Replace draft hood with barometric damper to reduce stack heat loss?

D107

Old draft hood for Weil McLain gas boiler--will a barometric damper reduce the heat loss from the stack effect of the hood which is basically wide open? Is there enough room in the horizontal run to install the damper? (see photo)

HVACNUT

I don't think you'll reduce heat loss. The hood is designed to pull room air while the boiler is idle.
I would check warranty issues.
Is there a spill switch on the left side?
I don't see one.
If you decide to change the flue, use a tee, not an elbow. Put one run end into the breach. Bull down to the boiler. Draft regulator at the other end of the run with a spill switch.
I recommend cutting down the regulator side of the tee close to the bull.
Set draft to manufactures specs.
Furnace cement at the breaching(s).

ChrisJ

HVACNUT said:

I don't think you'll reduce heat loss. The hood is designed to pull room air while the boiler is idle.

I would check warranty issues.

Is there a spill switch on the left side?

I don't see one.

If you decide to change the flue, use a tee, not an elbow. Put one run end into the breach. Bull down to the boiler. Draft regulator at the other end of the run with a spill switch.

I recommend cutting down the regulator side of the tee close to the bull.

Set draft to manufactures specs.

Furnace cement at the breaching(s).

If that's a WM I'd love to know what their specs are for draft because I've yet to see any such thing. They may give a minimum for the input to the drafthood, but that's all I've seen.

EBEBRATT-Ed

The boiler was built, tested, listed and approved with the draft hood. That's why they ship the draft hood with the boiler.

If you alter it you take all the risk

D107

This is a 1981 Model CGM-7P1 Series 7.
2007: stack temp: 382; amb temp: 76; outside temp: 72 deg; O2: 11.7%; CO2: 5.2%; CO 21ppm; 0% COR CO: 49ppm; Eff: 77% Excess Air: 113.7 Measured in draft hood: not valid
2008: Gas valve lowered from 3.1 to 2.65. Eff:75.5%, ExcAir: 67.3%;Primary temp: 67.6deg; Stack Temp: 488; O2: 9.0%; CO2: 6.7”; CO 8ppm; CO air free: 6.7%; Draft 0.
Having it tested/tuned next week.

EBEBRATT-Ed

You have to take the combustion test between the draft hood and the boiler

CJS

I have to agree with Ed, you are NOT supposed to modify the draft hood on this atmospheric boiler. It was tested with the hood on it and will void any warranty and may cause you to be liable if anything should happen. A double swing gas barometric damper is used for a gas conversion job when an oil burner is replaced with a gas power burner.

D107

Thanks, well obviously at 36 years old --installed long before we got here--we must be way past any useful warranty, but the liability would be a concern. But I will have a spill switch installed; no low water cutoff either.....

EBEBRATT-Ed

better add a carbon monoxide detector. It would be smarter to replace the boiler if it's that old

D107

Yes we do have a good low level detector right in the boiler room-- as well in two other places-- but would not a spill switch go to the next step of shutting the boiler down--good if we happen not to be home at the time?

HVACNUT

I think @EBEBRATT-Ed meant also, not opposed to.
Definitely do the spill switch.
Also consider replacement to a high efficiency boiler with an indirect water heater. At 36 years old, it's time. I'm surprised those B&G 100's are still there.

newagedawn

barometric dampers are not for gas fired equipment, only oil fired equipment, i would not recommend changing it, what is your issue to think it needs to be changed?

EBEBRATT-Ed

He thinks maybe too much excess air, low Co2 heat loss up chimney

D107

@newagedawn: that the barometric damper would lose less heat up the stack than the draft hood. But the advice here is solid so I've given that idea up. @havcnut: those BG 100s were replaced long ago by Taco 007 and Grundfos 3-speeds. We have other major house repairs looming so we've been holding off on boiler replacement--though we have just finished insulating our house, first with dense pack cellulose in the walls and now closed cell foam in the attic, so I'm looking forward to monitoring savings just from that. In terms of new boilers, an atmospheric would give us only 10% efficiency increase. Mod-Con wouldn't be that much more expensive--given that atmospheric would require a new stainless steel chiimney liner, but our concern is the limey water here and existing series piping. That's a whole other thread.

Steamhead

Instead of a barometric draft regulator, you need a stack damper which closes off the flue above the draft hood when the burner is off, reducing off-cycle losses. These have been standard equipment on atmospheric gas-fired boilers for a couple decades now, and your boiler may even have the proper hookup already installed.

These units MUST be installed professionally since improper installation can result in combustion products entering the room.

Given your situation, when it comes time for a new boiler I'd stick with cast-iron and a chimney liner.

captainco

Is it still the 1970's??

The Weil-Mclain boiler was approved with a drafthood and a maximum of 5' vertical pipe and 2 90's. It was also approved in a positive pressure mechanical room. Any violation yet?

In the early 1930's gas barometrics were created. Prior to that oil barometrics were used on gas. There was no such thing as a drafthood. Because oil barometrics were single acting they allowed pilots to blow out during downdrafts. That is when drafthoods were invented. However drafthoods were designed to go on top and to be moveable.

If one would check the CPSC recall list for unsafe equipment you would find boilers with drafthoods at the top of the list.

Barometrics connect equipment to their flues(isn't that a novel idea) where drafthoods disconnect. Barometrics control combustion air and venting and drafthoods prevent this from happening.

Barometrics do not reduce flue loss, they reduce flue dilution and minimize condensation. Because they control combustion air, they allow the equipment to be tuned to a known amount of air rather than random selection.

This boiler is underfired and needs to be because combustion air is not controlled. Wait, controlling combustion air violates industry standards (holes in walls do not control combustion air)!

Installing a tee instead of the 90 is a correct way to install the barometric with a spill switch.

Weil-Mclain, as does all other manufacturers do not allow any modification to their boilers in writing. However, what they might say off the record is slightly different.

I have gone head to head with them and they said they cannot condone it but what we do is our responsibility.

If you don't understand combustion, venting or combustion air then you shouldn't be doing it! If we are afraid of liability we are in the wrong business! In my classes I tell my students if I am on a job with anyone and there is equipment with a drafthood, it must be removed and replaced with a barometric and safety spill switch or I will have no part of the job. Yes, I am liable for everything I teach and recommend!!

Tim McElwain

Amen to that Jim!

captainco

And to you Timmie for promoting and teaching intelligence to the HVAC industry.

ChrisJ

@Tim McElwain @captainco

On a typical double swing barometric, such as the Field Controls MG series, what is the range of draft they operate at. I.E. from closed, to wide open?

They claim they come calibrated for 0.01" WC with no added weights, however obviously the draft must vary to open the door, therefore the draft cannot be perfectly consistent. But I've never seen a published range.

At what point is it 0.01" and if it's near closed, what draft will you have when the door is almost wide open due to excessive draft?

Is it 0.01" barely open and 0.5" near full open?

captainco

The Field gas barometrics are sensitive to .005" W.C. Because they can't get a 3" barometric to control that low, they don't make one. They are set for -0.02" W.C. in the closed position, which is usually the minimum draft most equipment requires. They swing open when draft exceeds this. They can only handle draft with good control up to -.08" W.C. to -.10" W.C. If draft is higher, like in a 50' flue, a blast gate or neutral pressure plate needs to be added to bring the barometric in control.

Unlike oil, gas barometrics are set up with a combustion analyzer not a draft gauge. After installed, you do a combustion test below the barometric. If the O2 and CO are stable it is set perfectly and will not change. If the O2 is falling and the CO is rising, the barometric is open too far and needs to be weighted closed. This is assuming draft is higher than -.02"W.C.

Remember draft is created by the temperature difference of the flue gases versus the outside temperature. Barometrics keep the flue gases hotter therefore they allow a greater draft to be created. Because the draft below the barometric is always -.02" W.C., the amount of combustion air drawn into the burner remains constant. The amount of flue gases vented remains constant. Wow! controlled combustion air and venting!!

ChrisJ

captainco said:

The Field gas barometrics are sensitive to .005" W.C. Because they can't get a 3" barometric to control that low, they don't make one. They are set for -0.02" W.C. in the closed position, which is usually the minimum draft most equipment requires. They swing open when draft exceeds this. They can only handle draft with good control up to -.08" W.C. to -.10" W.C. If draft is higher, like in a 50' flue, a blast gate or neutral pressure plate needs to be added to bring the barometric in control.

Unlike oil, gas barometrics are set up with a combustion analyzer not a draft gauge. After installed, you do a combustion test below the barometric. If the O2 and CO are stable it is set perfectly and will not change. If the O2 is falling and the CO is rising, the barometric is open too far and needs to be weighted closed. This is assuming draft is higher than -.02"W.C.

Remember draft is created by the temperature difference of the flue gases versus the outside temperature. Barometrics keep the flue gases hotter therefore they allow a greater draft to be created. Because the draft below the barometric is always -.02" W.C., the amount of combustion air drawn into the burner remains constant. The amount of flue gases vented remains constant. Wow! controlled combustion air and venting!!

But that's impossible, if the draft below it was always constant, the door would have no reason to open, ever. Therefore, the more that door opens, the more draft must be present.

No?

captainco

The draft above the barometric causes it to open. At -.02" the barometric is closed. At -.03" above it, the damper opens and relieves the excess -.01" draft and maintains -.02" below it.

Make sense yet? If not I will keep trying.

ChrisJ

captainco said:

The draft above the barometric causes it to open. At -.02" the barometric is closed. At -.03" above it, the damper opens and relieves the excess -.01" draft and maintains -.02" below it.

Make sense yet? If not I will keep trying.

Not sure how I understand there being a pressure difference across the tee that the damper is mounted in?

I would expect the pressure at both ends to be similar. No?

In order for the draft to "pull" on the damper door, it must also pull equally on the smoke pipe to the boiler that is literally right next to it, no?

captainco

No! It is simple how it works and almost magical. The flow just pulls the damper in and causes the excess draft to pull through the barometric. I guess you could call it a venturi effect but I am not sure if that is the exact reaction.
If you did put a draft gauge below the barometric you would see that the draft is stable regardless of what is above it. Or if you put the draft gauge below it and manually change the position of the barometric, then the draft below it would change. But draft doesn't tell us if we are venting it just shows we should or shouldn't. Only O2 and CO can tell if gas is venting.

Since 1980, helping contractors modify drafthood equipment, I am still amazed how simple a device is so functional and dependable, tens of thousands of times. I have taught contractors in over 45 states how to do it and 6 Canadian provinces from Nova Scotia to British Columbia.
Do manufacturer's understand? No! Code officials? No! Utilities? No! Contractors? A select few!

ChrisJ

captainco said:

No! It is simple how it works and almost magical. The flow just pulls the damper in and causes the excess draft to pull through the barometric. I guess you could call it a venturi effect but I am not sure if that is the exact reaction.
If you did put a draft gauge below the barometric you would see that the draft is stable regardless of what is above it. Or if you put the draft gauge below it and manually change the position of the barometric, then the draft below it would change. But draft doesn't tell us if we are venting it just shows we should or shouldn't. Only O2 and CO can tell if gas is venting.

Since 1980, helping contractors modify drafthood equipment, I am still amazed how simple a device is so functional and dependable, tens of thousands of times. I have taught contractors in over 45 states how to do it and 6 Canadian provinces from Nova Scotia to British Columbia.
Do manufacturer's understand? No! Code officials? No! Utilities? No! Contractors? A select few!

Not a surprise.
I'm still waiting to see an HVAC tech that understands what a run capacitor and start capacitor do, and why.

captainco

A start capacitor shoots the gun and the run capacitor goes around the track. Right?

bob_46

2Chrisj... Eli the ice man. Voltage leads current with inductive reactance. Current leads voltage with capacitive reactance. This enables you to shift the phase in the start winding relative to the run winding and make single phase power look like two phase power to the motor. Is the proper terminology run capacitor or run condenser? Whats the difference between a capacitor and a condenser? Don't they both look like this -I (- ? When I was in Cub Scouts I built a crystal set, it had a variable condenser. I still have it, it still works.

ChrisJ

bob said:

2Chrisj... Eli the ice man. Voltage leads current with inductive reactance. Current leads voltage with capacitive reactance. This enables you to shift the phase in the start winding relative to the run winding and make single phase power look like two phase power to the motor. Is the proper terminology run capacitor or run condenser? Whats the difference between a capacitor and a condenser? Don't they both look like this -I (- ? When I was in Cub Scouts I built a crystal set, it had a variable condenser. I still have it, it still works.

Capacitor and condenser are the same thing.
Condenser is generally an older term, but it was used a lot years ago.

You're correct regarding the run capacitor, but, what about the start capacitor?

bob_46

Caps in parallel add. The potential relay puts the start cap in parallel with the run cap increasing it's size. This is HVAC 101 stuff Chris that every pipe fitter apprentice is taught .

ChrisJ

bob said:

Caps in parallel add. The potential relay puts the start cap in parallel with the run cap increasing it's size. This is HVAC 101 stuff Chris that every pipe fitter apprentice is taught .

Correct.

But why?
What is the added capacitance doing for us? And why not just leave it in the circuit? How is both the run capacitor and start capacitor sized, and why?

ratio

Deep Magic.

I've always though that the hard start cap just provided a quick source of electrons whilst the armature got up to speed without having to drag down the supply; but I haven't had much luck with my always-thoughts lately.

ChrisJ

ratio said:

Deep Magic.

I've always though that the hard start cap just provided a quick source of electrons whilst the armature got up to speed without having to drag down the supply; but I haven't had much luck with my always-thoughts lately.

Nope.
The cap doesn't act like a reservoir, not in an AC circuit.

ratio

<sigh>

Jamie Hall

The additional capacitance from the start capacitor serves to increase the phase angle between the two sets of windings, which increases the torque on the rotor. One cuts it out when up to speed, as it also messes with the power factor reflected back, and with the efficiency of the motor...

And I love barometric dampers. Such an elegantly simple contraption. Also other somewhat similar rigs -- such as the pressure controlled dampers on coal fired boilers, which regulated the intake opening in accordance with the pressure (more open for less pressure) and were able to maintain an astonishingly constant steam pressure. At considerable expense in efficiency, however, it must be said!

Which makes me wonder just a bit -- is there such a rig to control the temperature on wood fired hot water boilers? Seems to me there could be.

HVACNUT

> @Jamie Hall said:
>
> Which makes me wonder just a bit -- is there such a rig to control the temperature on wood fired hot water boilers? Seems to me there could be.
>
>
> Yeah, that accordion looking, squeezey air blowing thing.

D107

Thanks for all the great info and thinking in this thread. I was waiting for the boiler to be checked out first with a service call to make sure that the new spray foamed attic wouldn't create an air intake problem, which apparently it hasn't. O2 9.6, CO 0-1ppm, Eff 74%, CO2 6.4, Stk temp 527, EA 73.3. Draft -.01 just above boiler. Even with boiler running steady state, there still is a draft up the draft hood.

I am hoping CaptainCo could post a photo or diagram of the setup he recommended:
"Use a tee, not an elbow. Put one run end into the breach. Bull down to the boiler. Draft regulator at the other end of the run with a spill switch. Cut down the regulator side of the tee close to the bull. Set draft to manufactures specs. Furnace cement at the breaching(s)."
The boiler company seems quite competent and up to date, the owner follows heatinghelp--always a good sign. The tech thought maybe a motorized vent damper might work, not sure if I want to add electric to that mix.

FYI boiler is way oversized for 40Kbtu heat loss (Net IBR output 143.5 mbh) but we've decided given other upcoming projects to stay with this for a few more years. Putting in new atmospheric to gain 10% savings doesn't make sense to us--but probably savings would be higher since it would be correctly sized. We've spent ourmoney insulating the house. But we'll be installing a low-water cutoff, spill switch and some kind of vent damper that can eliminate stack loss when boiler is idle.

captainco

This is what the bullhead or horizontal installation would look like, whether on top or in the rear. In this picture the drafthood is blocked off. On your boiler the drafthood would be removed.

I have contractors all over the country that have done this with 100% success.

D107

Thanks.
So if I'm understanding this there would be no second 90EL pipe, the run would go right into the chimney--the second 90 would be the terracotta flue itself. So one could look through the barometric right into the chimney. My only question would be, given no damper in the direct path, would this setup stop the off-cycle stack loss at all?

captainco

Off cycles losses are not usually as high as previously reported. Normally the draft in a flue when equipment is not operating is very low. Most studies or reports I have read on flue damper savings is 2%-3% at most. They can prevent downdrafts during the off cycle which could be a benefit.

Yes, you look right into the barometric into the flue.

D107

I think the flue damper savings you reference are for loss of heat from the boiler; my original concern was loss of room conditioned air up the flue via the draft hood which is completely open to the room. It would seem that as per your diagram, a barometric would cut down considerably on the loss of room air both due to its angle and the small amount of air that can enter when it is closed off-cycle.

ChrisJ

I know if I open my B vent to my basement when it's 20 degrees out, the amount of draft going up the pipe is decent. -8F outside it's impressive, that's with no boiler attached and 50 degree air in the basement.

I don't know for sure how much actual money is going up, but being people are worried about drafts around electrical outlets, I'm thinking that 6" hole in my roof is pretty substantial.