Boiler flue duct work transition piece and sealant

[John Beemer]

I have a hot water boiler and the ductwork has rusted where it connect to my chimney on the exterior wall.

The chimney was leaking but that has been fixed and now I need to buy what looks like a transition piece made of galvanized steel. Also, I'm not sure what sealant they used when sealing it against the wall. Is the sealant mastic? Any idea what the best sealant for this type of job?

The outside diameter of the transition piece looks to be 7" and the duct work coming out of the boiler looks to be 4". I'm not sure if this transition piece was fabricated or if this is something I could buy locally or online.

Please see attached pic.

Thanks for any advice.

JB

 

[Dana]

Is the boiler relatively new?

A mid-efficiency (82% AFUE or better) boiler with a 4" output usually needs a 4" chimney liner to draft properly. If it's being patched into a 6" - 7" single-wall vent into a large terra-cotta lined chimney it's likely to have flue-condensation and backdraft issues. The leaking chimney may have been due to degradation of the mortar by the acidic flue condensation.

Let's take a step back...

What is the model number and the BTU-in /DOE-BTU out of your boiler?

What size is the flue?

It looks like a 4" water heater vent is sharing the same flue- what is the BTU rating of the water heater's burner?

Sealants for flue pipes need to be high temp and fireproof, not any random mastic will do. Purpose made high temp silicone, or furnace/stove cement will work. Most box stores usually carry at least a few products that are suitable.

 

[Sponsor]

 

[John Beemer]

I have a natural gas RBI spectrum boiler. It is non-condensing.
http://www.rbiwaterheaters.com/commercial-boilers/non-condensing/spectrum.html

The output says 83,700. I've attached a pic of the model.
I have to measure the top of the chimney but it is terra-cotta and it's probably 7 inches inside diameter.
The water heat uses the same terra-cotta flue. The chimney has 2 flues because it is shared with my neighbor,

The crown needed to be replaced and a few layers of bricks needed to be re-mortared.

If I were to line the chimney with a 4 inch flue, would I share that 4 inch flue with the hot water heater?


I know my neighbors will not spend any money lining the chimney. The homes are 70 years old and I think the chimney had the original crown.
I have a 30 gallon hwh. Input btuh = 30,000. Here is link to the hwh. It needs 3 inch flue.
https://products.geappliances.com/appliance/gea-specs/GG30T06AVG00

 

[John Beemer]

The flue pipe coming off the boiler measures 5 inches.
I just read this from the boiler manual.
http://mesteksa.com/fileuploads/Literature/RBI Water Heaters/Spectrum IOM (SPMIOM-12)_3.pdf

"A thorough inspection of the masonry chimney must be
performed to ensure that the chimney is clean, properly constructed,
lined and sized. Exterior masonry chimneys should not be used
unless properly lined to prevent condensation and draft problems.
Table 4 lists the equivalent breeching and flue sizes required for
the boiler/water heater."

The manual says to use a 5 inch flue but I also have a hot water heater going to the chimney.

Will a 5 inch flue work if I have a hwh heat connected also?
When I looked down the chimney, it didn't look completely straight. It looked like terracotta sections weren't lined up correctly.
If I need to line the chimney, the smallest liner would be best. I definitely couldn't have an insulated line installed.

 

[Dana]

Between the 30K-in water heater & (ridiculously oversized) 100K-in boiler you have 130,000 BTU/hr of burner to manage. A typical flexible liner chart indicates that a 5" round would be sufficient to handle both, assuming it's more than 25' from the flue port on the chimney to the chimney top, and a horizontal run of no more than 10'. If it's only 15' to the chimney top it may need to bump to 5.5".

How much horizontal 4" vent do you have, and how tall is the chimney?

The fact that it's an 83.7 % mid-efficiency boiler means that the exhaust temp is lower, and less buoyant than on a big 78-80% burner, which means if the terra cotta liner is too big it will be prone to flue condensation and back-drafting.

How big is the terra cotta liner?

A common solution would be to replace the water heater with an indirect fired water heater running as the "priority zone" for the boiler, which would be more efficient on an annualized basis, but not in the summer (due to the gross oversizing factor.)

An 83,700 BTU/hr output boiler is way oversized for most houses that have glass in the windows and doors that close. It could heat my 1920s antique 2x4 framed 1.5 story sub-code 2400' house +1600' of insulated basement to 68-70F indoors at an outdoor temperature of -75F, a temperature not seen in my neighborhood since the last ice age.

If you were to consider replacing the boiler, run this load analysis first, then this radiation analysis on the zone radiation to help figure out the optimal size & type. (Most boiler installers don't have the time and may not have the experience, and will either size it to max-out the radiation, or on some brain-dead BTU/hr per square foot of conditioned space basis, both guaranteed to oversize it at some sub-optimal level.)

 

[John Beemer]

The vent coming off the boiler is 5".
There is about 4 feet of horizontal before it goes int the chimney.
The 3" hwh has about 7 feet before it goes into the chimney.

The measurement of the terracotta was 7" x 6.5"
It looked like each piece of the terracotta was turned 90 degree to the next piece. The chimney is about 23 feet high. I can measure it tomorrow.

My house is only 1,200 square feet and the basement is not heated.

I haven't had any problems with the boiler and I'd rather not replace it since it works even if it's not as efficient as it could be.
What size boiler would you recommend for a 1,200 sq foot home? I'm guessing a smaller boil would has a smaller vent.

Thanks,

jb

 

[John Beemer]

Dana, thanks for the links above. I'm learning a ton about boilers.
According to this site, https://www.hunker.com/13408622/what-size-boiler-is-needed-per-square-foot-of-a-house
they recommend 33.475 BTUs of power output per square foot. That's around 40k btu which is half what my boiler puts out.

Does that mean my radiators get hotter fast or am I spending close to twice as much money in natural gas to heat my house?
The boiler is 17 years old.

Thanks for your help,

jb

 

[Dana]

Dana, thanks for the links above. I'm learning a ton about boilers.
According to this site, https://www.hunker.com/13408622/what-size-boiler-is-needed-per-square-foot-of-a-house
they recommend 33.475 BTUs of power output per square foot.

That recommendation is utter crap, and would oversize the heat at my house by more than 2x! If my house were in a location/climate as temperate as yours it would be oversized by more than 3x!

Further down in that same article they eventually get around to:

Calculating Your Exact Needs

HVAC professionals use a method called Manual J Load to calculate the exact boiler needs for a space. This scientific method requires assigning values to factors such as home construction materials, insulation levels, size of rooms, number of windows and creating an overall value for your home based on this.

A Manual-J done correctly using the most aggressive (rather than conservative) assumptions possible is the gold standard, and will usually oversize by no more than 1.15x.

Correlating fuel-use data with outdoor temperature data (heating degree-days), is an actual measurement, and will usually come in lower than a Manual-J, but is more accurate if using wintertime only data, on periods that consistently have at least some heat load(= no days over 65F.)

A 1200 square foot house with at least some wall & attic insulation and clear-glass (not low-E) double-panes (or clear storm windows over single panes), and no foundation insulation will have a likely heat load is less than 20,000 BTU/hr @ +17F (the 99% outside design temp for Andrews AFB). Run a fuel use load calculation to find out. ASHRAE recommends an oversize factor of 1.4x, so with a 20K heat load the "ideal" non-modulating boiler would have an output of about 28,000 BTU/hr. There are a few cast iron boilers that would b small enough, but with big cast iron radiators there would need to be protective measures taken to avoid destroying the boiler with condensation on the heat exchanger plates. A condensing boiler would be a better choice, to take advantage of those cool temps for higher efficiency.

With modulating condensing boiler the minimum firing rate is more critical than the maximum firing rate, as long as the max still covers your design load with some margin. Your load is probably low enough that it might be easier to right-size ductless or ducted heat pumps. But if replacing the boiler, a modulating condensing boiler with a very low (under 8000 BTU/hr) minimum firing rate that adjusts the output temperature in response to the outdoor temperature (called "outdoor reset") would use less fuel and provide higher comfort levels than a smaller, right-sized cast iron boiler.

That's around 40k btu which is half what my boiler puts out.

Does that mean my radiators get hotter fast or am I spending close to twice as much money in natural gas to heat my house?
The boiler is 17 years old.

Thanks for your help,

jb

It does mean that the radiators get hotter faster, but getting hotter faster also results in temperature overshoot- it's only useful for deep overnight setbacks.

It's using more fuel than a right-sized boiler, but not twice as much. AFUE testing is done at a presumptive 1.7x oversize factor, and is usually within 1% of the boiler's steady state efficiency or raw combustion efficiency. But when 2-3x oversized the boiler operates at an extremely low duty cycle, and suffers increasingly significant standby losses, since the boiler fires up, runs for several minutes is still hot at the end of a burn retaining a lot of heat, but may not cycle on again for hours. The idling losses at 3x oversizing are big enough to reduce the "as-used" efficiency to 10-15% or so below the steady-state or AFUE efficiency. At 4-5x oversizing (roughly where you are) it will probably be more than 15%. So instead of 82-83% efficiency your boiler is probably delivering no more than 70% efficiency, with most of the wasted heat going into the boiler room. If you're keeping the boiler (I might, if it's still testing at a raw combustion efficiency of better than 78%), installing a retrofit heat-purging boiler control such as the Intellicon HW+ or HydroStat 3250 Plus will reduce those idling losses to only a few percent. See System #3 in Table 3 of this document, the only boiler tested that used a heat purging controller, and compare it's performance to the others at 3x oversizing.

A ~7" x 7"terra cotta liner 23' extending 23' above where the vents enter is a bit oversized for 130K of 80-83% efficiency burner, but isn't going to be a disaster. If it's on an exterior wall there will be some backdrafting potential on cold days, but if it runs up the middle of the house in a chase surrounded by conditioned space it should be fine.

 

[John Beemer]

Thanks Dana for all the info. I'm still reading up on everything you posted. My chimney is on an outside wall and is shared with my semi-detached neighbors house. I'm probably going to keep my boiler since I don't have any problems with it but I will get the Hydrostat 3250-Plus. Will I also need the Hydrolevel OS-100 Outdoor Sensor Kit?

The 5" aluminum flex liner looks like it will work since both my boiler and hwh are natural gas. The stainless liners are 3 times the price. If I upgrade my boiler in the future, I may need to get a smaller diameter liner if I get the modulating condensing boiler.

The smallest mod con boiler I could find was 46k btu.
https://www.ecomfort.com/Triangle-Tube-CC-50S/p79459.html

 

[Dana]

The outdoor sensor would only be appropriate if the boiler were closer to the right size and the water temp could be limited to some lower temp when it's warm outside. That might improve comfort if the system is prone to big overshoots of the thermostat settings, but may in your case increase the number of burn cycles, especially if you have more burner than your existing radiators can emit. Measure up your radiators to estimate the total equivalent direct radiation (EDR) square footage.

At 170F average water temp (180F out, 160F return) cast iron radiators put out about 150 BTU/hr per square foot EDR. If you drop that to a bare minimum 140F AWT that drops to about 90 BTU/hr per EDR'. For a gas burner going much lower than that risks excessive flue condensation or even condensation on the heat exhanger plates, but the odds are you may never need more than 140F AWT (145F out, 135F return) to heat the house, in which case the outdoor reset control feature using the outdoor sensor becomes useless. Read and understand the setup & operation manuals before buying any heat purging boiler control.

It's also worth scoping out how the system is currently operated, measuring the output and return temperatures at the boiler multiple times during a call for heat. If the entering water temp (EWT) at the boiler is under 130F for much of the burn it is going to have condensation issues, but there may be features in the near-boiler plumbing to keep that from happening.

 

[John Beemer]

I measured the square footage of all the radiators and came up with 430.
I also used this form to calculate a heat loss.
https://hvac-talk.com/vbb/attachment.php?attachmentid=722461&d=1472759155

The btu input needed came out to 34,873.

I will keep an eye on the AWT.

Thanks for all the help.

jb

 

[Dana]

At 180F entering water temp you get at most 150BTU per square foot EDR, per the nomograph in this document. So the biggest cast iron boiler that makes any sort of sense would have a DOE output of 150 BTU x 430' = ~65,000 BTU/hr. (If you cranked the temp up to 195-200F you'd get maybe 75,000 BTU/hr out of those rads, but that would be ridiculously hot.)

That heat loss calculator form is an ultra-crude I=B=R type load calculator, and only valid for 0F, not your local 99% outside design temp of +17F- +18F. But even that is calling for only a ~35K boiler, or or a modulating boiler. If it turns out the crude heat loss calculator is anything near reality, an input of ~35KBTU/hr in an 80% efficiency burner comes out to a design load of 28,000 BTU/hr. With 430' of radiator that's 28,000/430= 65 BTU/hr per square foot EDR, which the rads can deliver at an AWT of 125F (say 130F out/120F return), which would be in the condensing zone with 90%+ combustion efficiency even at design condition- mid-90s or higher during the vast majority of the heating season. Run a Manual-J or a better IBR type load calculation, but no matter what your system is likely to be an excellent candidate for a modulating condensing boiler, when the time comes.

That small Triangle Tube boiler has a low turn down ratio- it only modulates down to 13,500 BTU/hr-in. While it would work fine with your radiation, you'd get more modulation and fewer burn cycles out of an HTP UFT-080W or a Lochinvar KHB-055 or -085, all of which modulate down to under 9000 BTU/hr (there are others.) The HTP is also quite a bit cheaper than the Triangle Tube. At condensing temps it's output would be ~7600 BTU/hr, which is about 1/4 of your design load, whereas the Triangle Tube's minimum output would be ~13,000 BTU/hr only more than 1/3 of the load, or almost half. That makes a real difference in how much of the season the boiler modulates up/down with load vs. cycling on/off.

 

[John Beemer]

Thanks Dana. I'll look at getting a HTP UFT-080W or a Lochinvar KHB-055 or -085. I might do that in the spring. If I get the HTP, I could also get one of these https://www.htproducts.com/superstor-contender-waterheater.html.

I could then get rid of my hwh.

 

[Dana]

Thanks Dana. I'll look at getting a HTP UFT-080W or a Lochinvar KHB-055 or -085. I might do that in the spring. If I get the HTP, I could also get one of these https://www.htproducts.com/superstor-contender-waterheater.html.

I could then get rid of my hwh.

If you are planning on replacing the beastie boiler with a modulating condensing boiler in short years there isn't much value to installing a narrowing flue liner. The chimney isn't likely to be close to collapsing or anything if there isn't visible and considerable degradation on the exterior or during a flue inspection, and one more heating season isn't going to push it over that edge. As long as nothing else is using the flue it's usually OK to use it as a vent chase for the plastic combustion air & exhaust venting used by mod-cons.

It will be worth insulating the basement walls (including the chimney) to the current IRC code minimum for you climate zone. (In zone 4A that would be continuous R10, such as 1.5-2" of foil faced polyiso foam board mounted to the walls with 1x4 furring through-screwed to the walls with masonry screws. There are alternative methods and a number of details to get right to avoid turning it into a mold-farm, so check back when considering it.) With air sealed insulated walls the basement is warmer in winter even without the standby losses of a big cast iron boiler, and will require less mechanical dehumidification in summer to keep the "musty basement smell" under control.

 

[John Beemer]

Thanks Dana. I was checking out the polysio foam and the 1.5" is 10R and the 2" is 13R.

Would it be better to attach the polysio foam board to the 1" furring strips or to attach the polysio to the side of 2x4 framing?

I might frame the walls with 2x3 since my basement it so small and the walls won't be load bearing. There would be more of an air gap if I attach the polysio to the framing and it might save me a little space although attaching to the furring strips would probably be faster.

I'm currently framing out a bathroom wall in the basement so this would be a good time to get some polysio foam board. What details should I adhere to in order to avoid turning it into a mold-farm?

Thanks again,

jb

 

[Dana]

Thanks Dana. I was checking out the polysio foam and the 1.5" is 10R and the 2" is 13R.

Would it be better to attach the polysio foam board to the 1" furring strips or to attach the polysio to the side of 2x4 framing?

I might frame the walls with 2x3 since my basement it so small and the walls won't be load bearing. There would be more of an air gap if I attach the polysio to the framing and it might save me a little space although attaching to the furring strips would probably be faster.

I'm currently framing out a bathroom wall in the basement so this would be a good time to get some polysio foam board. What details should I adhere to in order to avoid turning it into a mold-farm?

Thanks again,

jb

As long as the 1.5-2" polyiso is continuous and air-tight, it doesn't much matter whether it's on the foundation side of a studwall or screwed/clamped in place with 1x furring. Be sure to keep the bottom edge off the slab- it can wick ground moisture even if the slab isn't visibly damp, and be sure to air-seal the top seam. If the studwall will have fiber insulation it needs to be tight to the foam board, but if the stud bays are empty it doesn't really matter. Be sure to NOT install any interior side vapor barriers such as vinyl or foil wallpaper, 6 mil polyethylene etc.- standard interior latex paint on wallboard is preferred, since it allows any moisture that finds it's way to the space between the foam board and insulation a drying path. A fiberglass tub/shower surround in that area would be OK, even though it's a vapor barrier, just don't install true vapor barriers anywhere else along the foundation wall.

Installing a strip of 1" EPS under the bottom edge of the polyiso and under the bottom plate of the studwall is an adequate capillary break to keep the wood and polyiso from taking on moisture from the slab. Tape the wall foam to the strip of floor foam with housewrap tape. Keep a 1/2" clearance between the EPS and the foundation wall as a drain channel for any bulk water incursions/seepage coming through the wall.