What would you do? (hb)

heatboy

of 126MBH and it falls right in the middle of two boiler selections, 116 and 146 MBH outputs. 13,000 total feet of tubing (1/2" & 5/8") split into 8 heating zones and the house is built like a tank. We all know not all zones will be running continously, so very seldom(?) will I need total capacity. To make it more interesting, If I move my design temp 60 miles East (Allentown 10°F to Newark 14°) the load falls to 117MBH. The home is halfway between the two points in Central Joisey. Which way are you inclined? Roll the dice or CYA? I do have a 92 gal indirect to think about also.

hb

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Dan Peel

Look Up

I'd opt for the larger boiler - especially with the winter we just came through. That said the worst that's going to happen with the smaller boiler is you can't maintain the stat setting for a couple of hours on the coldest day of the year at four in the morning. 92 gallon? go up. Enjoy...... Dan

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J.C.A.

How about....

Programming a few stats in the biggest offending zones to beat the demand, and going with the lower size. Your right about the worst case scenerio seldom being called on , but this winter WAS a test eh ?

I think setting the biggest or most used zones to get themselves up to temp a bit earlier than the H.W. demand would work pretty well . Comments gents ? Chris

Paul Pollets

I'd go

with the upsized boiler. I like to fire a 92g tank with 150K BTU's so I can recover from 50-140deg. in 12-14 min. with a 26-64 pump. I've noticed the horizonat tank recovers a few minutes faster. BTW, are you using a vitodens, vitola or vito100?? Or none of the above?

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bigugh_4

there is 20%

Between the boiler sizes. there is 4% between the weather estimates. We know there is a 5% fudge factor in the heat loss statement. That comes down to 11% not accounted for. Let us assume that since it is a radiant floor system,there is a flywheel effect we do not know. I'd say that the smaller boiler should do fine, and like JCA says, programable T/S's sure could make life ok there!
jmho
bigugh

kevin coppinger_4

are those numbers...

DOE or IBR numbers? If some of the zone piping is running through heated areas you could figure more on the DOE or smaller boiler size since the DOE ratings figure for that in the rating...kpc

TGO_13

I think I'd

take a close look at the domestic needs. With a 92 gallon tank you must have a pretty large load, If so, I would probably go with the larger boiler. tekmar make some controls that could help with any potential short cycling.
JMHO

[Deleted User]

Roll the dice...

I'm sure every one of us has been in one of our fer sure guaranteed properly sized boiler rooms during design conditon, or near there, and how many of us have seen the boiler running full time, like it's theoretically supposed to.

Not me. Closest I've seen was around 66% duty cycle on systems I know for a fact were properly sized. That tells me we are oversizing our physical plant on the order of 1/3, and in a lot of cases 50 percent or better.

Unless there is an issue with DHW demands that you didn't tell us about, I'd go for the lesser of the two. It'l operate MUCH more efficiently than the big one.

Now, if it's a modulating boiler like the Vitodens, then it doesn't matter, cause the boilers gonna adjust to the lessened demand anyway. Right?

Right on. Modulation is the only way to fly...

ME

ScottMP

I would be concerned

About recovory with the indirect.

Also what about fututer additions to the home. Fours years from know they decide to put on a great room and the boiler now has to be replaced ?

Scott

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Boilerpro_3

\"built like a tank\" and Heat load

I think this is the key to your quandry. If the house is heavy masonry construction, I'd go with the smaller boiler. From studies done with solar, it takes about 24 hours for 4 inches of concrete to change temperature significantly. If you have 12 inch brick walls, that design day cold temp won't reach the interior space for about 3 days. That mean's that the wall load won't change until about 3 days of extreme cold. Usually, it warms up during the day some and you will swing above design temp before that three days is over. Take a look at the advanced cooling load calculations (such as the CLTD calcs) and you can see the significance of mass on how quickly loads change.

Also, what is you building balance point. This is the outdoor temp in which the home can heat itself without the heating system turning on. When someone is at home, lights will be on, cooking and showers will generate heat, the sun will be out. All of these will add additional heat to the space. In old uninsulated homes, it usually could keep the home about 5F degrees warmer than outdoors, so to maintain 70F inside, the heating system would need to come on when it was 65F outside for the average daily temp. Newer homes, or any home with lots of solar gain for instance, usually don't need heat until its 60F outdoors average. Apartment structures, with thier high internal gains, don't usually need heating until its in the low to mid 50s.

In my area, with a 98% design temp of -5F, to maintain 70F inside, I use a 70F delta tee for older uninsulated frame homes. For new, well constructed insulated frame homes, (standard or poorly constructed newer insulated homes often are so leaky that they aren't much more efficient than old uninsulated homes, IMHO) I use a 65F delta tee. For masonry structures, I keep the boiler size on the light side.

Internal gains and the thermal flywheel effect is probably why we see many boilers that are sized right only firing at 66% on design days.

Boilerpro

hot_rod

Or you could

consider other brands that match the load closer We won't tell!

How about the condensing type that would modulate, and step to high fire for rapid DHW recovery? Then all bases are covered. Or is this oil?

I think I would let the DHW load effect the final choice. Lots of the manufactures will allow a little adjustment to fine tune the boiler output if you really want to dial it in that close.


hot rod

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