Sizing boiler

Boilerpro_5

Unless you are very careful about your assumptions, our loads will calculate high. In addition, heat load calcs make a few assumptions that are completely unrealistic.
1) They are instantaneous. In other words the thermal flywheel affect of the building mass and furnishings is ignored....something that is included in proper cooling load calculations. Homes with lots of mass (masonry costruction), can have boilers "undersized" and still heat just fine because that real low temp at 5:00 in the morning on the coldest day of the year is never seen inside the home because it has not transmitted through the walls until 2 to 3 days later (Heat "travels" through heavy masonry about 4 inches every 24 hours due to the mass, according to solar mass designers). For my area the design low is -4 but the diurnal(sp), daily outdoor termperature swing is about 17 degrees, so the average temp on the design day is about 8 degree warmer. +4 F is probably a much better design temp than -4 for bulding conduction loss. I would stick with -4, however, for air leakage losses as this is more instantaneous.
2) There are no windows that let in solar heat or it is perpetually night. Where most of us live (the guys up in Alaska and north have a different life) the sun helps heat the home. The more windows on the south and the better insulated and tightened the structure, the lower the load becomes on the equipment.
3) There are no internal gains. No lights, no people, no showers or baths (yikes!), no cooking, no computers or TV's, etc. Oh course none of this is true, except when the space is unoccuppied, and them there is no need to keep the space at 70F, 45F is just fine.
4) The home is floating above the ground. This is especially applicable to brick construction. The ground stays about 55F at 5 to 6 feet down. The warmth from the ground is conducted upward through the structure and moderates the impact of exterior temperatures, so less supplemental heat is needed. This helps explain why typically very poorly insulated brick homes really do not cost more to heat that a typical older insulated frame home. (lower infiltraion in a brick home probably also is a factor)

These last two items are reflected in your building balance point, the temperature at which solar heat gains and internal gains equal the heat loss. Back in the old days this was probably about 65F average daily temp(remember the effect of that daily outdoor temp swing) becasue homes did not retain heat well. However, in homes designed with some recognition of the sun and/or have tight, well insulated envelops this average balance point temperature can drop into the mid 50's pretty easily. In fact a high mass structure can mimic this effect to a certain extent.

Let's take the typical 70F indoor temp at -4 outdoor design and apply some of these changes to compensate for reality. Typically it is designed as a 74F temperture difference.

The effect of daily temp swing brings the design temp up to +4 for conduction, still -4 for infiltration, but thermal mass should make up the extra infiltration loss, so +4 should be pretty accurate.
The home is well insulated and someone lives there, so internal gains allow the house to stay warm until the average daily temp drops to, lets say 58F (daily high of 66,daily low of 50.....sounds about right for an insulated tightened up home, doesn't it?)
Ground contact heating should be good for some improvement even in a frame home, as the basement receives benefit



With these changes for reality, we are now designing for a 58F - 4F = 54F temperature difference instead of a 74F temperature difference or 20/74 = 27% lower heat loss. Ground contact tempering should add some additional savings, maybe 3 or 4 percent for a frame house, and probably closer to 10% for a brick home.

Taken all together, this would account for much of the experiences of those that have found real life loads to be dramatically lower than manual J heat losses.

If you want to look at these ignored factors some more for larger structures, take a look at Henry Gifford's article in the recent PM. With what he has written regarding his successful analysis and modifications of steam systems and careful investigation of heat loads (in both caes moving way beyond the best of the best norms) , he is probably one of the sharpest in the business.



Boilerpro


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Mark_7

Sizing boiler

How close do you guy's like to size a boiler to the targeted btu of a house? Ex: house has 81,254 target load, Mpo 147 net of 112,000 or Mpo 189 net of 145,000. Will have smart 50 indirect w/priority. House has new addition of staple up plates/radiant tube and old side is cop. baseboard.

Steamhead (in transit)

As far as I know

every heat-loss calc program has a fudge factor built into it. I've installed boilers whose Net ratings exactly matched the calced loss, and they still appeared to have plenty of headroom.

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Uni R_2

Target load

If your target load is a typical manual J, you would probably have a true load that would be less than even a boiler netting 70,000 btus. So assuming it was 70,000, an MPO with a net of 112,000 btus is 60% oversized and the next bigger one is more than twice as big as it needs to be on a design day.

My Manual J was 57K net and turned out to be 37K gross (done by clocking meter at design). Using the same ratio, yours would be 53K gross. I think anything around mid 60s or higher would work well for you in case you greatly understated the infiltration and or possibly the insulation. A Monitor FCX (76,000 net) or the Peerless Pinnacle Oil (64,000 net) would probably be the most efficient options in oil that I could think of.