\"net BTU\" versus \"gross BTU\"

Steamhead

to size the new boiler. Give the customer the same size and it should work fine. And sometimes it actually does.

Old coal boilers were often oversized so the firebox would be bigger, so the owner or custodian would not have to shovel coal as often. The Label Method guys either don't know this, or have forgotten.

We usually use the Net rating to match the boiler to the system, since it factors in the usual piping/pickup allowances. But if your system has bigger pipes than usual (as on a Vapor system) or the insulation is gone (not a factor on your system) the usual 1.33 pickup factor may not be enough. In that case I like to use a factor of 1.5. Here's how to do it:

Measure up your radiation in square feet EDR. Convert this value to BTU per hour, and add 50%.

Then select your boiler by the Gross (or DOE) rating.

Remember, if the boiler is slightly oversized you can down-fire it. But if the boiler is too small, it may not heat the building.

<A HREF="http://www.heatinghelp.com/getListed.cfm?id=157&Step=30">To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"</A>

GP

net BTU vs. gross BTU

Okay guys bear with me. This is new to me...

I'm replacing an old coal boiler, low pressure (2.5-3lbs) (that has been converted to oil, to gas) in a 1930's cathedral church. I figured my BTU ratings on the radiator system.

(I had 6 contractors come to give bids, only 1 has said the radiator system needs to be surveyed?! I've just been quiet and played dumb when they get here. When the one guy mentioned this, inside I said to myself, "Yes! Someone who is going to do it right!"

Everyone else just looks at the boiler and reads the plate..."Oh. Well, this is a 2.8 million BTU unit. We'll set you up with a 3 mil. That shoud cover things..." They don't even look at what radiators I have!

Anyhow, with the initial survey that I HAVE done, I only come up with about 700,000 to 800,000 BTUs based on all my radiators. What am I missing?! Dan, you called this my net BTU, but not my gross BTU. Somebody please elaborate. Tell me how to get to gross BTU - that is, what should my boiler size be?!

Thanks in advance,
Gene.

Andy Morgan_2

net and gross

Gene,

If you are referring to boiler output, net is BTU's that the boiler will give you after you factor the boiler efficiency int the equation. For example, a 100,000 BTU Gross boiler firing at say 80% eff, will give you 80,000 BTU net.

Good Luck!

Andy Morgan

R. Morgan Mechanical, LLC

GP

Okay, what if we come at it from another diretion. Let's say all of radiators total up to 800,000 BTU - based on total square footage of radiators. (Maybe you are saying the same thing.)

How do I calculate what size boiler I need?

Additional note: on the "main runs" of the pipe (all the pipe up to each radiator) are insulated.

Thanks,
Gene.

Bill NTSG

maybe.....

...this will help. copy & paste

http://www.heatinghelp.com/newsletter.cfm?Id=64

GP

Yes it helps... But

Yes it helps... But from what I just read:

Take the 1.33 factor AND:

1. I caluclate the bulding heat loss

However, this was already done. The radiators are in place now. I'm not going to add any, right?

2. Take the installed radiation load in BTU - which I did and I come up with (let's say) 800,000 BTU (I'm tweaking it a bit with E.D.R. book).

3. And multiply by the 1.33 factor.

I get 1,064,000 BTU as my gross.

Is it that simple? I feel like I missing something here. How is it that 5 out of the 6 contractos said I need to replace my unit with a like machine?! (replace the 3 million BTU unit with another 3 million BTU?!) Only one guy has come in and said I need a 1.5 mil and not a bit more.

Gene.

Einsiedler

forgot something

Gross Output x Efficieny = NET Output @ Sea Level !!!! (0-2000ft)

Dont forget to de-rate another 10% for every 2000ft above sea level or you may find your self short changed.


Regards,
Terry

Bill NTSG

Steam - heat loss

Gene, with steam the boiler does not care so much about the heatloss of the building or the windows or insulation [or the lack thereof] It cares about connected load. The amount of iron connected to the boiler that the steam has to fill. EDR equivalent direct radiation. 240 btu pr sq ft. The absolute only way to size a replacement boiler is to match the piping and the radiators. I know Dan has another article on this topic [I will try to find it] So you already surveyed your radiators and converted the sq ft to BTU's ? The 1.33 pick up factor makes allowance for cold pipes [the steam will condense faster with cold pipes] and boiler loss [up the chimney] And yeah , it is pretty much that simple. If you need to size your radiators there are many resources for the numbers like Burnham's Heating Helper and Dan Holohans books The Lost Art of Steam Heating and EDR [every darn radiator] or Golden Rules of heating. Maybe someone else can do a better job of explaining. I know what I am trying to say but ........................

flange

the one you are replacing is giving you a gross rating. chances are it too was oversized. you have already figured out the actual load of the rads, you need to allow for exposed piping if any, and the 1.33 should cover you. the new boilers are so much more efficient than what you currently have, upward of 80%, versus more than likely less than 60%. believe the numbers they dont lie. a properly sized boiler will have longer run times in the dead of winter, while saving you money as they are more efficient versus constant starts and stops.

David Efflandt

gross input is not gross output

Maybe instead of gross you should talk input, output and net load. For example even conservatively figuring 1.5 loss factor instead of 1.33 and 80% efficiency:

800K x 1.5 / 0.8 = 1.5 million BTU gross input (fuel burned)

But to calulate that accurately you would need to know the actual efficiency of the boiler (if not 80%).

Wayco Wayne

I would

look into going with multtiple boilers. There are great controls out there that can rotate the lead/lag so the boiler wear evenly, or you can even put one high efficiency in the lead and use less expensive boilers behind to make sure of capacity. It also gives you redundancy if one boiler has problems you are not totaly without heat. It makes life better IMHO. WW

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Boilerpro_3

Gene, My speciality is churches

I responded to your cast iron vs steel post also. As to boiler sizing, I have replaced 3 million plus btu plus boilers with less than a million before. Oversizing in church boilers was probably done for many of the same reasons as Steamhead said (he's one of the best in the steam business). However, another reason is that historically most church structures were held just above freezing most of the week (about 40 to 45F) and then brought up to temp quickly for services. This had two consequences.

1) The boiler had to have a very large pick up factor in order to bring all this very cold metal up to temp. If you read the fine print in the boiler sizing info this falls under "unusual pick up allowances".
2) The radiation in the structure is grossly oversized. More radiation allowed the space to heat up faster.

I would approach the sizing of the new heating plant in this way.

Calculate the actual building heat load and compare it to the radiation capacity. If the radiation is greatly oversized, we should look at reducing the amount of radiation capacity.
This often can be done by simply valving off or removing some radiators where appropriate (a room by room heat loss will be necesary). The zoning configuration of the system will also effect how well this can be done. In large rooms such as sancuaries, a large amount of radiation can be removed and not effect heating balance. In zones with multiple rooms, radiation may not be able to be removed because it would adversely effect heating balance.

Another approach is to convert the system to hot water, if feasible, because you can run the system at the same temp as steam (about 215 water temp) or reduce the water temp, and thereby, decreasing the output of the radiation.

I've done both of the above depending on the system. In high ceiling spaces like churches, water systems have a definite advantage in control because they can modulate heat output into the room by modulating water temps and avoid hot air stratification at the ceiling. Also, the heating plant is usually much smaller and is easier to maintain, reducing costs consiederably.

I also only use modular boilers for churches that are step fired for heating plant replacements. And here's why. Most churches provide full heating to only a small portion of the structure for weekday meetings, offices, etc. The rest of the structure sits with thermostats set at only 45F for freeze protection. A rather small amount of heating capacity is required to fullfill this 6 1/2 day a week need.
On sunday morning however the whole structure needs to brought up to temp requiring lots of heating capacity to bring all that cold stone, concrete, pews, etc up to temp. As a side point, however, we don't want to bring the space up to temmp too quickly, because the air temp will be 70F, while the pews and everything else will still only the 50F....not very comfortable.

Using modular or multiple boilers (two different things in boiler codes) that are step fired wil allow one or two small boilers to run near full capacity, and therefore maximum efficiency, during the week, and then bring on extra capacity for Sundays. Applying this concept has consitantly cut fuel usage in the churches I've worked with over 40%, ranging from about 35% at the minimum and about 75% at the maximum. The boilers being replaced were form 12 to 50 years old.

For additional resources that are church specific, you may want to contact Inspired Partnerships. They have an office located in Chicago. Also they will probably be able to provide you with information about a group called Interfaith Coalition on Energy, which deals specifically with energy use in religious structures.

Give me a call if you like at 1-815-857-2339. I'm about 2 hours west of Chicago, and grew up in the Beverly Hills area out on the south side.

Boilerpro