Oversized Boiler.... eating oil

BennyB

I just moved into a 1950 cape in Westchester NY, about 1900 sq. ft. The heating system uses hot water radiators with an indirect hot water tank. The boiler.... original. It's rated at 300,000 BTU, but was probably listed that for the original coal fuel source. It's been converted to Oil. I'm using 10 gallons a day easy, and it isn't even very cold yet.

I need to replace this sucker, and am open to any suggestions. Gas. Biofuel. Shutting the thing down and putting a pellet stove in the front room. Anything.

Any suggestions? I got a quote for a new Burnham oil boiler for around 8 grand. The natural gas equivalent was closer to 11 grand because of the new hook up needed. Is gas much cheaper to operate?\



Thanks

Robert_25

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It is against the rules to discuss pricing on this forum. The cost of one fuel vs another is fine, but not the contractor's quotes.



10 gallons per day in a 1900 sq. ft house, yikes! As for oil vs. NG, it depends on NG pricing in your area. Has NG been run to the house already?



Did either of the boiler quotes you received include lining the chimney?

BennyB

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My apologies about the pricing. Thanks for letting me know.

NG has to be run to the house, that's why it was more expensive. The boiler units were equivalent.

And NG appears slightly less expensive than Fuel oil, for sure. I was also wondering about maintenance issues in general.

The quotes include new flue piping to the existing chimney, but not a new liner. I'm guessing since you asked, that a new liner is important? Can you explain why?

Thanks.

Jamie Hall

Ten gallons per day

is a bit excessive...



However, even five gallons per day might be a bit much in this weather (not when it gets cold, though), and that's probably the best you could expect to achieve even with a very good mod-con.



Which leads me to: insulate the house.  Tighten up the windows (are they original?  Do they have storm sashes?  Double pane).  Look for draughts and get rid of them.  A little work along those lines can go a long way...

Jean-David Beyer

1900 sq. ft. rated at 300,000 BTU

"1900 sq. ft. The heating system uses hot water radiators with an

indirect hot water tank. The boiler.... original. It's rated at 300,000

BTU,"



Do your quotes specify that they will use a 300,000 BTU/hour new boiler?



Westchester County may be a little colder than Monouth County, NJ where I live. Design temperature around here is 14F. New York City is 15F. So let me guess it is 13F in Westchester county. But that is only a guess.



My 1150 square foot house calculates out at 30,000 BTU/hour when it is 0F outside, and that seems to be correct. Assuming your heat load is proportional to mine and dependant only on floor area, you might need around 50,000 BTU/hour.



I have a W-M Ultra 3 80,000 BTU/hour (input) boiler that will modulate down to 16,000 BTU/hour. It is too big, but W-M do not make any smaller mod-cons. I am not pushing any particular brand of boiler, though if you can afford one, you should probably get a mod-con, especially if your radiators are large enough.  A mod-con really pays off if you can run with low (less than 130F) return water temperatures. So you would want to use outdoor reset even if you need much higher temperatures on the coldest few days each heating season..

BennyB

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The new boilers were for ~175,000 btu.

I'm a newbie. What's a mod-con?

Jean-David Beyer

What's a mod-con?

It is a modulating condensing boiler.



The modulating means that the firing rate of the boiler can be changed dynamically by the controls of the boiler to supply just the amount of heat needed instead of running at full blast that would be needed only a few days a year. This permits much longer cycle time for the burner and that promotes higher efficiency for several reasons. One reason is that if the boiler temperature is lower, the gradient across the heat exchanger is higher so the heat flows from the flame to the water more effectively. Another reason is that when the boiler is running at a lower temperature, the condensing takes place more effectively. The modulation is controlled, in large part, by a feature called outdoor reset that lowers the water temperature as the outside temperature increases and less heat is needed by the house.



Condensing is a scheme to raise the efficiency of the boiler. You do not want condensing to occur in a boiler not designed for it because it tends to corrode the internals. If you burn something like natural gas, ideally what you get out is some heat, and a lot of carbon dioxide and steam. In practice you get some other stuff (carbon monoxide, carbonic acid, nitrogen-oxygen compounds) too. Now if you condense the steam to water, you get 540 calories per gram of steam condensed. If you could do that completely, you could get a little over 10% more efficiency from your boiler. In practice, you cannot get it all, but if you are doing primarily snow melting, you could probably get about 98% efficiency from one of these. The deciding factor is if you can get the return water temperatures low enough to get the most condensing. As far as I can tell, it is now routine to get 92%  efficiency from one of these, and higher if you are running radiant heating where lower temperatures are sufficient to heat a building because of the large surface area of the emitter. My house has a concrete slab with copper tubing in it. I use a supply temperature of 75F to heat my house until the outdoor temperatue goes below 50F, where I start increasing the temperature of the water until it gets down to 6F outside, at which point the water has risen to 120F. So I get condensing all the time. I have a chart that shows the efficiency I can expect because of this. At 75F, my boiler will run at around 97% efficiency. At 120F, it probably runs at about 87%.



"The new boilers were for ~175,000 btu."



If my assumption from before is correct, that is between three and four times the size you need. But you should not rely on my assumption. My guess is that the contractor did not calculate the heat loss of your house. And if he did not, he is unqualified to do this job. The first step, if you are going to do anything, is to have a heat loss calculation done. And not the way my former contractor did. He paced off two sides of my house and came up with a number and recommended a 105,000 BTU/hour boiler, and my house needs only 30,000 BTU/hour.



By the way, my house is also a Cape Cod built in 1950.

Gordy

High oil use

 That pans out to right at 30 btus an hour per sf output. How much hot water do you use? 

Jean-David Beyer

That pans out to right at 30 btus an hour per sf output.

 "How much hot water do you use?  "



If you mean me, I get 26 BTU/hour per square foot. 30 is close enough, I expect.

I use very little hot water. There is only one of me and I am not a teen-age girl who takes long showers. I have a 36 gallon indirect and I never run out of hot water. I would not need even 26 BTU/hour unless it gets down to 0F outside for several hours. Design temperature around here is 14F.



If you mean the O.P., I get 158 BTU/hour per square foot for the 300K boiler and 92 BTU/hour per square foot for the 175k boiler.

bob eck

new boiler

if staying with oil look at how old the oil tank is. does the oil tank need to be replaced? Have a heat loss done on the house. I would look at going with Nat gas condensing gas boiler. look at the triangle tube prestige boiler line. www.triangletube.com with a condensing boiler and outdoor reset control you will only heat the water in the system to just the right temp to heat the house. as it gets colder outside the boiler system temp will go higher to meet the heat loss of your home. with the condensing boiler it is vented sidewall with PVC pipe. get a heating professional to help you. Happy boiler hunting. Merry Christmas

BennyB

Thanks

Good information everyone thanks.



So my takeaways are this: My system is currently way oversized.

I need to have my contractor do a thorough home heat loss calculation to get the right size for a new unit.

Getting a boiler with a outside temperature sensor and condensing efficiency is worthwhile.

Last question, if the home heat loss it say 35,000 btus and I have an 30 gallon indirect hot water tank for a family of 4, 2 baths (we don't use much water)... What's the consensus on the correct btu input for a gas-fired boiler?

 

BennyB

One more thing

I do currenly have a pellet stove. Nice little machine. Does a good job heating in mild-cold weather. Right now I have the boiler heat shut off entirely, using it only for 3 hours in the morning. It still short cycles and definetely short cycles when just heating the hot water.

Gordan

Those assumptions...

...would put you right in my ballpark. After much mulling over, I decided to go with the smallest Prestige - PS60 - which modulates between 15 and 60 kBTU/hr input. After a year of hot water usage history, I'd say it works out well; it's enough for one never-ending shower or to fill up a decent-sized tub and then recover in time for rinsing off. That's enough for us now; I'll let you know in about fifteen years how it works out with two teenagers (as opposed to a baby and a toddler.)



I keep my indirect at 150 degrees and temper it down to 120 at the tank outlet with a thermostatic mixing valve. Also, what I have is a 26 gallon reverse indirect (boiler water in tank, domestic water in coil) which may act differently than whatever you have, so your mileage may vary.

Gordy

JDB

I was refering to OP's consumption at 10 gal per day (at least). Of the present boiler. 

138000 btus per gal #2 x 10 gal=1,380,000 btus per day/1900 sf = 726.38 btus per sf / 24 = 30.26 btus per sf per hour.



 Since this would be present boilers Gross output, and give it an efficiency of say 70% ( which may be generous) then that would put the input at about 21 btus a sf including hot water use.

 But that includes OP's hot water use. Which we do not know how many people etc.



 So if you take 21 btus sf x 1900 sf = 39,900 btus per hour heatloss, and Domestic use. We do not even know what temps the structure has been at. If its been mild........Bottom line needs to do a heat loss calc.



I think his new boiler size recomendations are highly sized.



Gordy

Jean-David Beyer

So my takeaways are this:

I may have forgotten to mention that I am not a heating professional. I am not.

Let me address each of your points separately.



1.) My system is currently way oversized.

This seems very likely. Of course I do not know if you have all outerwalls floor to ceiling single pane glass with 18 foot ceilings, etc. I am exaggerating here, but I think you will see what I mean.





2.) I need to have my contractor do a thorough home heat loss calculation to get the right size for a new unit.

Yes. He should have done it already. If he did not, I would be suspicious of him. Perhaps he did and did not tell you. You could ask for a copy of his calculations.





3.) Getting a boiler with a outside temperature sensor and condensing efficiency is worthwhile.

This is tricky. At the risk of contradicting myself in earlier posts, this is not certain, just likely. What is going to matter is how much condensing you are actually going to get. If your radiators are too small, you are going to have to run very hot water through them most of the time. And since the amount of condensing you get will depend on how cold the return water to the boiler is, you might find you get little condensing. You might still come out ahead because in the fall and spring your heat requirements are much less than in the end of January and early February. In my house, I get condensing all the time in my radiant slab zone, and all the time in the baseboard zone as long as the outside temperature is over 14F (which happens to be the design temperature around here). The reason I get that is because I replaced the 3 foot lengths of baseboard there with 14 foot lengths. You could figure out a lot of this stuff yourself if you read this book:



http://www.heatinghelp.com/products/Hot-Water-Heating-Books/26/96/Modern-Hydronic-Heating-Third-Edition-br-by-John-Siegenthaler



You may get sticker shock when you see the price, but it is well worth it. Even if you never do any of the work yourself, it will give you a great understanding on how these systems work, and you will be better able to evaluate the abilities of your contractor. It seems to me, as a homeowner, by reading this web site, that there are some very good contractors out there, but that they are few and far between. And it is very important to get one of them.



One thing to consider: if your existing boiler is oversized by a factor fo 3 or 4, your radiators may be oversized as well. This is good. That would mean that you could probably do very well with lower water tempertures and then a condensing boiler would pay for itself in a reasonable time.



4.) Last question, if the home heat loss it say 35,000 btus and I have an 30

gallon indirect hot water tank for a family of 4, 2 baths (we don't use

much water)... What's the consensus on the correct btu input for a

gas-fired boiler?

My guess is that you will need more than 35,000 BTU/hour based only on floor area, something like 50,000 BTU/hour . But this guess is not to be relied on; you would need the correct heat loss. My boiler has separate thermostat inputs for each of the two heating zones, and another for the indirect hot water heater. The hot water heater has the highest priority, so when it calls for heat, the heating zones are shut off and all the heat goes to the hot water. The manufacturer of my boiler also supplies indirect hot water heaters and they give a calculator to figure out what size indirect is needed and what size boiler for a given size heater you need. When I figured out mine, their smallest indirect (nominally 40 gallons, but I think it is 36) was enough, and their smallest boiler would suffice to run it with my domestic hot water load. To summarize, I did not have to make any allowance at all for the domestic hot water. Look at it this way. When my domestic hot water heater calls for heat, the heat to the house is shut off. 10 minutes later, the domestic hot water heater is satisfied, and heat to the house goes back on (if needed). I never notice the interruption.

JohnHenry_2

Here in Seattle

Oil is 3 times the price of Natural Gas per BTU. I just switched to a mod/con from my original (1930 coal converted to oil) boiler and I figure I'll save between 4 and 5 THOUSAND dollars per year on fuel costs.



Twice the efficiency coupled with 1/3 cost/BTU = WIN!!



Benny, with your insulation level, I suspect you'll recover the extra cost of gas vs oil in a year.



Energy future pricing shows the ratio of cost/btu staying the same or widening in gas's favor in the near future.



Something else to consider: with a mod/con you have the ability to decommission the old flue for the oil burner by direct venting. I my case it's allowing me to remove a chimney in the center of the house giving me space to reconfigure my kitchen and dining room and move some doors.



BTW, you won't believe the comfort level these modern, computer controlled, heating devices provide. The difference is absolutely astounding.



My 2 cents says: Get the gas and find an excellent installer to do the work. He's not going to be the cheapest bid but you'll get the good value with a modern, properly installed heating source.



Here in Seattle, the gas company discounts the work to install the gas depending on how many BTUs will be used. For me, that meant it was free gas install (425 meter). Possibly there is the same or rebates in your area to help defray the cost?

Jean-David Beyer

OK.

Yes, I was assuming he would be using the full capacity of the boilers. You are right. Even if they were the correct size, they would be using that only on design day. I also assumed they were running at 100% efficiency because I did not want to deal with the correct value, that I did not know.



My guess, and I remind everyone that I am not a professional, is that he probably needs closer to 50,000 BTU/hour than 175,000. But there is no escaping the need for a correct heat loss calculation. Guesses will not do.

Jean-David Beyer

Here in Monmouth County, New Jersey ...

"BTW, you won't believe the comfort level these modern, computer controlled, heating devices provide. The difference is absolutely astounding."



I got that too. I never thought about it, but my differential  used to go from about 66F to 75F in my radiant slab zone with my old constant-temperature boiler. With outdoor reset on my new mod-con, it holds 69F all the time.



"My 2 cents says: Get the gas and find an excellent installer to do the work. He's not going to be the cheapest bid but you'll get the good value with a modern, properly installed heating source."



I agree. The tough part is to find that excellent installer.



"Here in Seattle, the gas company discounts the work to install the gas depending on how many BTUs will be used. For me, that meant it was free gas install (425 meter)."



In May 2009, the gas company installed the gas line free. It would have depended on just how far I was from the street, but my lot is only 65 feet wide, and the run was only about half that. They used a gizmo called a mole and pushed the pipe through. They dug a hole in the street and another were the meter was to go. I did have to pay the town $150 for a permit to have them dig a hole in the street.

Jean-David Beyer

That is interesting.

I used 35.68 therms of gas from October 18 to November 16. According to an on-line calculator, that amounts to 3,570,389 BTU. 28 days, so 127,513 BTU per day or 5,313 BTU/hour. So 4.62 BTU/hour per square foot (1150 square feet house). Some of that gas use was for domestic hot water, but I use very little of that. The reason the number is so low is that it has not really been very cold out. It is 52F outside at the moment. For the month of November, there were about 445 degree days according to an on-line web site. I never figured out that stuff before.

JohnHenry_2

Finding the installer

The OP should be able to find a decent installer with help from this board. He's already learned that any contractor that doesn't do a heat loss straight away should be removed from his list of suitors.



I would also think that engaging an installer that frequents this board would generally be a good bet as that installer isn't working in a vacuum and probably isn't married to bad ideas...