Can a Natural Gas Boiler be too Big?

BigPackFan

My HVAC contractor feels my boiler is much too large for my house (I was complaining about my high energy costs). It is a 180K BTU output. His heat loss calculation determined I need about 85K BTU (or 110K if I also add indirect domestic hot water). The efficiency of the current boiler was tested to be 81%. Some questions:

[1] Does an "oversized" boiler really lead to increased energy costs (i.e., will I save money by "downsizing" - BTW, my company did when they let me go......)?

[2] I have old, single pane windows with triple track storms; they are drafty. A buddy suggested I'd be better off investing in new windows. Any comments on that statement?

[3] If I go for a new boiler, what's the better option : (a) super high efficiency condensing type (i.e.,W-M Ultra) or (b) high efficiency Dunkirk, Burnham, etc.?

[4] The "unofficial" quote for a W-MU & indirect domestic hot water was $8300. Seemed high but I assume that's driven by the Ultra high efficiency boiler. Not sure how much less expensive a Dunkirk (or equivalent) would be.

Thanks everyone!

Earthfire

comments

1. YES 2. Do this also 3. Depends on your system (but I feel the Ultra is an excellent all around boiler)4. sounds real reasonable. Price should be dictated by the value of the installation and the installers knowledge, experience, and commitment to the customer,not just the shelf price of materials.

Bill Nye

#2

I will comment on #2. No matter whose boiler you have the energy will be lost through your windows and walls. You will get an immediate return on your investment and continue to save as long as you live in your house.

You can not invest in any stock market and get the return you will get by tightening up the house. Then you may need an even smaller boiler.

It depends though I have had quotes on replacement windows in my home and some of them could have bought my fuel for a couple of years! I had to do it my self, one quote was almost as much as I paid for the house!

Insulation is a relatively cheap investment, even proffesionally installed it is a bargain. You could give up one Saturday and get your brother-in-law to help and do the entire attic or basement fo under $500 I would think.

Boilerpro_3

Instead of replacing windows

weatherstrip what you have. With good storms and well weatherstripped windows the thermal performance can equal or outmatch that of most new windows. However, it takes alot of time to weatherstrip windows, so the investment tends to be time or money.
Air tightening is you biggest bang for your buck, and then insulation. After you've done all this then you can install an even smaller boiler. In my own home I have gone from a 270,000 btu input cast iron boiler to a 100,000 btu/hr input Dunkirk 95% AFue Quantum Leap, which, BTW, is still too big for my 2800 sq ft 1906 frame home.


Boilerpro

Greg Swob

Ask if the firm performing your heatloss calc

will do a 'courtesy' alteration in a variable or two. I do this frequently. By that I mean, it is usually a simple task to go back in heatloss software and adjust: attic insulation levels, window types, etc. See how much or little these factors alter the final heatloss/heat gain. At times ,I've advised owners while they were at it, to upgrade insulation and the cost was often taken care of with savings by equipment downsizing. While it may be cost prohibitive to perform all that your home needs at one time, you will have a basis to go on. You may need a smaller boiler yet and this cost savings may help buy a few windows and/or insulation for you. One customer took their heat loss calcs showing 'before and after' to their bank and it helped seal the deal on their remodel loan. Sometimes I've find the heatloss varied just a little with more attic insul., yet the A-C load decreased dramatically, allowing smaller tonnage equipment. It's nice to know the entire pricture so long as you don't end up with an overwhelming pile of data to digest. Greg

MikeB34

yes but...

Just the first Q.
Too big, and the heat exchanger never reaches its optimum heat transfer rate before the burner shuts down. This is true for any system.
Longer runs will result in less fuel used as more of the enrgy you consume will go to heating your house and not to heat up the heat exchanger.
I know its counter intuitive, but its true. A run of 20-30 minutes/hour is good. Ideally it should not turn off and then you are balancing the heat produced with the heat loss of your home.

Mike

Cliff Brady

80k BTUs for DHW should be enough

for normal hot water usage. A very large bath tub may be an exception. 80K BTUs is still double what most 40 gallon gas water heaters will do. To get the best of both worlds, a wall mounted Munchkin T80 will provide 80k for heating and 110k rampup for DHW demand. With the optional Vision-1 control the Munchkin will modulate the flame up and down to provide just enough heat to keep the house warm at the current outdoor temperature.

doug_13

Get the heatloss software

from http://www.hvac.cc/ for 49 bucks for 3 months. It is easy to learn and you can play with all the variables. Load the walls, windows, ceilings, basements, change to air tightness, enter the compass direction of the sides of the house. And then play- add insulation, fix air leaks and see what the effect is. The most time consuming thing is listing the detail of your house, i.e., Living room west wall, 9x12, 2x4 with R13, window size, window types etc.
You can go from no insulation to R19 in the ceiling with a push of a button and watch the BTU's change. It's just cool software and Don was very helpful even to a building owner.

(OK Don, you can send the money now to ....)

DD_2

It sounds like you have 3 options:
1. Downsize boiler
2. Insulate
3. Replace windows

Definitely do 2 before 3, and don't confuse them. Blown-in cellulose in the attic, and in the walls if you can afford it. Be sure you seal the air leaks first. Get an insulation contractor. Hint: ask him about the blower door test. If he blanks out, you've heard enough. For windows, weather-stripping and storm windows are much more cost-effective than replacement windows.

BigPackFan

Folks - Thanks for all the replys.

Tom Meyer

Just little old me's thinkin' on this:

(1) Without a doubt bigger is NOT better. Your boiler should be fitted like a pair of shoes. Have a professional heat loss performed anf get the boiler sized for your heat load. We could go "ad nauseum" as to why just replacing your boiler with the same size, isn't the best thing to do, and why some boilers are oversized.

(2) Look at it this way. Insulation is put there to keep the heat inside where you want it. The walls are R-19 to slow down the transfer of heat from the warmer inside to the colder outside. If you have R-1 windows, you are cutting holes into the R-19 walls and putting basically NO resistance to heat transfer. You may as well leave the windows open for all the good ineffective window sets are worth. I think you're buddy is right on target. Spend the money on good U-value windows. R = 1/U for some help making R value comparisons.

(3) We recommend boilers based on the application and water temps. You should have someone take a look at what's best for your particular application.

(4) Sorry, can't help you with this one.

Tom Meyer
Senior Designer/Trainer
Precision Hydronics Corp
www.precisionhydronics.com

R. Kalia

missing data

What people can't tell you is whether you'll gain or lose on the whole deal. Re your #1 and #3, of course getting a smaller boiler and a more efficient boiler will lead to reduced energy costs, but whether replacing a working boiler is a good investment is something you'll have to calculate. If, for example you spend $8000, and your current gas bill of $800/yr goes down to $550 (30% drop), well, you're only getting a return of 3% which is probably less than you could make by investing the $8000 elsewhere. In other words you haven't saved $250, you've LOST money on the deal. (Some people will divide the net cost by the dollar amount saved per year and say this is the "payback period", but this is nonsense, because it ignores the time value of money.) You lose even more if you move within a few years.

The same applies to windows. Of course new double-glazed windows are a good thing, but are they worthwhile? Only you can determine that. Windows cost about the same in N. Dakota and Louisiana, but they save more energy in N. Dakota. In most of the US you lose money by getting new windows; it is better to caulk them if they are drafty (even single-pane windows should not be drafty).

However, if you need new windows, get energy-efficient ones, just as if you need a new boiler anyway, make sure it is not oversized. Generally no contractor loses a customer by recommending a too-large boiler or furnace, but they will if it is too small to keep the house warm. So many contractors build in a huge unnecessary safety margin. But it's a sloppy and wasteful thing to do.

Re your question #4: SHOP AROUND!