Boiler sizing/Pickup factor in the 21st century.

SteamingatMohawk

While I understand the use of pickup factor from a historical basis, I have a question. It seems to me, generally speaking, replacement boilers at the same capacity as the original can be overcapacity for today's "conditions" (e.g., less radiators installed than originally, better windows, maybe even more insulation). That being said, how important is using the historical capacity, vs. going through the evaluation of what is needed for today's buildings I have seen that concept mentioned occasionally on HH. Can the pickup factor be reduced as well?

Jamie Hall

First off, remember that physics is not affected by the century you are in.

What does make a difference is custom and building styles. So… for any heating system, it is important to consider what the actual heat loss of the structure is. That's always been the case, but older structures had higher heat loss, in general, per square foot of usable space, and for some — built during the extreme fashion for fresh air a century or so ago — had very high "infiltration" losses to cope with.

So step one.is to determine the heat loss. Now step two varies with the heating system, since steam, hot water, radiant, and forced air all have quite different requirements. In any case, you have to remember that the heating system is… a system. No one part is independent of the rest of the parts. If you have an existing system to work with, it is very likely that the power source — boiler or furnace or heat pump — will be limited or even determined by the rest of the system which is already in place. In steam, for instance, the power output of the boiler must match the power demand of the installed radiation (and is likely to be greater than the power required shown by the heat loss)(you mention removed radiators — you will need a smaller boiler, for instance — but new windows or more insulation are irrelevant for steam). Hot water radiation may also limit the boiler, not so much in size but in the temperature needed to actually produce the power output needed from the radiation. Forced air systems may be controlled by duct work considerations. And so on.

Pickup factor — only applicable to steam — is an unhappy term, and the subject of a good deal of debate. I personally would much rather look at it as an allowance for steam use which isn't included in the radiation calculation, such as uninsulated pipes. Assuming that the system is otherwise in good shape — insulation on the pipes, at least adequate venting, reasonable pipe runs, etc., it seems that an oversize factor of no more than 10% will do. It is critically important to remember that more boiler output will not heat the space any faster — it is the output from the radiation which governs.

Now there are two aspects two historical considerations, and I'm not sure which one you are looking at. Historical data, used with caution (a lot of caution) can help in figuring the building heat load. It shouldn't be the only source of information, but it's helpful. On the other hand, historical data in the form of looking at what equipment is installed (particularly boiler size) can be very misleading.

EdTheHeaterMan

I would have to ask Buck Rogers in the 21st century about this.

His friend Flash Gordon may also know.

But that is enough of the comic reference. I believe that the standard numbers developed from the 20th century may be outdated, however those old buildings are still in existence. So the old sizing pick up factor still applies. 

If designing a new system, where all the connected piping is located in the conditioned space, then much of that pickup factor in hydronic design is not lost to a non conditioned space (like a ventilated crawl space), then it stands to reason that a 15% piping and pickup factor is a little overkill.  But on the other hand, some builders are still building homes with ventilated crawl spaces, so that needs to be considered and perhaps a 20% pickup factor is more in order.  

And I still believe that a heat loss calculation is not Rocket Surgery.   If you are 5 to 10% off, you are not going to miss the celestial target by a couple of light years.  The scalpel will not knick the artery, and the building will not be under heated if you select a size a little larger than necessary.   

Selecting the 138,000 BTU input boiler in a building that requires 110,000 BTU on the design day, is not a crime.  As long as they keep making “modulating input systems”, then selecting the proper size boiler is not as critical as it is compared to ON OFF single stage input boilers.

SteamingatMohawk

@EdTheHeaterMan said "But that is enough of the comic reference. I believe that the standard numbers developed from the 20th century may be outdated, however those old buildings are still in existence. So the old sizing pick up factor still applies." 

The posed question was simple enough.

@EdTheHeaterMan hit the nail on the head. While the century old houses, like mine, may still be around, I am reasonably sure (without proof) improvements to the buildings have been made such as additional insulation in the building and on the piping, replacement windows and doors and anything else that reduces heat loss.

That's the point to ponder.

As usual, thanks for the feedback.

Jamie Hall

The real trick with older houses — particularly very old houses — is to really get a handle on what "improvements" were made — and how much difference, if any, they really made. And then what the actual heat loss numbers might be. Example: old double hung windows may have been replaced with fancy modern windows. Or old storm windows. Or new storm windows. And may or may not have been adjusted. Makes a big difference (those listings of things to do are in order, by the way, as to reducing heat loss). What are the walls made of? Was any insulation used? If so what? When?

Gets to be lots of fun. With steam it's easy — look at the radiators and ask the occupants: can you get the house to where you need it in terms of heat? — and go from there. Other systems… um… not so much.