.
Is there data on Power vs BTU? Is it directly proportional?
Thanks

 

[sophiecentaur]

BTU is a unit of Energy and Energy is Power times time. You should always aim at comparing like with like.
Afaik, AC units work at as constant Power level and the rate of heat they shift (per hour, say) will be proportional to the AC Unit's power times the proportion of each hour it's on for (thermostat duty cycle).
The effect of heaters and coolers will depend on the temperature difference they are working with - and even more so, in the case of a Heat Pump (AC), which performs better over a small temperature difference.
Could you be a bit more specific about your original question? What practical circumstances are you talking about?

 

[russ_watters]

Air conditioners power should vary with thermostat and fan/Airflow rate levels.
Is there data on Power vs BTU? Is it directly proportional?

It doesn't vary a ton (get it!?), but it does vary a little: any decent air conditioner catalogue spec should list performance at varying conditions. Pick one and google and we'll go from there...

 

[russ_watters]

BTU is a unit of Energy and Energy is Power times time.

That is true, but it is common to truncate BTU/hr to BTU.

 

[sophiecentaur]

That is true, but it is common to truncate BTU/hr to BTU.

Really? So it's like Pounds and Pounds force. The imperial units really do need sorting out (or just to be left behind).

 

[techniker]

BTU is a unit of Energy and Energy is Power times time. You should always aim at comparing like with like.
Afaik, AC units work at as constant Power level and the rate of heat they shift (per hour, say) will be proportional to the AC Unit's power times the proportion of each hour it's on for (thermostat duty cycle).

1. Does power not vary when you adjust air flow rate or thermostat level?

The effect of heaters and coolers will depend on the temperature difference they are working with - and even more so, in the case of a Heat Pump (AC), which performs better over a small temperature difference.
Could you be a bit more specific about your original question? What practical circumstances are you talking about?

2. ACs work at maximum power when air flow rate and thermostat levels are maximum, and cool with the stated BTU/hr.
When air flow rate and thermostat levels are not max, BTU/hr will not be max. Let us take BTU/hr to be 50%, what will power be? If it is higher than 50% then running the AC at lower than max will lower efficiency.

 

[russ_watters]

Really? So it's like Pounds and Pounds force.

Yeah, and you can also get on a bathroom scale and read "kilograms".

Example:
https://www.google.com/webhp?source...spv=2&ie=UTF-8#q=air+conditioner+btu&tbm=shop

 

[techniker]

It doesn't vary a ton (1) (get it!?), but it does vary a little: any decent air conditioner catalogue spec should list performance at varying conditions (2). Pick one and google and we'll go from there...

1. OK, if the AC runs at lower than max, for 50% BTU/hr, it will have lower efficiency if power does not decrease by 50% (ratio of BTU/hr / hp).

2. I will look for that.

Thanks

 

[russ_watters]

1. OK, so an AC running at lower than max will lower efficiency.

Wait, lower than max what? At lower than max outside air temperature, it runs more efficiently, not less.

 

[techniker]

Wait, lower than max what? At lower than max outside air temperature, it runs more efficiently, not less.

max air flow rate/fan and max thermostat (=lowest temperature level in the control panel/remote control).

 

[Chestermiller]

Here's another one . When we talk about pounds pressure, what we mean is pounds per square inch (psi).

 

[russ_watters]

max air flow rate/fan and max thermostat (=lowest temperature level in the control panel/remote control).

Typically, more air means more efficiency, but there is a limit where fan energy starts going up faster than cooling energy goes down.

For the thermostat: warmer setpoints in the house make for higher efficiency with the system.

 

[techniker]

Typically, more air means more efficiency, but there is a limit where fan energy starts going up faster than cooling energy goes down.

For the thermostat: warmer setpoints in the house make for higher efficiency with the system.

This is beyond my knowledge level at the moment.
Maybe I should have said "ratio of [BTU/hr] / hp" instead of efficiency, and take room and outside temperature as constant.

 

[russ_watters]

This is beyond my knowledge level at the moment.

Naa, we'll get you there.

Maybe I should have said "ratio of [BTU/hr] / hp" instead of efficiency...

Same diff. That's a power over a power.

...and take room and outside temperature as constant.

And varying what? Are you referring to the "normal/max" setting on an air conditioner? My assumption is that that only varies the fan speed and if that is the case, the efficiency is likely higher in the "max" setting.

 

[techniker]

Naa, we'll get you there.

Thank you mate!

Same diff. That's a power over a power.

True

And varying what? Are you referring to the "normal/max" setting on an air conditioner? My assumption is that that only varies the fan speed and if that is the case, the efficiency is likely higher in the "max" setting.

Yes. This is why it is better to run an AC at the max setting even if you do not need this much cooling.

 

[sophiecentaur]

Yeah, and you can also get on a bathroom scale and read "kilograms".

Example:
https://www.google.com/webhp?source...spv=2&ie=UTF-8#q=air+conditioner+btu&tbm=shop

Hah. Nice try but it tells you what you actually want to know - your Mass - usually by inferring it from the force on a spring. But you don't know what's in the instrument. It could actually have a proper balance inside it and then the kg value would be direct. What would we do if our planet was significantly more oblate, I wonder?
This unit business is really quite important and Imperial measurements really don't make a lot of sense. The use of decimal currency was a sensible choice. The UK was dragged kicking and screaming in that direction a mere 40 years ago and the pint is now the only worthwhile unit that's historical.
But even with SI, we have people talking about kW per hour and a 10A battery.

with the stated BTU/hr.

An AC unit is a heat pump and the "stated BTU/hr" will apply only under certain conditions. Give it a divergent operating temperature range and the rate of shifting heat can be very different. It's very hard to quantify what's really going to happen.

 

[techniker]

OK
That is good to know.
Thanks

 

[sophiecentaur]

Good to know but it doesn't help your problem, I fear.
AC is expensive, the way that people seem to use it, even under moderate conditions. People living in Arctic regions treat building and heating with strict discipline. I get the impression that more people in extremes of heat do not tend to treat it as an Engineering problem but are prepared to throw money at it. Apart from the need for a pitched roof to shed snow, I would think that houses in both extremes should be built in much the same way.

 

[techniker]

Good to know but it doesn't help your problem, I fear.
AC is expensive, the way that people seem to use it, even under moderate conditions. People living in Arctic regions treat building and heating with strict discipline. I get the impression that more people in extremes of heat do not tend to treat it as an Engineering problem but are prepared to throw money at it. Apart from the need for a pitched roof to shed snow, I would think that houses in both extremes should be built in much the same way.

I agree.

 

[David Lewis]

Cooling power out can be divided by electrical power in to yield the Coefficient of Performance (COP), a dimensionless number.

COP is distantly related to Seasonal Energy Efficiency Ratio (SEER). The differences are that SEER calculates average efficiency over a typical summer (e.g. 125 90⁰F days x 8 hours/day with indoor 80⁰F at 50% humidity) and, due to inconsistent units (BTU/W-hr) SEER needs a conversion factor to express it as a COP.

 

[russ_watters]

Cooling power out can be divided by electrical power in to yield the Coefficient of Performance (COP), a dimensionless number.

COP is distantly related to Seasonal Energy Efficiency Ratio (SEER). The differences are that SEER calculates average efficiency over a typical summer (e.g. 125 90⁰F days x 8 hours/day with indoor 80⁰F at 50% humidity) and, due to inconsistent units (BTU/W-hr) SEER needs a conversion factor to express it as a COP.

To make life even more fun, engineers often prefer units of kW/Ton.