What price range should I expect to replace existing boiler?

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Sluggo

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I have a newer (10+ year old) house that is well-insulated. The heating system is radiant in slab floors. My existing gas-fired boiler uses about 95k Btu/hr, and it seems well-sized. Regular hot water for the house is supplied by a standard gas-fired water heater. I'm considering replacing the boiler as it has needed some service calls lately and I am curious what a good quality boiler replacement would run (total parts and labor). I am also aware that there are some combo units that provide heating as well as water heating, such as Rinnai, Triangle Tube, etc., so I am also curious to know what the installed cost of one of these fancier units might run. Thanks.
 

Dana

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Unless yours is one of the largest and crummiest houses built since 2000 in WA there's no freakin' way your heat load is anywhere near 95,000BTU/hr (with the windows & doors closed, anyway... :) ) The fact that it's not even being used for heating the domestic hot water is almost criminal. Many newer houses could be heated with the burner the size of what's in the hot water heater.

In western WA the 99% outside design temps are in the 20s F, and a typical heat load to floor area ratio of code-min new construction runs about 9-10BTU per hour per square foot of conditioned space (with many exceptions on both sides of that number to prove the rule.) In eastern WA (particularly at elevation) the design temps range from mid positive single digits well into the teens, and heat loads run in the 11-12 BTU/ft^2-hr. So if this house is a well insulated 8000-9000 mansion it might actually need a 95K boiler. The average house won't need more than a 50K boiler, and there are many smaller or better insulated houses where a 50K boiler would be oversized.

Sizing the boiler correctly for the heat load is something that can only happen at boiler-swap time, which is typically only once every 15-35 years. Right sizing it is important for comfort, efficiency, and reliability/longevity. So let's figure it out, OK?

Since you have a heating history on the place, with a ZIP code and a couple of wintertime gas bills with the EXACT meter-reading dates an fuel use it's pretty straightforward to estimate the actual heat load of the house based on how much fuel is used per heating degree day. The ZIP code is necessary to figure out a reasonable weather station to pull data from at degreedays.net. For a "...house that is well-insulated..." it's better to use 60F as the heating degree-day base, but if it's a crummy enough house to need a 95K boiler base-65F would be more appropriate. With the fuel use & total accumulated HDD between meter readings, and the boiler's nameplate efficiency, the boiler becomes the measuring instrument. Only wintertime billing periods should be used, since that's when the solar gains and other uses of natural gas inject the smallest error in the measurement.

The short answer to the cost issue is "it depends", and one of those dependecies is the competence of the installer, and how the heating system is currently configured. At the very low end it might come in at about $8K USD, but depending on how screwed up the original system design was (and I'm assuming some level of incompetence here, based on boiler size), it could run as high as $15K or even $20K, depending on the complexity of what needs fixing, assuming you're installing a modulating condensing boiler.

How many zones?

How big is the house?
 

Sluggo

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Unless yours is one of the largest and crummiest houses built since 2000 in WA there's no freakin' way your heat load is anywhere near 95,000BTU/hr (with the windows & doors closed, anyway... :) ) The fact that it's not even being used for heating the domestic hot water is almost criminal. Many newer houses could be heated with the burner the size of what's in the hot water heater.

In western WA the 99% outside design temps are in the 20s F, and a typical heat load to floor area ratio of code-min new construction runs about 9-10BTU per hour per square foot of conditioned space (with many exceptions on both sides of that number to prove the rule.) In eastern WA (particularly at elevation) the design temps range from mid positive single digits well into the teens, and heat loads run in the 11-12 BTU/ft^2-hr. So if this house is a well insulated 8000-9000 mansion it might actually need a 95K boiler. The average house won't need more than a 50K boiler, and there are many smaller or better insulated houses where a 50K boiler would be oversized.

Sizing the boiler correctly for the heat load is something that can only happen at boiler-swap time, which is typically only once every 15-35 years. Right sizing it is important for comfort, efficiency, and reliability/longevity. So let's figure it out, OK?

Since you have a heating history on the place, with a ZIP code and a couple of wintertime gas bills with the EXACT meter-reading dates an fuel use it's pretty straightforward to estimate the actual heat load of the house based on how much fuel is used per heating degree day. The ZIP code is necessary to figure out a reasonable weather station to pull data from at degreedays.net. For a "...house that is well-insulated..." it's better to use 60F as the heating degree-day base, but if it's a crummy enough house to need a 95K boiler base-65F would be more appropriate. With the fuel use & total accumulated HDD between meter readings, and the boiler's nameplate efficiency, the boiler becomes the measuring instrument. Only wintertime billing periods should be used, since that's when the solar gains and other uses of natural gas inject the smallest error in the measurement.

The short answer to the cost issue is "it depends", and one of those dependecies is the competence of the installer, and how the heating system is currently configured. At the very low end it might come in at about $8K USD, but depending on how screwed up the original system design was (and I'm assuming some level of incompetence here, based on boiler size), it could run as high as $15K or even $20K, depending on the complexity of what needs fixing, assuming you're installing a modulating condensing boiler.

How many zones?

How big is the house?
The house is 2800 square feet, with four zones. It's somewhat sprawling and the walls have a lot of insulated glass (about 25% of the wall area). Looking at past bills during the worst winter months we used about 9 therms of gas per day, which I penciled out to 12.5 BTU/sqft/hr. Of course, that includes gas for hot water and cooking in addition to space heating.
 

Dana

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Without knowing the average outdoor temp during the billing period, the 9 therms/day is kind of meaningless. Your math may be good arithmetic (not shown), but it's not the correct analysis. Let's run a quick & dirty straw man calc on this.

Assume average temp was 40F, that's 20 degrees below a presumptive 60F heating/cooling balance point, and 25F below a presumptive 65F base temp.

Given that it may be an overglazed house, as a straw-man calc lets use 65F, so call it 25HDD. (A ratio of 15% of window to FLOOR area is pretty typical- do you have more than 0.15 x 2800= 420 square feet of glass? That would be about 35-40 typical double-hungs.)

Assume the boiler has a nameplate efficiency of 82%. 9 therms is 900,000 BTU, but burned in an 82% boiler it delivers only 0.82 x 900,000 BTU = 738,000 BTU into the heating system (the rest went up the flue.)

738,000 BTU used over 25 HDD is (738,000/ 25= ) 29,520 BTU/HDD. With 24 hours in a day, that becomes (29,520/24=) 1230 BTU per degree-hour.

Assume your 99% outdoor design temp is 25F (roughly the outside design temp in Seattle/King County).

With a heating/cooling balance temp of 65F, and an outdoor design temp of 25F, the design temp is 65F- 25F= 40 heating degrees.

So the implied heat load at an outdoor temp of 25F is then 40F x 1230 BTU/F-hr= 49,200 BTU/hour.

And that's about half the output of the boiler, which would make the boiler ridiculously oversized.

But, for only 2800' of space that would be 17.6 BTU/ft^2-hr, appallingly high for a house described as "well-insulated". (That's a higher ratio than my sub-code 1920s antique 2400' house has at 0F outdoor temps.)

So, look up the real HDD for the real dates, your real 99% outside design temp, and the real nameplate efficiency of the old boiler, and run the math. I'll still bet it's nowhere near 95,000 BTU/hr. If it's even half that it means you probably have very high infiltration rates or an uninsulated foundation or some other large heat leak.
 

John Molyneux

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I expect there's some regional variation in cost, but we're just getting natural gas here in my neighborhood in the Northeast and lots of neighbors have been getting estimates for new heating systems. Dana's range seems about right. I had a Bosch Greenstar 57 installed for about $9,000 and they did a great job. That was just for a straight boiler replacement and chimney liner. I already had a suitable indirect hot water tank. Folks that are looking at combi's or adding an indirect are generally looking at $10-12,000, but every situation is unique.

During my initial research here and elsewhere I became convinced that the quality of the design, installation and support is more important than whichever brand you choose -- and making sure you get the right size unit. I heat 1400 square feet with a design day temperature is 5F. My heat loss is less than 30,000 BTU/hr and the 57,000 BTU boiler will do just fine.

I've had lots of conversations recently about why it costs so much when you can go online and see that the wholesale price of the boiler itself is only $3,000, or whatever. I had the same question until I saw how much labor (and craftsmanship) is involved.
 

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Sluggo

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Without knowing the average outdoor temp during the billing period, the 9 therms/day is kind of meaningless. Your math may be good arithmetic (not shown), but it's not the correct analysis. Let's run a quick & dirty straw man calc on this.

Assume average temp was 40F, that's 20 degrees below a presumptive 60F heating/cooling balance point, and 25F below a presumptive 65F base temp.

Given that it may be an overglazed house, as a straw-man calc lets use 65F, so call it 25HDD. (A ratio of 15% of window to FLOOR area is pretty typical- do you have more than 0.15 x 2800= 420 square feet of glass? That would be about 35-40 typical double-hungs.)

Assume the boiler has a nameplate efficiency of 82%. 9 therms is 900,000 BTU, but burned in an 82% boiler it delivers only 0.82 x 900,000 BTU = 738,000 BTU into the heating system (the rest went up the flue.)

738,000 BTU used over 25 HDD is (738,000/ 25= ) 29,520 BTU/HDD. With 24 hours in a day, that becomes (29,520/24=) 1230 BTU per degree-hour.

Assume your 99% outdoor design temp is 25F (roughly the outside design temp in Seattle/King County).

With a heating/cooling balance temp of 65F, and an outdoor design temp of 25F, the design temp is 65F- 25F= 40 heating degrees.

So the implied heat load at an outdoor temp of 25F is then 40F x 1230 BTU/F-hr= 49,200 BTU/hour.

And that's about half the output of the boiler, which would make the boiler ridiculously oversized.

But, for only 2800' of space that would be 17.6 BTU/ft^2-hr, appallingly high for a house described as "well-insulated". (That's a higher ratio than my sub-code 1920s antique 2400' house has at 0F outdoor temps.)

So, look up the real HDD for the real dates, your real 99% outside design temp, and the real nameplate efficiency of the old boiler, and run the math. I'll still bet it's nowhere near 95,000 BTU/hr. If it's even half that it means you probably have very high infiltration rates or an uninsulated foundation or some other large heat leak.
Dana-
Thanks for all your work on this. I just checked the blueprints and there is a total of about 850 sqft of glass (windows, glass doors, and skylights). I did say it was heavy on glazing, and I know that is where a lot of my heat loss goes. The walls are fully insulated 2X6, there is plenty of attic insulation, and the slab sits on 2" of styrofoam The receptacles and slab are sealed with foam, and everything seems well-caulked, so I don't think I have more than the average amount of infiltration. The boiler has about an 83% efficiency: 96KBtu in, 80KBtu output. There is also a large, unheated, attached garage, but I assume that doesn't affect the calculations much.

A typical winter bill shows 30 days with a total of about 270 therms and an average temp per day of about 40F. I am in the Greater Seattle Area.
 

Dana

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So it looks like the straw-man calculation was pretty close to the reality number. Since it's an 83% boiler rather than 82% it bumps the calculated design load to the calculated 49,200 BTU/hour x 83/82= 49,800 BTU/hr.

That works out to about 18 BTU/ft^2-hr @ 25F, which is a fairly LOW performance building envelope. Some might even call it crazy high for a house that size, but with roughly twice the amount of glazing of the typical house it's not entirely out of the question. If the boiler is out in the garage, outside of the insulation, that's about a 10-15% hit in system efficiency, since the standby losses of the oversized hunk o' cast iron don't accrue to the house, which would put the real load for the house at something like 42-45K. That's still at least 15 BTU/ ft^2-hr, which is still not great, but perhaps less crazy. Assuming IRC 2o12 code-max U0.35 windows, at 850 square feet, at an indoor temp of 68F and an outdoor temp of 25F, that's a 43F delta-T, and a window loss of U0.35 x 43F x 850'= 12,793 BTU/hr just in window losses. If they are U0.50 clear-glass double panes (no low-E) it's 18,275 BTU/hr just in window losses. That skews the load significantly to the high side of typical. (In my neighborhood installing that much window would require a code variance, which would probably only be granted if there were improvements beyond code elsewhere to offset the hit in building efficiency that comes with that much glass.)

Only a calibrated blower door test would be able to tell you how leaky the house is. If the studbays are stuffed with low-density R19s the house is going to be leakier than if it were stuffed with R21 fiberglass or R23 rock wool, unless the cualked the framing to the sheathing in every stud bay, installed an EPDM sill gasket or a bead of caulk or expanding foam under the bottom sill plate, and a bead of caulk between the top plates of the framing. Plumbing stack chases, fireplaces, recessed lights (even the WA required gasketed air-tight type recessed lights) typcially leak far more air than most people think.

With a 50K whole house load cut up into 4 zones, the smallest zone has to have a design day heat load less than 12.5K. When buying a modulating boiler, ideally the minimum modulated output would be no more than half the design heat load of the smallest zone to keep it from short-cycling on zone calls. But by virtue of the fact that your radiation (the concrete slab) has substantial thermal mass that wouldn't likely be a problem unless the minimum output was 2-3x the design heat load of the zone (TBD.) Suffice to say, you still need to look at the minimum firing rate, not just the maximum. If you blindly went ahead and installed a 90-100K mod-con boiler, the min-fire output would likely be 30K, which is comparable to or higher than your average wintertime load, and much higher than your seasonal average load.

Assuming a whole house load between 40-50K (it's certainly not more than that) with slab radiation you should be looking at condensing boilers in the 55-60K input range, ideally with a min-fire output less than 15K (or even under 10K).
 
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Sluggo

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Dana-
Thanks so much for all the calculations and comments. You've given me a terrific amount of useful info, and I feel much better prepared to talk with heating contractors and to select one based on how well they respond to questions I can now ask, and how knowledgeable they are about the effects of these various parameters.
 
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