Should I Go with Larger or Smaller Boiler????

joe_66

Size youre boiler to the connected load approx 700 btu per ft for reg copper tube baseboard and only about 550 for ci.Don't add for an indirect a priority control would be more efficient.A Viessman vitola vb2 18 sounds just right.Forget AFUE its a joke worry about thermal efficientcy make sure youre boiler wont condensate or have to deal with thermal shock and dont go with a low mass boiler long burner cycles are more efficient.AFUE dosent say a boiler will preform the same on every system.

[Deleted User]

Should I Go with Larger or Smaller Boiler???? 75K BTU Heat Loss

I am having a new boiler installed this week. I used the Slant Fin s/w to determine the amount of heat loss in the house. About 75,000 BTU/hr Load. I am tossing between going with larger 125K BTU input unit and a 100K BTU unit.

I am looking to save as much as I can on heating cost, but I also do not want a house that is awlways cold. Normally keep the house around 68 degrees. Live in upstate NY.

Two story house. Seperate zones. We rarely have heat on both floors at the same time.

Any Suggestions would be greatly appreciated.

pitman9

Look to the Net IBR rating on the boiler

It should meet or exceed your heat loss. If there isn't a net IBR rating multiply your heat loss by 1.15 and use the net output rating of the boiler.

Above assumes this is a water boiler.

Constantin

Well, let's see...

Hi Mike, I am a fellow mere homeowner, but I went through a similar excercise as you did with my home not so long ago.

I'd select a hot-water boiler on the basis of the boiler being able to produce 75kBTU/hr DoE output on a design day if all the pipes are in the conditioned space. If the supply pipes are outside the space (i.e. unheated, uninsulated crawlspace or basement, etc.), I'd derate the boiler as suggested above.

If this system is steam heat, there are a lot more factors to consider, such as system type, the number and size of radiators, etc. Size any eventual indirects to the hot potable water load, not the other way around. JMO and YMMV.

Empire_2

Good amswers guy's.

Also dont forget that the heat loss should take into affect that there are unheated and or uninsulated spaces. Also, Just as an added thought, you said that you do not heat or at least keep other zones cooler. Inside walls are where the lightest insulation is. Doesen't pay to knock a room or zone down 10 to 15*. It's like when mom and dad had junior go off to college and totally closed off the upper flat on the house, well that upper flat if kept considerably colded is a , (cooler than normal space) but a colder adjoining space that needs to be heated also. I hope this makes sense. My brain thinks faster that I can type...

Jay_17

Also look

You may also want to look at how much radiation capability you have, pretty easy if you have baseboard, maybe harder to figure out if you have something else. Point being that if you only have radiators to emit 40K btu then a bigger boiler isn't going to do anything but cycle more often.
This is kind of assuming it is retrofit, if you are installing radiation too then of course you can do what you want.
Jay

[Deleted User]

Radiation

Jay,

We have a total of 100 feet of baseboard radiators. 50 feet of aluminum/copper fin baseboards on second floor. 50 feet of cast iron baseboard on the first floor.

Does this help.

thanks

Joe Brix

@180° boiler temp

That much baseboard can only put out 80K BTU using a generous 800 BTU/ft output factor? So why bother with anything larger unless you were going to expand the house or if you plan to use an indirect HW tank and you want good recovery. The biggest fuel saving comes first from right-sizing the boiler.

Jay_17

Ok

Well, typical copper tube baseboard is 700 btu per foot at 180 degrees. CI baseboard is probably different depending on make, and I don't know a typical value, but if you go with the same value you would have around 70K btus of output.
If someone out there can give a better idea of CI baseboard outputs that would be great, but making the above assumption it sounds like the 100K boiler will be enough, and extra. You could increase the output by upping the water temp (in an emergency), but copper fin won't get much above 900 btu I think, so you may be able to use all 100K if you had to.

Look at it this way, as long as you can supply the amount of BTUs the radiators can put out, then you're ability to heat the house will be the same as it was before. The oversize boiler would still have to cycle even when the heating system is running full tilt, it doesn't give you the ability to put any more heat into the house.
The correct size will run continuously under full output, with the radiators able to emit all it can produce, and cycle less under normal conditions, which is more efficient.
Just out of curiousity, what was the old boiler size?
Jay

[Deleted User]

Old BOiler Size

201K BTU Weil McLain. Not sure why they put that big of a unit in. Probably 25-30 years ago.

Winter gas heatings bills in excess of $800/month last year.

Jay_17

DHW?

Is your hot water separate, or do you have an indirect tank?
If you have an indirect tank on a zone there may be justification for the extra size.
Yowzers, that is a bill, well, probably not completely out of line, we had 250+ per month for around 1000 sqft, you probably have more than 2x that footage.
What boilers are you considering?
Jay

[Deleted User]

Boiler

Hot Water Heater is seperate.

We are looking at Laars JVS, New Yorkerer, and Crown. All in the 100K BTU range.

Contractor is pushing Laars ince he says he installs them all the time and never has an issue. I have read some negative thinhs about Laars, but the impression I get is that if installed correctly they are very good. Joanne at Laars has also been very helpful.

The additional cost for a 90%+ efficient boiler was about 4-5K. I do not see the value in this. Also supposed to be additional maint costs.

Jay_17

We must both be sitting bored at work, I am.

Hmmm, well this is getting beyond the initial question, actually I hope someone else chimes in to back up my comments. Anyway, I am not familiar with those boilers, I am surprised that there would be that much cost difference, although a condensing boiler would need different venting etc. The actual boiler cost diff wouldn't be that much, in fact I would say you could get a 100K BTU munchkin for around $3000 or so (just the boiler, and that isn't contractor pricing), of course something like a Buderus would be a few K more. I really don't know enough specifics to say any more with any certainty, there may be aspects to your system that I don't know about.
Anyway, hope I helped.
Jay

Joe Brix

I agree with Jay

With all the new condensing wall hung boilers like the TT Prestige, NYT Trinity, Munchkin, etc. seems like a shame looking at 80% efficent CI boilers. It's your money but with the price of gas going up, I'd ask some more contractors about other options.

heatboy

Unless you don't..............

trust your figures, the 100 MBH is more than sufficient. I would choose the Crown over any of the others you mentioned. I have yet to size to anything other than the DOE output, unless the unit sits in unconditioned space (like outdoors). Slightly smaller is always better than too big. Well, when we're talking boilers, that is.

Jay_17

One final comment

Ok, one last comment,
If it were my house, with the info you gave me, I would consider how the system performed on the coldest day I can remember. If it had no problem keeping the house warm I would consider these two options, either try to match as well as possible the amount of radiation I had, so I still had the same capability, or even think about undersizing a bit and living with a bit of a cold house during the worst times. If there is another source of heat like a good wood stove then option 2 is very attractive.
Of course if those cold times will stress the marital bonds, then don't risk it, it probably won't make enough of a difference. This might also make your contractor shake his head and give you grief.
That is the beauty of a modulating boiler of course, best of all worlds, small most of the time, big when you need it.
Ok back to my cold house....
Jay

Tim_24

Double check

the loads. Just because the computer program spit out a number doesn't mean that is is accurate. The previous boiler was sized quite differently. You gave the location, but no other details of your house. Obviously old enough to require a boiler replacement. Not that I am advocating "square-footing" the job, but an experienced heating professional can make a good spot judgement about the sizing of equipment. Is your house 1500 sf or 3300 sf? 2x4 walls with no insulation or very well insulated?

Steamhead (in transit)

Mike, are these boiler ratings

the "input", the "DOE Heating Capacity" or the "Net" ratings?

[Deleted User]

Ratings

Input: 100K BYu/hr
Heating Capacity: 84K
Net IBR: 73K

Thanks

joel_19

IBR

IBR is oversized for anything but steam boilers DOE is the way to go with hot water