A Question for Steamheads

Mike T., Swampeast MO

Do you find gross oversizing of steam boilers to be as nearly universal as gross oversizing of boilers in old hot water systems?

TGO_54

Steam boilers



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TGO_54

Steam boilers

Hi Mike,
On oil fired systems (what I see most of) the problem is underfiring older boilers. The service techs reduce the nozzle size and the house no longer heats properly. The customer is then told to convert to FWH cause "steam don't work".

When doing service on these systems we will bring the firing rate up to match the connected load and check/repair the venting.


When doing a replacemant, we will oversize a boiler (slightly) and then underfire it to match the load of the connected radiation as closely as possable. You can check with the manufactures to find how far down you can go without problems on their boilers.

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Steamhead

Most are oversized

usually it's a question of how much.

gehring_4

question

Technically, what is considered oversized? That is, what percentage of a boiler's rated output must be connected in order for it to not be considered oversized? 80%? 90%?? or what?

I am just curious because a boiler's rating will jump in increments as the number of sections go up, but rarely will that ever match exactly to the connected load of radiation in the house.

so, I am just curious to hear what the rule of thumb is regarding this.

Mike T., Swampeast MO

Thanks Gentlemen!

Would be great to hear more experience.

Steamhead: Your steam system upgrade update was perfectly timed.

Boiler manufacturers provide an EDR-based rating for steam boilers. EDR ratings for radiators are easy to find and extremely accurate--after all, those ratings CAME from steam, not hot water.

It seems to me unforgivable to oversize a steam boiler when it's easy to measure the need and match such to the boiler's rating.

A properly sized steam boiler driving a steam system is quite capable of providing nearly the net efficiency of the boiler to the system. Provided the system is reasonably balanced to the spaces involved, the worst things you get by weatherizing (decreasing heat loss) are greater space temperature fluctuation and magnified imbalance in mild outdoor conditions.

On the other hand, a water system suffers TERRIBLE efficiency problems the more it is oversized. You may be able to get better-than-steam comfort, but you've thrown a huge number of BTUs up the flue.

This is yet another place where rated efficiences vary GREATLY from reality.

THP_6

Oh my yes!!! I think most times in the field most mid sizes replacement (100hp or so) are always base load times 2. That must be a rule of thumb for a lot of people.

Boilerpro_3

Oversizing....

It seems in many smaller systems the boilers are not oversized as much. Usually 40 to 50%. However, in larger homes or commercial, oversizing can get way out of hand. That factor of 2 seems common. My worst to date, however, was in a 2,200 sq ft home a few blocks from me. They called saying they needed the boiler replaced. I stopped by and the first thing I saw was a commercial gas meter on the side of the home. Then I nearly tripped down the stairs into the boiler room, as the shadow of the boiler fell over me. There sat a Weil Mclain LGB, 680,000 input running at full bore. I replaced it with a 200,000 input, bills dropped about 40 to 50% and you no longer could here the one pipe steam radiator vents howling through the 12 inch brick walls when the boiler kicked on.

About five years later I get a call from another large old home just down the street from that one. The retired women along in years had put out quite a sum of money to have the steam boiler replaced a few years before. She complained about the high fuel bills, the boiler was cycling on and off, and she left the basement door open to let all the heat come upstairs.
I went down and found a 300,000 input weil mclain EGH set up with a power venter exiting directly through a boarded up window with no clearance around the single guage pipe. The thermostat they installed did not have the anticpator properly adjusted and the scale didn't go up high enough to properly meet the boiler needs. Measured up the radiation and told her she only needed a 175,000 input boiler (A burnham power ventor would have been the ticket. She called the installer( a 40 to 50 year veteran to boilers in the area) and told him what I had done to size the boiler. He told her I didn't have any idea what I was doing. Oh course the piping was wrong too, and he said he's been doing this for forty years and I didn't know what I was talking about.
So much for experience and talk about apprenticeship training going all wrong. Let's just keep passing on our mistakes to future generations.

Boilerpro

Steamhead

Well,

traditionally you wanted to get as close to the EDR total without going under. However, Boilerpro is experimenting with slightly undersized boilers and we eagerly watch for his updates.

The above assumes that the piping system is well insulated and there are enough main vents. Problems in these areas can make a properly-sized boiler seem undersized.