Help! Boiler Sizing Question

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Old 04-03-13, 01:07 PM
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Help! Boiler Sizing Question

Hey y'all

I am converting to NG from Oil and am trying to figure out what boiler I really need for the house. The current boiler is 140 BTU Peerless WV-DV Boiler, with an indirect water heater and two heating zones (each with forced air handlers, fan coil).

Some calculators say 108 BTU, others say 140 and some say 60, so I am really unsure to what I need. Here are some details below on the house and I would appreciate any help here!

Details
- Built in 2010
- R30 Insulation Attic, R20 in Walls
- Located 40 Miles north of NYC
- Three Stories (1st, 2nd, 3rd floor attic room) along with a finished basement
- First Floor Size: 1100 SQFT, 2nd floor: 1100, Basement:600 sqft ft, Attic: 200 sqft
- Total SQ Ft roughly 3,000 sqft
- Lots of windows in the house
- 3 heating zones, with 2 air handlers (1 is split for basement 1st floor)
- Target heating temp ~68/70
- Smart50 indirect water heater
- Use roughly 140-150 gallons in oil a month during cold months

I dont plan to add any more rooms, so can someone recommend what size (BTU's) I should be targeting? I have to imagine my 140k oil BTU is really oversize
 
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Old 04-03-13, 02:36 PM
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You need to input the required data into the Slant/Fin heat-loss software, see this post for more information and where to download (free).

http://www.doityourself.com/forum/bo...alculator.html

You should be able to gather the data in a few hours. You need a solid estimate, not just a horseback guess.
 

Last edited by NJT; 04-03-13 at 04:48 PM.
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Old 04-03-13, 05:32 PM
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A 140,000 BTU boiler would have to be very oversized for you home.
Thats around 50 BTU per sqft.
If your towns construction practices follow national standards, your 2010 home should be very eff. I bet you could heat with 15 BTU per sqft.
Not knowing anything at all about location and construction I would quess a 60,000 BTU boiler would be fine.
Luckily, I don't guess in the real world. Do a heat loss, or pay someone to do it.
Go find a wholesaler that will do it if your contractor will buy your boiler from him. They might do it for free, just to ensure the sale.
 
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Old 04-03-13, 05:43 PM
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What's Barba say?

"Amateurs guess, Pros do the math!"

Since I am 'Everyman', I get to do both!

My horseback guess is less than 75K, and SlantFin will probably come in just under 70K.
 
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Old 04-03-13, 05:45 PM
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I said you don't need a horseback guess. But with a 3,000 sq ft house, three years old, and the insulation you describe, your 140 kBtu/hr unit is way oversized. Maybe by a factor 50% or probably more. But please do the heat loss calculation! And don't add any safety margin to the result, it's already in there. Match the new boiler's DOE OUTPUT rating to your calculated heat loss.

Does the manufacturer of your existing oil boiler/burner offer a gas conversion? If so, you might be able to downsize your existing boiler with minimum additional expense.

Congratulations for being able to join the natural-gas world. Since your boiler is only three years old, was gas on the immediate horizon when you installed the oil-fired boiler? If so, it might have been better to initially go with propane, and then easily convert to gas. But, whoever installed your oil boiler would have over-sized the propane boiler, too.
 
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Old 04-03-13, 08:17 PM
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Well, I just bought the house in August so I inherited the boiler. Oddly, the boiler was made in 2008, yet the house was built in 2010 so perhaps the contractor had one lying around.

As for the gas, there was and still is a gas line right on the street, about 30 feet from the house. Who knows why they didn't do gas, perhaps too lazy I dont know!

I am hoping to maybe sell my oil burner locally to recover some of the cost, but I think I am just going to get a peerless purefire and swap it in. Luckily, the current oil burner is direct vent with all of the manifolds, pumps, indhw all in place, so the conversion is fairly simple assuming the gas company is on top of things!

I will let you know on the slat fin calcs...going to use the building plans which should save some measurment time!
 
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Old 04-03-13, 08:27 PM
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You'll have a large hole in your wall when you remove that direct vent... Hmmm I think you could get a good penny for that oil boiler...

Best to get the concentric kit for the new boiler. It will probably fill the hole perfect...
 
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Old 04-03-13, 09:16 PM
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Can't I use the existing direct vent option for the new boiler? If I go with a pureless purefire, the manual says it is OK to use the metal exhaust/intake combo as long as you run PVC into a sleeve into it. I should be able to just use the existing, albeit it is a tad bigger than needed

Drum Roll...I ran the slat fin - took me 30 minutes, but I have the architectural drawings with all room sizes, window sizes, etc...all in front of me. I rounded up a bit and my BTU estimate was ~88,000. Now, this was for a 3150 sqft home according to my measurements, when the house is really more like 2950, so it is more like low 80's, high 70's. I also put in medium insulation when the attic is really R48 and assumed I heat the basement, when really I rarely need to turn anything on.

Ironically, this lines up perfectly with the purefire 88k model, which we were looking at.

Neat program at hand!
 
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Old 04-03-13, 09:23 PM
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the manual says it is OK to use the metal exhaust/intake combo as long as you run PVC into a sleeve into it
I would need to look at the manual.... Sleeve it? Your going to run a vent and intake through it? What do you do when you get out the other side?


 
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Old 04-03-13, 09:27 PM
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DO NOT "fudge" upward with a heat loss program as they already have a fudge factor built in. More likely your heat loss is going to be more on the order of 70,000 BTU/hr. at the design temperature. Since you are rarely at design temperature (outside) but a few hours a year, or less, the boiler WILL still be oversized at 88,000 BTU/hr.
 
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Old 04-03-13, 09:35 PM
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So...there was a bug, as it had the bath taking up 21k BTU...fixed that and we are talking 70k BTU now via slant fin.


As for the vent. The existing oil boiler has a vent like that PVC one, same shape and all, just in metal. Perhaps they can just use that one?

Still baffles me why they put in a 140k oil burner? No wonder it burns oil like crazy!
 
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Old 04-03-13, 09:42 PM
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Old boiler for reference. I dont think there is a fitting that will attache pvc to metal pipe for boiler condensate venting......





I bet with your 2010 house, and if you do the calc right, you might be at a 50k boiler IMO.....

You said your over radiated and only need 140 f water on the coldest day right?
 
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Old 04-03-13, 09:49 PM
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Nope, I am over forced air (fan coil), so need 180 degree water.

Bummer on the vent, but at least they can foam around it or something when the PVC goes in.

How long you suppose it takes to rip out the oil WV-DV and install the purefire burner, assuming the gas line is set outside?

I figure the boiler is ~3k, then mayber 2.5k for labor and piping?
 
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Old 04-03-13, 09:55 PM
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Nope, I am over forced air (fan coil), so need 180 degree water.
Not my area but I dont think a modulating boiler will benefit you.
 
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Old 04-04-13, 07:11 AM
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All...is that true that a modulating boiler is not worth the expense for a forced air system? Also, do I need a bigger boiler BTU wise if I have forced air vs radiators?

I imagine that in cold weather the modulation will not be very useful, as it will modulate between 150-180 ish for heat, though I can see some serious benefits for May - October when it is only used for the hot water heater.
 

Last edited by gsimmons2005; 04-04-13 at 07:27 AM.
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Old 04-04-13, 08:46 AM
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If a modulating condensing boiler is not below 130F you lose the eff. A regular boiler would be best IMO..

I believe for comfort you dont want less then 140f water through a fan coil. That would be during the shoulder season with a slow fan speed. That 140f water you can get with a standard boiler thats 2012 doe compliant....


Others will chime in I am sure, so sit tight.
 
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Old 04-04-13, 10:11 AM
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Thanks...curious to see what the group says here, especially with an indirect connected.

Oddly enough, the price difference between a modulating boiler and regular boiler at 80k BTU is almost the same, once you take into account the extra aquastat equipment needed on regular boilers and the additional 500$ incentives for 90% modulating boilers from the gas company.
 
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Old 04-04-13, 12:20 PM
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Keep the $500 buck incentive. You will still be behind IMO.

Take a slant fin s 60 here... What is probably all you need, but could be wrong...

You must do a heat loss calculation if you want to size right.

http://www.slantfin.com/images/stori..._10.pdf%20.pdf

But these boilers could be bought locally to you. These sell from rutherford NJ. Here is an example of the s 34. The s 60 which they do not have any at the moment sells for $1300.

You can pick up and save the couple hundred on delivery.

Slant Fin Sentry s 34 DP Natural Gas Hot Water Boiler | eBay

This one was from a yonkers supplier. So you would be $2000 ahead of the game.

Slant Fin s 60 DP N Sentry High Efficiency Gas Boiler | eBay


These units have the 2012 compliant aquastat. water temps as low as 140 only on the shoulder season. Works off input from your t stat. Best controls I have found out there for this compliancy....

Just saying....
 
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Old 04-04-13, 12:28 PM
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I agree with Lawrosa, in your particular installation there is little to no benefit in using a mod/con boiler. While you could use a mod/con you would really need to have a sophisticated control system with a variable speed motor on the blower serving the hot water coil. Sophisticated controls, both on the air handler and the boiler, are costly and more prone to malfunction.
 
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Old 04-04-13, 01:21 PM
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Interesting!

Stupid question, with these cheaper boilers, it looks like the DHW functionality is limited and only works well when there is a separate circulator on the IDHW. Right now I have a zone control with on system circulator. It may be better to add a circulator instead while he is putting this in.

I would really like to get the 85% level though instead of the 83 on the slant fins because of the 500$ rebate (jumps to 1100 for 90%+). There are some Burnham boilers at 85%...anyone know of any that are 85% and cheap? Otherwise it makes sense to just get the purefire for 2700, get the 1000 credit and its cheaper than the regular boiler at 80%. Also, when you add in the controlers that are built into the consensing ones (and not on the slant fins), it doesnt make financial sense
 

Last edited by gsimmons2005; 04-04-13 at 02:02 PM.
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Old 04-04-13, 04:00 PM
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Otherwise it makes sense to just get the purefire for 2700, get the 1000 credit and its cheaper than the regular boiler at 80%. Also, when you add in the controlers that are built into the consensing ones (and not on the slant fins), it doesnt make financial sense
Not really. For DHW use a taco zone controller with DHW priority. $80 bucks I think...

Additionally your talking longevity of a cast iron boiler with minimal moving parts to a unit that may cost higher maintenance. The pure fire is SS heat exchanger, fans, switches, sensors,....etc... You need to take them apart yearly and clean the exchangers as well as the condensate line/trap... New gaskets...etc.

Also must pipe primary secondary piping. More materials. It has critical install criteria.


it doesnt make financial sense
oh but it does.... I say do the math.......

Just saying......My opinion of course...
 
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Old 04-04-13, 09:47 PM
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This thread is great and will save me lots of money. If the slant fin calc says something like 70k ish, do I need to get a 70BTU boiler, or something higher that when multiplied by the effeciency (85%) will be 70k?

Lastly, anyone know of any coventional boiler brands that have built in aquastats and zone controls? Or at least can recomend a good aquastat for forced air boiler operation? I have a becket aquasmart with outdoor reset that works well for my oil boiler, should I just get the gas version of that?

Anyone know about Burnham boilers? or should I just go slant fin victory (though the closest size is 90k, which is a bit large)

I also dont understand how I will save money with a lower BTU Boiler? If my current aquastat often uses the boost feature to heat the water to 180 or past the outdoor reset temp, wont the new boiler do the same and take longer to heat the water since it has a lower BTU? Or is it going to take the same time but not burn off soo much fuel?
 

Last edited by gsimmons2005; 04-04-13 at 10:19 PM.
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Old 04-05-13, 06:52 AM
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gsimmons2005,

I'm somewhat in a similar situation choosing between conventional and mod/con boiler. I like the conventional boiler for all the reasons that NJ Trooper mentioned. However Iím with you on the price. With state/federal incentives some mod/cons are as cheap or even cheaper compared with good conventional boilers.

I want a closed combustion boiler and that makes the choice between conventional boiler pretty small. And all of them require expensive exhaust piping, pushing mod/con advantage even further.

Iím still not 100% convinced regarding which way to go. Right now Iím leaning towards peerless PF-50. But I will review my choice when I actually have to have it installed. Prices might be different at that point, maybe new choices, new incentives, etc.

Here is my thread that you might find helpful: thread
 
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Old 04-05-13, 09:42 AM
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Will the victory boilers (conventional) work with my existing intake/exhaust since the flu gases are so hot as it does not condense them? The setup picture is below of my current oil burner. Just trying to get a flavor of what I need in terms of venting for a slant fin conventional boiler instead of modcon before the plumber tries to take me a for a ride
 
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