# A discussion on BTU's



## UN1TED-WE-PLUMB (Oct 3, 2012)

I'm trying to learn the different BTU formulas to figure out water temp rise with tankless heaters and other applications. I've seen them in posts here before but I'd like an explanation of the different values used in the formula and I'm sure other interesting questions will arise with this discussion on BTU's. 

I will start with the definition of BTU: 

A BTU is an abbreviation for British Thermal Unit, which is the amount of energy required to raise 1 pound of water 1 degree Fahrenheit at sea level. A BTU is a relatively small amount of energy... about equivalent to the heat generated by striking 1 stick match.

This is a BTU calculator I found on the web. 









I chose these values because yesterday I had a complaint on a tankless heater not giving enough hot water. Well at 6 gpm it was putting out cold water since it was exceeding the output of the heater with the desired temp rise. I found the water pressure exceeding 90 psi which was contributing to it so I installed a new prv and set pressure at 55psi which did help but also explained to the customer that she will have less hot water in the winter months. So that is what sparked this topic of conversation for me. 

First question I have off the bat is... In the formula above, what is a ton and how is it calculated? I know tons are used in HVAC and I've always wondered what it is referring to..
In addition the calculator above calculates everything for you so don't know if I'm supposed to divide or multiply etc.


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## grandpa (Jul 13, 2008)

I think the best data for tankless is the manufacturer's spec sheets. They always show in LARGE PRINT the gpm for 40 degree rise, but in small print for 70+ degree rise. This is often the biggest disappointment with tankless.


For tank water heaters, you can get a good number for HOURLY recovery rate, using 1 gallon per hour per 1000 BTU/h. So a 40,000 BTU WH would give you 40 gallons per hour. The gallon per hour figure is useful in commercial situations...laundrys, restaurants, but you can see for a residential, it is only part of the picture. That WH would only recover 2/3 gallon per minute, but of course you have the stored hot water to give you several minutes of showering.

Most manufacturers will give you detailed design help for tank or tankless installs.

In air conditioning, and maybe heating, a ton is the weight of air heated or cooled. Rough calculations are 1 ton = 12,000 BTU/h. In your calculator, you can see that 195,000 BTU/h divided by 12 gives you 16.25 tons.


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## OldSchool (Jan 30, 2010)

It takes 1 btu to raise 1 lb of water 1 deg F in 1 hour


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## 778 Plumber (Jan 17, 2013)

1 btu= amount of energy required to raise 1 LB of water 1 degree F

it takes 144 btu per LB of ice to change to water
it takes 970 btu per LB of water to change to steam
it takes .53 btu per LB to raise ice 1 degree f
it takes .48 btu per LB to raise steam 1 degree f

ex. If you wanted to raise 1 LB of ice from 0*F to 250*F
^t= change of temperature 

1 LB x .53 BTU/lb x 32^T = 16.96BTU
1LB x 144 BTU/lb = 144 BTU
1LB x 1 BTU/lb x 180^T = 180 BTU
1LB x 970 BTU/lb = 970 BTU
1LB x .48 BTU/lb x 70^T = 33.6 BTU

Add the btus together:

It takes 1345 BTUs to raise 1 LB of water from 0*F to 250*F


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## U666A (Dec 11, 2010)

Now for practical application...
Use gpm instead of #
Factor in efficiency.

Hang on, I'll call the ex wife and see if I can pick up my syllabus...


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## UN1TED-WE-PLUMB (Oct 3, 2012)

grandpa said:


> I think the best data for tankless is the manufacturer's spec sheets. They always show in LARGE PRINT the gpm for 40 degree rise, but in small print for 70+ degree rise. This is often the biggest disappointment with tankless.
> 
> 
> For tank water heaters, you can get a good number for HOURLY recovery rate, using 1 gallon per hour per 1000 BTU/h. So a 40,000 BTU WH would give you 40 gallons per hour. The gallon per hour figure is useful in commercial situations...laundrys, restaurants, but you can see for a residential, it is only part of the picture. That WH would only recover 2/3 gallon per minute, but of course you have the stored hot water to give you several minutes of showering.
> ...


Very good info:thumbsup:

Why do you say the WH would only recover 2/3 gal/min?


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## dclarke (Dec 22, 2012)

So how many BTU = 1 MBH ? What does MBH stand for ?

Sent from my Galaxy Nexus using Plumbing Zone


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## rjbphd (Feb 5, 2010)

dclarke said:


> So how many BTU = 1 MBH ? What does MBH stand for ?
> 
> Sent from my Galaxy Nexus using Plumbing Zone


Million btu


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## U666A (Dec 11, 2010)

Ok... Um... 1,000,000?


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## UN1TED-WE-PLUMB (Oct 3, 2012)

I don't remember ever seeing MBH before so it's new to me. 

I I've got this figured out. 
1 BTU= GPMx(outlet water temp-inlet water temp)x500. 
So...5 gpm x (120-42=78) x 500 = 195,000 btu/hr. 

The water heater in question would sustain temp at 5 gpm but at 6 gpm is where it fell short. It was also set at 130*. 
6 x (130-42=88) x 500 = 264000. So that water heater would need to be 264k BTU to meet that demand. It is only 195k BTU so thy is why they were getting cold water when under a heavy load.


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## UN1TED-WE-PLUMB (Oct 3, 2012)

Another question.. Why the 500 value? What does that represent?


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## RealLivePlumber (Jun 22, 2008)

rjbphd said:


> Million btu


:no:

M = 1000.


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## UN1TED-WE-PLUMB (Oct 3, 2012)

So if I wanted to figure this way... We already know how to figure btu when we know the gpm but what about if we wanted to figure gpm based off of BTU and inlet and outlet temp? 

So.. To determine the gpm that will be available at any given inlet temp say 42*, and any given btu say 195k and we want 120* outlet. What would the formula be?


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## UN1TED-WE-PLUMB (Oct 3, 2012)

Math is not my strong point btw. 

I figured it out. 

If we have 195k btu/500/78=5
78 being our temp rise. 5 being our available gpm. This question arises again, what does 500 represent?


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## rjbphd (Feb 5, 2010)

RealLivePlumber said:


> :no:
> 
> M = 1000.


Ur right... my bad


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## justme (Jul 4, 2012)

rjbphd said:


> Million btu


1 mbu is a 1000 btu's

I've been seeing MBU a lot more on heating equipment lately.


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## 778 Plumber (Jan 17, 2013)

UN1TED-WE-PLUMB said:


> I don't remember ever seeing MBH before so it's new to me.
> 
> I I've got this figured out.
> 1 BTU= GPMx(outlet water temp-inlet water temp)x500.
> ...




1 BTU = heating 1 LB of water 
1 US gallon = 8.33 pounds

So now we can use gallons instead of pounds in my other formula posted earlier. 500 comes from 8.33 BTUs required to heat 1 lb of water X 60 minutes in an hour.

So: 

BTUs Req'd = 

GPH x (Difference water inlet and outlet) x 8.33lb/gallon of water
____________________________
Thermal Efficiency

So to use your other numbers (Formula works with GPH, so 6 GPM x 60 = 360GPH)

360 x 88 x 8.33 = 263894 BTUH <-- IF the WH was 100% efficient, which it isnt. We'll use 75% efficiency


263894
______ = 352,000 BTU
0.75


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## rjbphd (Feb 5, 2010)

K.. in metric... k is thousand... m is million


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## 778 Plumber (Jan 17, 2013)

rjbphd said:


> K.. in metric... k is thousand... m is million



"The unit MBtu was defined as one thousand Btu, presumably from the Roman numeral system where "M" stands for one thousand (1,000)."


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## rjbphd (Feb 5, 2010)

778 Plumber said:


> "The unit MBtu was defined as one thousand Btu, presumably from the Roman numeral system where "M" stands for one thousand (1,000)."


 That's from english system.. c... 100. M. 1000... mm million


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## 778 Plumber (Jan 17, 2013)

rjbphd said:


> That's from english system.. c... 100. M. 1000... mm million


I guess I cant speak for the US as I don't know if you use a different abbreviation. In my gas codebook it uses a number and M to represent total thousands of BTUs. 3,500 MBTU = 3,500,000 btus


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## rjbphd (Feb 5, 2010)

778 Plumber said:


> I guess I cant speak for the US as I don't know if you use a different abbreviation. In my gas codebook it uses a number and M to represent total thousands of BTUs. 3,500 MBTU = 3,500,000 btus


 As we are going gobal here.. that's equal to how many pesos?


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## Tim`s Plumbing (Jan 17, 2012)

Here is how to convert horse power into btu`s 1 hp = 2,545 btu`s.
In large high pressure steam boilers they are often listed in hp not in btu`s.
So a 40,000 btu water heater would be 15.71 hp thats`s more than the average lawnmower.


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## rjbphd (Feb 5, 2010)

Or 746 watts per horsepower...


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## OldSchool (Jan 30, 2010)

Get yourself the app convert units


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## dclarke (Dec 22, 2012)

im glad i can add some confusion to the mix with this MBH. i was just wondering. i believe it is Millions. the boiler i had to change a relief valve was 3.5 MBH i believe and it was quite large.


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## justme (Jul 4, 2012)

dclarke said:


> im glad i can add some confusion to the mix with this MBH. i was just wondering. i believe it is Millions. the boiler i had to change a relief valve was 3.5 MBH i believe and it was quite large.



lets clear this MBH stuff up 

http://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html


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## justme (Jul 4, 2012)

http://www.engineeringtoolbox.com/boiler-capacity-d_1115.html


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## UN1TED-WE-PLUMB (Oct 3, 2012)

778 Plumber said:


> 1 BTU = heating 1 LB of water
> 1 US gallon = 8.33 pounds
> 
> So now we can use gallons instead of pounds in my other formula posted earlier. 500 comes from 8.33 BTUs required to heat 1 lb of water X 60 minutes in an hour.
> ...




Very helpful info. :thumbsup:


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## UN1TED-WE-PLUMB (Oct 3, 2012)

Tim`s Plumbing said:


> Here is how to convert horse power into btu`s 1 hp = 2,545 btu`s.
> In large high pressure steam boilers they are often listed in hp not in btu`s.
> So a 40,000 btu water heater would be 15.71 hp thats`s more than the average lawnmower.




That's cool as shiot! They rate it in hp:thumbsup:


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## UN1TED-WE-PLUMB (Oct 3, 2012)

Typical tankless install

Meter is under sized.. It's only good for 250k btu
Now let's add up the load. These are estimations but very close to actual numbers. 

2 furnaces 80k ea =160k
Gas log 40k
High end cook top 80k
Navien tankless heater 199k. 
Total load 479 cf/hr. 
That's a total of 479k btu on a 250k btu meter.


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## RealLivePlumber (Jun 22, 2008)

We evaluate and contact the gas co. They will put in a larger capacity meter. A few times they had to upgrade the service. 

Doesn't everyone installing tankless do this?


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## plbgbiz (Aug 27, 2010)

I have been told more than once by the techies with our NG utility that the meter's ability to deliver far exceeds the rating plate.

The most recent was this one that was providing 1.3 million BTU to a commercial building. Everything was running just fine.

When I spoke to the ONG rep he said they can easily run double the capacity of the rating plate. :blink:


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## plbgbiz (Aug 27, 2010)

RealLivePlumber said:


> :no:
> 
> M = 1000.


M being the Roman numeral for 1,000.


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## Gettinit (May 9, 2012)

UN1TED-WE-PLUMB said:


> Typical tankless install
> 
> Meter is under sized.. It's only good for 250k btu
> Now let's add up the load. These are estimations but very close to actual numbers.
> ...


Call in and verify. The meters capacity depends on pressure being supplied to the structure.


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## UN1TED-WE-PLUMB (Oct 3, 2012)

Interesting...

I know wht the 5 psig means but what does it represent on the meter?

Also what does MAOP mean?


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## UN1TED-WE-PLUMB (Oct 3, 2012)

plbgbiz said:


> I have been told more than once by the techies with our NG utility that the meter's ability to deliver far exceeds the rating plate.
> 
> The most recent was this one that was providing 1.3 million BTU to a commercial building. Everything was running just fine.
> 
> When I spoke to the ONG rep he said they can easily run double the capacity of the rating plate. :blink:



I can't believe that meter was meeting such demand.. If these numbers don't really mean much then how can we accurately tell what we are working with? 

Also if that's true then why even bother with a 425cfh meter on a larger home?


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## UN1TED-WE-PLUMB (Oct 3, 2012)

RealLivePlumber said:


> We evaluate and contact the gas co. They will put in a larger capacity meter. A few times they had to upgrade the service.
> 
> Doesn't everyone installing tankless do this?


I do.


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## Gettinit (May 9, 2012)

UN1TED-WE-PLUMB said:


> Interesting...
> 
> I know wht the 5 psig means but what does it represent on the meter?
> 
> Also what does MAOP mean?


Max allowable operating pressure I believe.


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## Gettinit (May 9, 2012)

UN1TED-WE-PLUMB said:


> I can't believe that meter was meeting such demand.. If these numbers don't really mean much then how can we accurately tell what we are working with?
> 
> Also if that's true then why even bother with a 425cfh meter on a larger home?


The difference between inches to 2# to 5# is a lot. Also, just like in gas regulators there are orifices that can be changed to meet volume requirements. If not mistaken the rating plate refers to its Max if it were quarter pound system.


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## UN1TED-WE-PLUMB (Oct 3, 2012)

Gettinit said:


> The difference between inches to 2# to 5# is a lot. Also, just like in gas regulators there are orifices that can be changed to meet volume requirements. If not mistaken the rating plate refers to its Max if it were quarter pound system.



Gotcha:thumbsup:

That makes sense. So if your on a 2psi system then it would obviously be capable of handling a higher load. 

I could probably figure out what kinda load it would handle at 2 psi too then.


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## UN1TED-WE-PLUMB (Oct 3, 2012)

So technically the picture of my meter IS undersized since it is a low pressure system.


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## Gettinit (May 9, 2012)

UN1TED-WE-PLUMB said:


> Gotcha:thumbsup:
> 
> That makes sense. So if your on a 2psi system then it would obviously be capable of handling a higher load.
> 
> I could probably figure out what kinda load it would handle at 2 psi too then.


What is is set up for now? There are several different ways to attack this one if it is a quarter pound setup.


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## UN1TED-WE-PLUMB (Oct 3, 2012)

It is 1/4 lb. low pressure.


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## UN1TED-WE-PLUMB (Oct 3, 2012)

I asked a guy that was splitting a meter for me one time how much pressure is actually going to the meter.. He said anywhere from 25-50 psi.


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## Gettinit (May 9, 2012)

UN1TED-WE-PLUMB said:


> It is 1/4 lb. low pressure.


I would have to say it was undersized. There is not a problem starving equipment if a combination of things were not running. I could be wrong though. I have seen meter companies tweak the pressure a bit to make it work since some equipment can handle more than a quarter pound without needing a regulator but I would not condone it.


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## RealLivePlumber (Jun 22, 2008)

UN1TED-WE-PLUMB said:


> I asked a guy that was splitting a meter for me one time how much pressure is actually going to the meter.. He said anywhere from 25-50 psi.


Depends. Around here, newer plastic services are 60psi plus.

Older stuff is 1/2" wc.

Which reminds me. I gotta tell y'all a story about a friend of mine.....

I'll put it over there in a bit.


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## Plumbus (Aug 4, 2008)

RealLivePlumber said:


> Depends. Around here, newer plastic services are 60psi plus.


The meter doesn't get exposed to such pressures. It's protected by it's regulator. The meter shown has an MAOP of 10psi. The delivery pressure tables in my code book only go up to 5psi, which makes me doubt if you can get post regulator pressures higher than that.


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## Gettinit (May 9, 2012)

Plumbus said:


> The meter doesn't get exposed to such pressures. It's protected by it's regulator. The meter shown has an MAOP of 10psi. The delivery pressure tables in my code book only go up to 5psi, which makes me doubt if you can get post regulator pressures higher than that.


You can.


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## 778 Plumber (Jan 17, 2013)

Yup you can. I think ours goes up to 10 or 25 psig, cant remember exactly and I dont have the book in arms reach. We can only use up to and including 2 psig for residential applications here in BC


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## UN1TED-WE-PLUMB (Oct 3, 2012)

Am I missing something?

AO Smith rates their Promax 50 gas at 40k btu and 58% efficient with 41 gph recovery at a 90* rise. And 91 first hr gallons. 

So. 40000x.58=23,200 btu being converted to energy. 
23200/500/90=0.52 gpm recovery. 
0.52x60=31.2 gph recovery. 
81 first hr gallons. 

So the math comes to 31 gph recovery and 81 gallons first hr delivery. 10 gallons short of manufacturers claim. 
Either I'm missing something or they are inflating the numbers.


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## grandpa (Jul 13, 2008)

rjbphd said:


> Million btu


Although one would expect under strict metric usage for M to be million (abbrev. of Mega), in the current context, the usage is 1000 ( from Roman numeral M for 1000). That usage is archaic and confusing , but still is widely used in the HVAC industry.

http://www.unc.edu/~rowlett/units/dictM.html


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## rjbphd (Feb 5, 2010)

grandpa said:


> Although one would expect under strict metric usage for M to be million (abbrev. of Mega), in the current context, the usage is 1000 ( from Roman numeral M for 1000). That usage is archaic and confusing , but still is widely used in the HVAC industry.
> 
> http://www.unc.edu/~rowlett/units/dictM.html


I'm gonna need a magnum of wine to aborb all this!


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## Plumbus (Aug 4, 2008)

778 Plumber said:


> Yup you can. I think ours goes up to 10 or 25 psig, cant remember exactly and I dont have the book in arms reach. We can only use up to and including 2 psig for residential applications here in BC


If so, how would you size a line using 10 psi? If you used NFPA 54, Table 6.3
your accuracy would be off because of the disparity between NG and propane btu value per su. ft.


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## Gettinit (May 9, 2012)

Plumbus said:


> If so, how would you size a line using 10 psi? If you used NFPA 54, Table 6.3
> your accuracy would be off because of the disparity between NG and propane btu value per su. ft.


Get the gas company to print or email you their charts.


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