Which rating and why? Please!

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Whem selecting a boiler after doing a heat-loss, which rating do you use when selecting a boiler. The mfr's site lists I=B=R Water Ratings and DOE Heating Capacity! Which one do I use?

Thanks much in advance!
Bob

Brad White_142

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My own answer to you would be, how sure are you of your calculated heat loss? How much cold piping are you expecting to heat up every morning or how long do you want to wait for your house to come up to temperature?

Personally, I think using the I=B=R rating results in over-sized boilers by at least 15 percent. (The I=B=R rating assumes 15% of the boiler goes to heating a cold system using a term called "piping and pick-up").

With good tight sizing to the heat loss, proper control (outdoor reset to keep ahead of things, to anticipate if you will), the Gross or DOE output should be plently.

As an example, say you have a home heat loss of 82 MBH (thousands of BTU's per hour). If you have your best selection of a 100 MBH input boiler rated at 85% efficient, you get 85 MBH DOE output, simple enough. About as good as it gets.

The I=B=R rating for that same boiler would be 73.9 MBH. Thus, using this, your thinking would be, "hey, I am short by over 11,000 BTU's!". You naturally do not want to fall short so you go to the next boiler size up, a 125 MBH input beast. Now you are 25% oversized (106 MBH output versus 85 MBH) on the coldest day.

What favors have you done for yourself?

Edit- I agree with Gary on steam. The 1.33 factor is worth taking into account especially if your piping is not insulated. (And why not?? )

GW

piping loss

The input lets say is 100,000. Let’s say it's an 80% afue for easy math. You have 80,000 BTU to work with, 20,000 flew the coupe. 80,000 is the DOE heating capacity. The IBR considers the piping losses, I believe it's 15% for hot water jobs, and pretty sure it's 33% for steam. For hot water jobs, I personally don't consider these numbers, because whatever heat I lose in the basement isn't "lost", it's offsetting the temp diff between the basement and the living area thereby lowering the heat loss through the floor. But don't take my advice, if in doubt use the IBR numbers.
For steam, yes, use the numbers


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Todd_12

Pickup factor

Doesn't the pickup factor become a moot point after the system is hot? It would seem that on a cold day or under design load, the piping would already be hot so the pickup factor wouldn't be needed.
Unless trying to come out of a setback mode, this wouldn't be of any "real" help - Is this correct?

EBEBRATT-Ed

The only time I see pickup being needed on a hot water system would be when starting a new system on a design day in the winter in an unoccupied building with no lights or other internal heat gain. Other than that it is just over sizing.imho



Ed

Keith M

you should consider

You really should consider the method you used to determine heat loss, the type of system and design of system.

We all know steam and water are treated differently. I will discuss hot water systems only and residential only.

The IBR heat loss method I was taught stated "wood floors" over basements where the boiler and heating pipes are located are treated as having no heat loss. Jacket and piping losses maintain the temperature of the floor and eliminate heat loss through these floors. The 15% piping and pickup is used here, it is not lost.


Is the system a series loop baseboard with "smaller" diameter pipes where the pipes do not contain large amounts of water? Not much "pick-up" in these systems.

Does the system have a large water volume content? These systems can certainly have significant "pick-up".

Are the heating pipes located within heated spaces? In other words does the "piping loss" contribute to heating the space.

Some boilers do not carry a IBR rating and do not list a Net IBR rating. You should always know if the boiler you have has a Net IBR or just Heating Capacity.

While hot air furnaces list the heating capacity and not IBR Net, the duct losses are generally added in when calculating the heat loss for warm air. Many hydronic heat loss programs do not add in piping losses. Years ago I was a member of ACCA. While ACCA is certainly a fine organization, the method they used at the time really oversized boilers. If my memory is correct they said do the heat loss, include duct or piping
loss and size boiler using Net IBR. Yikes!

I guess the bottom line is hire a pro.

Rob_32

can we add an indirect to this discussion?

Interesting stuff. Can we add an indirect water heater to this?

In sizing my system, for example, I have a 50k BTU/hr design heat loss, 60k BTU/hr possible fin-tube radiation output at 180F, and a plain vanilla 40-gal indirect. Seems like indirects of this size want to see a (DOE) output of around 115k-140k BTU/hr for rated recovery (mine I believe is 120k).

I chose to size the system (single stage boiler, neither modulating nor technically condensing -- Burnham Revolution with 84k DOE output) closer to the space heating load than the indirect. I use a DHW priority control in an attempt to effect better recovery given that I'm ~30% below what the indirect manufacturer wants to see. Running the tank hot and tempering down, too. I attempt to mitigate the oversizing relative to space heating by using outdoor reset.

That sound about right for a non-modcon application?

What would one do if constrained to a standard CI boiler, or is this question moot now that modcons and low-return-temp-tolerant boilers are becoming more widespread? Primary-secondary with outdoor reset?

Thanks.

Mike E_2

Think about this when looking at indirect BTU needs. The vast majority of homes in this country are using gas fired traditional water heaters - usually 30/40 gallon. They have an input of around 35,000BTU, and a running efficiency of about 75%. So that comes out to 26,250 BTU/hr going into the water.

Using the 84k from the Revolution RV4, you'll be over 3 times the capacity of a standard water heater. Yes, you won't have the full rating of the indirect, but most homes in America get by on far less.


I also agree with the others that the 1.15 factor in hot water systems isn't really necessary. The 1.15 is to help in a cold start-up. The only time that you need full output is during design conditions, and when it is that cold outside, you most likely won't be having any cold starts. So in the spring/fall when you have cold starts, a system that is sized to DOE numbers at design conditions will basically have a 1.15 factor available since your house doesn't need full output.


Michael

Brad White_9

It is

moot in a hot water system run by outdoor reset and with continuous circulation. On a steam system this is less true because each time it cycles, the pipes fill with room-temperature air on the down-cycle. Not as cold as a cold start but there still is some lifting to do. Well-insulated pipes can mitigate this a good deal. In other words, you can get away with less than 33% reserve. Each system is different.

Todd_12

Thanks Brad!

I have used the DOE capacity in the past to select a size and never ran into a problem. I think by the time you add all the fudge factors in, the units tend to be at least one or two times the "true" size.