Weil Mclain CGA 6 Chimney vent question

[Cpeters]

I have a 8" clay lined chimney which vented a 140,000 btu burnham and a Rheem HWH. The boiler flue was
7" and the HWH 3". The new vent size is 6". I looked at the nfpa code and it said clay liner was ok. Can anyone point me to where I can find info on this? Thanks.

 

[Dana]

So, you really went for beyond-ridiculous oversizing factor, eh? The CGa-6 has enough output to heat a 10,000+ square foot house at 0F outdoor temps, and probably enough BTUs per hour to heat your house at an outdoor temperature of absolute zero (theoretically, since the natural gas itself would already be long since frozen). ASHRAE recommends 1.4x oversizing for the load at the 99th percentile temperature bin, which is usually enough to cover the coldest hours of the past century. At your likely 5x-6x-7x oversizing the only advantage is the speed of recovery from overnight setbacks (assuming you have enough radiation to even deliver that much heat), and with an atmospheric drafted cast iron boiler that amount of oversizing knocks between 10-15% off the AFUE numbers for your as-used efficiency. (AFUE testing is at 1.7x oversizing.)

A right-sized narrowing liner may or may not be a local code requirement, but is ALWAYS a good idea at 83% efficiency and higher. An 8" x 8" clay liner can handle over 300,000 BTU/hr of ~83-84% efficiency burner if it has a reasonably high duty cycle, but at your oversizing factor the average duty cycle won't be high enough to warm much terra-cotta liner to above the dew point of the exhaust in dead of winter. The lower exhaust temperature that comes with 83-84% efficiency compared to your old 80% or lower efficiency Burnham means it will be substantially more susceptible to flue condensation. If the chimney is on the exterior of the house and exposed to outdoor air temps this is substantially bigger problem than if the chimney runs up through the middle of the house surrounded by warm conditioned space air, with only a cold attic and a few feet of above-roof exposure, but even interior chimneys are prone to condensation when the duty cycle is that low.

Adding up the total BTU capacity requirement, the CGa-6 is 175,000 BTU/hr, the water heater is likely to be 35-45,000BTU/hr (look it up), which adds up to something on the order of 200-225K. Do not oversize the liner- you're looking for the smallest liner that fills the bill, since that exposes less condensing surface to the exhaust, and increases the stack velocity.

The CGa-6 is 175,000 BTU/hr, the water heater is likely to be 35-45,000BTU/hr (look it up), which adds up to something on the order of 200-225K. If the top of the flue is at least 20' above the vent port for the boiler, a 6" liner is about right. Single-walled or B-vent can be used if it's an interior chimney, but if it's an exterior chimney a single wall liner has to be stainless to tolerate the condensation (see p.8 of the manual.)

You might find this slide show covering the fundamentals useful to review (starting at around slide #25, but pay particular attention to slide #43 & #44)

 

[Sponsor]

 

[Dana]

Seriously, if it's not too late (though it probably is), drop in a smaller boiler (even if it's still 2-3x oversized for no rational reason), which will use a smaller-cheaper liner, and provide higher comfort. With some 5th grade math skills and some old gas bills figuring out the "right" size doesn't take very much time at all, and will lead to higher comfort and a higher average duty cycle (= less flue condensation), higher as-used efficiency, and lower up-front cost for the whole shebang.

 

[Cpeters]

So, you really went for beyond-ridiculous oversizing factor, eh? The CGa-6 has enough output to heat a 10,000+ square foot house at 0F outdoor temps, and probably enough BTUs per hour to heat your house at an outdoor temperature of absolute zero (theoretically, since the natural gas itself would already be long since frozen). ASHRAE recommends 1.4x oversizing for the load at the 99th percentile temperature bin, which is usually enough to cover the coldest hours of the past century. At your likely 5x-6x-7x oversizing the only advantage is the speed of recovery from overnight setbacks (assuming you have enough radiation to even deliver that much heat), and with an atmospheric drafted cast iron boiler that amount of oversizing knocks between 10-15% off the AFUE numbers for your as-used efficiency. (AFUE testing is at 1.7x oversizing.)

A right-sized narrowing liner may or may not be a local code requirement, but is ALWAYS a good idea at 83% efficiency and higher. An 8" x 8" clay liner can handle over 300,000 BTU/hr of ~83-84% efficiency burner if it has a reasonably high duty cycle, but at your oversizing factor the average duty cycle won't be high enough to warm much terra-cotta liner to above the dew point of the exhaust in dead of winter. The lower exhaust temperature that comes with 83-84% efficiency compared to your old 80% or lower efficiency Burnham means it will be substantially more susceptible to flue condensation. If the chimney is on the exterior of the house and exposed to outdoor air temps this is substantially bigger problem than if the chimney runs up through the middle of the house surrounded by warm conditioned space air, with only a cold attic and a few feet of above-roof exposure, but even interior chimneys are prone to condensation when the duty cycle is that low.

Adding up the total BTU capacity requirement, the CGa-6 is 175,000 BTU/hr, the water heater is likely to be 35-45,000BTU/hr (look it up), which adds up to something on the order of 200-225K. Do not oversize the liner- you're looking for the smallest liner that fills the bill, since that exposes less condensing surface to the exhaust, and increases the stack velocity.

The CGa-6 is 175,000 BTU/hr, the water heater is likely to be 35-45,000BTU/hr (look it up), which adds up to something on the order of 200-225K. If the top of the flue is at least 20' above the vent port for the boiler, a 6" liner is about right. Single-walled or B-vent can be used if it's an interior chimney, but if it's an exterior chimney a single wall liner has to be stainless to tolerate the condensation (see p.8 of the manual.)

You might find this slide show covering the fundamentals useful to review (starting at around slide #25, but pay particular attention to slide #43 & #44)

The CGa-6 has enough output to heat a 10,000+ square foot house at 0F outdoor temps, and probably enough BTUs per hour to heat your house at an outdoor temperature of absolute zero (theoretically, since the natural gas itself would already be long since frozen).

Dana, you are so far off on the 10,000 sf house in zero weather.

 

[Cpeters]

The CGa-6 has enough output to heat a 10,000+ square foot house at 0F outdoor temps, and probably enough BTUs per hour to heat your house at an outdoor temperature of absolute zero (theoretically, since the natural gas itself would already be long since frozen).

Dana, you are so far off on the 10,000 sf house in zero weather.

But thanks for answering the liner question.

 

[Dana]

Show me a 10,000' house that has glass in the windows and insulation in the walls that comes in over 150,000 BTU/hr @ 0F. I'm sure they're out there, but they're exceedingly rare- a 10,000' air-leaky Victorian with a complicated shape or something.

If you've ever done any heat load calculations you'd know that bigger houses have lower BTU/hr per square foot ratios than smaller houses, and that '70s vintage houses with even modest insulation and tightness levels come in under 20 BTU/ft-hr @ 0F. Almost any 10,000' house in NJ would have a heat load @ 0F less than the 146,000 BTU/hr output of the CGa-6. That's 14.6 BTU/ft-hr. Most houses that size would come in under 14 using Manual-J or even I=B=R methods.

Most 1977 vintage 2700' 2x4 framed houses (described in your prior thread) with 1977 vintage clear double-panes or single-panes + storm windows and at least R19 in the attic will come in under 50,000 BTU/hr @ 0F. Whomever came up with 50 BTU per square foot "region factor" for sizing heating equipment is a complete hack, incompetent, clueless, when it comes to estimating or calculating real heat loads. (They didn't even copy the DOE climate zone map correctly!)

Even a completely uninsulated single-pane windowed framed house or even a single-wythe brick with no insulation would have a heat load anywhere near that big at +10F to +15F central NJ type outside design temps, or even at 0F. A 2700' greenhouse with all single-pane glass could come in that high, but not the sort of house people would live in. Even 25 BTU/ft @ 0F is pretty rare to find in a house that size- fewer than 1 in 20. Typical would be about 15 BTU/ft @ 0F if it's pretty tight maybe 20 -22BTU/ft if it's air-leaky and has little to insulation.

Since you have a heating history on this place, you can easily calculate an upper bound on the heat load based on fuel use. If you're burning well over 1000 therms per MONTH in winter you might have a rationale for a boiler that big, but I'd hazard that even your all time record coldest month since the house was built was under 500 therms (under 350 gallons of oil in the worst month with the oil burner.) I would expect your actual use is more like 1000 therms/year, 1500 tops.