heat loss calcs trying to mess me up?

tk_3

My house was built in 1919 and is about 1800 sq ft. No sidewall insulation, good windows and door, 10" in ceiling and 3" under floor. Heatloss was about 60k. Installed a 68k boiler with an indirect. Family of 5. Working 11 years and doing fine. Cut the fuel bill 52% when I tore out a 6 yr old boiler along with 3 year old direct fired water heater. Trust the numbers. Having the second floor ceiling against a heated area will save a ton of heat loss. Don't waste your time doing another heat loss. You've done two. A sign of an insane man is one which does the same thing over and over expecting different results.

fatty

I did two separate heat loss programs on one space I'm looking to heat - and I simply can not bring myself to trust the results.
Let me describe the building. Built 1900 - 3 Floors, 10' ceilings, Brick with 1" air gap. The top floor has Forced Air, so I didn't include it in the heatloss... A dozen windows per floor, big 3'x4' windows.

The floors are about 15x55 each - so the estimate was for only the 1st and 2nd floors. Granted - the gc has put up stud walls inside with R13 over the brick - and has foam ins. in the bays, etc. Windows are new, good vinyls, foam sealed. Still I'm getting 50,000 MAX BTU/hr no matter how I swing this... I feel like I'm setting myself up for problems if I set a 50k boiler in here. Again, its to heat the 1st and 2nd floors only.


I dont mind undersizing... but seriously... the average calculation results 44k btu/hr and with some tweaking still doesn't go over 50

many thanks in advance for your time and consideration

(Cast Iron radiators, planned for 4 zones, 1 pump, outdoor reset)(existing defunct boiler looks to be a 150k btu, installed before renovation and insulation)

Jim Pompetti

Heat loads

Many times I have Performed heat loads and was suprised at the results .Thinking that can't be right , but over the years I've learned to trust the math.Weil has a load sheet that is really general , try to get a copy . It was support your heat load .

targetman

How are you treating the ceiling of the second floor in your heat loss calc? Under a heated space?

Boilerpro_5

Actually your number sounds a bit high

I just ran a quick calculation based on Weil Mclain's charts and end up closer to 40,000 for -4 design and these charts tend to be conservative.....they oversize. With a brick structure, you can undersize because the brick moderates the really cold outdoor temperature... a thermal flywheel good for about 24 hours per 4 inches of brick. Also since the building is soo well insulated (the combination for the walls should end up about R-19), people just breathing, cooking taking showers, etc. will keep the building warm using about 10 to 15F above ambient. So if your design day is -5 with 70F interior, you should design for about +5 to +10 with 70F interior. Also, since one side is protected by the adjacent structure, air leakage and heating loads are reduced. I bet a 35,000 output boiler would be enough. Just FYI, I live in a two story frame from 1906 with 1400 sq ft per floor, 9 foot down 8 foot up, full basement, R-19 walls, R-35 ceiling, and 700 sq ft of original windows (weatherstripped) with aluinmum storms and my heat load at -4 is about 45,000 btu/hr. It takes alot less heat than most realize, especially when working with hot water or steam. You may want to sell them a air to air heat exchanger, it sounds like they will need it.

Boilerpro

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Brad White

You mentioned two

separate programs. What are the relative, not avarage results? Did I miss that?

Overall the outcomes seem rational especially given the insulation and foam used.

Normally and as a benchmark only, I would take such a building, if uninsulated and in a zero-degree design area at about 50 BTUH per SF and when improved to about 30-35 depending on certain variables. (Not calculation factors, just how they work out).

The 45-50 MBH does not seem unreasonable to me as a gut-check.

Your use of two calculation programs and results reminds me of what my dad always says: "A man with one watch knows what time it is. A man with two watches can never be sure."

fatty

some sound advice

yes, thank you voices of reason.

the average (mean) between the calcs was 45000 about. i believe it was ~42000 and ~48000 as the results I think I have some pretty good options with the smaller boiler, saves a bit of money and it sure sounds efficient, although i guess the design of the system will determine that. i think its the 8" brick with the air gap that threw me for a loop.

i guess i'm good to go! any preferences for a small boiler?
thanks a brazillion

Brad White

8-inch brick

The 8-inch brick with an airspace I presume means two withes of brick with an airspace between? Maybe some bituminous parging? Plus a plaster wall finish on lath with some furring strips to form a second interrupted air space?

With that setup, I get a wall R value of about 4.47 assuming common brick and both air spaces in series. The u-factor would be about 0.2233 (about? yeah right...)

Insulate that sucker with a stud wall and R-13 and I get a wall assembly worth R-14.07 and a u-factor of about 0.071.

Over every 1,000 feet of wall with a 68 degree delta-T across it, is the difference between 15,185 and 4,828 BTUH, over 10,000 BTUH right there.

fatty

yep brick is mostly stretchers 2 widths, bonded every 8th or 9th row, so not so much thermal bridging as would be in a flemish bond or an english. the furring strips remain, setting the stud wall off the brick 1", less thermal bridging there too, so thats two 1" airgaps (almost).

there are 4 separate zones (3 apts and a lobby) for this building. a <40k boiler has 3/4 supply piping... none of the zones exceed 15k in heatloss - which means theoretically even 1/2 copper could supply each zone with enough heat.

[Lobby- 5800 Btu/hr, 1 3/4" spud CI rad]
[1A- 15000 Btu/hr, 5 1/2" spud CI rads]
[2A- 11400 Btu/hr, 3 1/2" spud CI rads]
[2B- 11400 Btu/hr, 3 1/2" spud CI rads]

I feel that I should still pipe 3/4" zones. It's a pretty high-head system for its size @ 22-24 head feet, due to reverse-return design, radiators and zones. plotting the curve points toward a taco 009 3/4. so lower delta and more pump power wasted... but still, trying to spread that heat out even

Plumb Bob

Possibly the calc sounds low to you because the previous boiler was way oversized. But one should never size a new boiler based on the old boiler.

Right-sized boilers are more efficient, but they come with some tradeoffs. They naturally take longer to heat the water to full temperature, so the residents can't turn up the heat quickly and casually when e.g. elderly relatives stop by. It may take hours for the thermometer to budge. For the same reason, one can't do nighttime setback.

But, allowed to run steadily, a right-sized boiler will keep the house warm, and those blasts of intermittent heat from the radiator will become less noticeable (they will go away completely if outdoor reset is used).

Brad White

Let's take a look at your pump heads first!

The notion of a 22-24 foot head loss for a small system gets me to "huh?" really quickly.

If this is multiple-zoned using zone valves, you do understand that the head loss is not the sum of all zones together but the head loss of the most resistive (usually longest) run.

With the heat losses you describe, none would warrant more than 1.5 GPM. Sure, 3/4" is plenty adequate as you say, 1/2" might do it also.

But for a 3/4" pipe to carry 1.5 GPM and incur a pressure drop rate of 0.85 feet per 100 feet, are you really running half a mile of pipe?

The radiator losses are negligible. I would be hard pressed to think of a single zone worth more than 8 feet of head assuming 100 equivalent feet out and the same coming back, plus peripherals.

This is why I think you may be adding each circuit's head loss rather than having the worst one define the circulator.

What was your methodology?

fatty

oh wow, saving my butt

geez brad you are right. i actually added the zones together, assuming if all were open then the head would add up. my methodology came from one of Dan Hollahan's books, which stated that 6 feet of head for each 100ft of pipe is a good rough estimation. but zones weren't mentioned as being a factor.

the longest run comes out to... about 8 feet of head. hey... pretty good that you could ballpark that without even looking. cripes!

... i guess i could see how 1/2 could work, assuming the boiler is sized right, the water would be circulating a large portion of each day. with a reverse return its not going to short circuit... i sure wouldnt mind using 1/2. just seemed odd to branch off a 1/2 supply with multiple 1/2 T's

j.cricket, thanks for the input - i plan on using a reset.

Brad White

Whew....

I for one am relieved...

The notion of reverse return may even make that head loss a little less, certainly practically self-balancing. As you probably know, reverse-return has each runout circuit with the same head loss as any other. If the radiators have the same pressure drops this pretty well defines "self-balancing". Such a deal...

I would keep it to 3/4" mains at least and 1/2" branches. Nice.

fatty

i put together a couple images to put visuals to all this: I do realize the home-made quality of it. but they are really just for my own benefit, so nobody else has to know about it... i hope.
man i love this stuff!

Brad White

Nice

Nice drawings-

I take it that your boiler BTU rating noted is output not input?

Have you thought about using manifolds and homeruns of Pex-Al-Pex by the way? Not to throw in a curve ball or anything. Also, have you thought about TRV's on the radiators?

Just a couple of ideas which may have merit. Nothing wrong with what you have, though!

Brad

fatty

output yes,

i'm a sucker for a curveball. i like the trv's conceptually but application-wise - i don't think i could use them, due to tenants having access to the controls. the system i have been working up is centered around the remote thermostats and 'waste' heat (planned it to run the pipes under the bathroom tile floors... not really a waste) i know the pex-al-pex doesnt shed much of its load on the way to the rads like the copper will, although i have all the tools to do pex already.
i feel ready to begin this project - with winter bearing down - there is a nice sense of... urgency.
bradford, thanks for lending me your ear. i cant explain how much i appreciate your advice. i look forward to bettering my brain - and this site has greatly facilitated the feeling that i can.

fatty

uh oh i just found tamper-proof locking trv's
now i'm going to have to swing at this one. i assume i can pipe the same copper as planned -
then eliminate the zone valves, zone control and ... then how would i integrate the reset? actually now i'm not sure what kind of control this system uses.
hm, so far i see it saves money, has better room to room temp control... less work. sounds like a hell of a plan.

Brad White

Will the tenants have primary control?

I think TRV's are a nice way to go- they prevent over-heating especially if the sun hits the glazed areas for long. They will add some cost but the Pex-Al-Pex will reduce the cost to make up for that. I would keep zone valves for main "call for heat" control if tenants have that option.

No change on the reset strategy- that does not change.

You can pipe Pex-Al-Pex right to the radiators but many use hard pipe to below the floor where the Pex-Al-Pex takes over. I used 5/8" for the runouts on a conversion of an old gravity HW system and it worked great.

If you go with what you had originally, no problem. Don't over-think it. Good Luck!

Brad