Oversized overtall chimney?

captainco · September 7

I was on a job once with a contractor and it took opening a double garage door to overcome the leakage of a house. We didn't tell the homeowner to fix his house or tell him his chimney was too big for his orphaned water heater. Excessive house leakage causes down drafting not excessive up drafting.

Wet chimneys loose about 40% of their capacity when wet, which is why caps are called rain caps.

Open door, cold and wet chimney and door open. Which way will the air move initially.

There is not one scientific study that proves chimneys can be too big. I can come up with hundreds of field situations where big chimneys were never the problem. Pressure and temperature control chimneys not size.

Bob Harper · September 7

Install a smoothwall 316 ss chimney liner sized for the input rate. Incorporate a barometric damper and set it using a draft gauge btw the baro. and the flue. Seal the upper level air leaks in the home. Run combustion analysis to tune the burner.

A manual fixed blade stack damper is a flow obstruction. BTW the barometric damper and the burner compartment, it can cause sufficient flow disruption to cause flame impingement, which leads to CO production and soot. Use with caution and only guided by testing.

Too high draft pressure can cause CO production so it must be regulated and the only thing that does that reliably is the baro. damper properly set. In extreme cases you may need to install a baro. larger than the vent connector diameter or add a second. On gas, you would use double acting baros. with spill switches attached.

Jamie Hall · September 7

Really now. You are going to use a fan to bring in more air… in hopes that a little less air will come through the cracks? Please…

dandub1960 · September 9

https://forum.heatinghelp.com/discussion/comment/1864233#Comment_1864233

So I've only seen barometric dampers used to limit draft on oil or wood stoves. They appear in my simple mind to break the pressure connection between the combustion appliance itself and the chimney by allowing dilution air into to stack (like a vacuum breaker of sorts). Whereas my atmospheric gas boiler has an open draft hood that forces the boiler and the chimney to function independently (up to a point).

My fixed blade damper is above the draft hood, so throttling the chimney. I would expect the boiler internal draft to carry on as before.

Maybe I'm picturing this wrong but if I put a baro on the chimney isn't it just going to kick open and suck up air like crazy until we mimic what is currently happening at the draft hood? Does the baro need to connect to an outside air supply?

Bob Harper · September 9

A draft hood de-couples the flue from the appliance. A baro. allows the appliance to be connected to the chimney but with the ability to regulate the draft pressure, which a hood cannot do. A fixed blade in the connector is a flow restriction and not a draft control.

A baro. is pulled open by high draft pressure. It allows cooler room air to enter, thus reducing the draft pressure down to the set point as determined by a counterweight that has been adjusted using a draft gauge. It does not induce but rather limit draft. A hood entrains air up the stack unregulated 24/7. The baro uses room air. The baro. also allows the attachment of a spill switch, which the hood does not or does a poor job at.

EdTheHeaterMan · September 9

https://forum.heatinghelp.com/discussion/comment/1864349#Comment_1864349

Maybe I'm picturing this wrong Probably not you have this pictured correctly but if I put a baro on the chimney isn't it just going to kick open and suck up air like crazy That is exactly correct until we mimic what is currently happening at the draft hood? Pretty much Does the baro need to connect to an outside air supply? No just connected to the vent pipe connecting the appliance to the chimney base. The draft hood would then be eliminated if you go this route. I have seen overdrafting conditions that require a second Barometric draft control in order to allow the appliance to operate within the proper range.

BTW Doing this will not solve the problem you have described in your original query. You need to eliminate the chimney if you do not want to or can't afford to do the other work sealing the structure. Eliminate the cause of the overdrafting and vent the boiler mechanically with metered air flow and in and out of the boiler room.

Or replace the boiler with a direct vent ModCon boiler that does not require a chimney.

dandub1960 · September 9

https://forum.heatinghelp.com/discussion/comment/1864365#Comment_1864365

So knowing what you know about my boiler situation what would you guys recommend as a replacement boiler? This whole exercise really started as a way to figure out proper sizing for a new boiler. I figured the whole heat loss / pick up factor estimation was just a lost cause for my very unique house. I'd like very much not to repeat the mistakes of the past (past two boilers anyway).

BTW I have been working at the home leakiness and insulation for a long time too, just reaching limits of what can be done I'm afraid.

EdTheHeaterMan · September 9

Based on the information you have provided, you have an old house that is not very tight construction like a house built today might be. That being said, there is a section of every heat loss calculation that covers something called infiltration. That is the factor that says how many air changes per hour you can expect to happen in your home based on how leaky or how tight the construction might be. I have completed many I=B=R form 1504WH and ACCA Manual J load calculations that can tell you that a home with the exact same dimensions as your existing home, that is built with today's construction specifications can change from a 48,000 BTU load to a 125,000 BTU load by just changing the infiltration factor from .25 for new construction to 4.0 for old leaky construction. The person that does your load calculation must actually look at what you have to determine that infiltration factor for your home's “AS BUILT” condition and get you the proper size replacement boiler size.

There are other ways to determine your boiler size by using a Degree Day calculation and your actual fuel usage. I have not used the calculation but I understand that it can be very accurate. Saves you from all that measuring of all the doors and windows, walls, ceilings and floors, then making a wrong guess at the infiltration factor.

Do you have the actual fuel usage of your boiler? That may be easy to get from your utility. You will need to subtract any gas used for other appliances like the water heater, clothes dryer and cooking. But that is easy enough. Then the national weather service has the degree day records for the weather station closest to your home. With that info, the Degree Day method might be your best chance at determining your replacement boiler size.

Can you provide that information here? I wouldn’t mind giving it a try. Perhaps there are others that may also give it a stab.

Bob Harper · September 9

Again, forget about the equipment until you fix the thermal envelope of the structure. If you do, it will take far less boiler to do the job. If you just throw more equipment at the problem the utility will thank you for wasting so much energy and paying for it. You don't have to rebuild the house but close the barn doors then think about the boiler. BTW, the standard is to have a blower door test done with infrared thermography, which will generate numbers on just how leaky the house is as a baseline. Once you tighten it up, you can re-test, calculate the improvement, which can then be used to calculate the savings and if further improvement would pay for itself. You can install a TurboBlaster 2000 with the flux capacitor and inverter raterfrazzer but if the building is a sieve you're wasting money.

Larry Weingarten · 1:40AM

Hi, Just so you know, I looked for the "inverter raterfrazzer" on Ebay and it's so expensive that they don't even have one for sale! Shell tightening just might be less expensive in the long run. 😇

Yours, Larry

ps @Bob Harper got it right about the blower door. If you can keep a blower door on site while work is being done, it will help guide you towards the most effective things you can do to bring leakage numbers down quickly.

Jamie Hall · 1:49AM

Just to throw a little sand in the gears… a blower door test is not always feasible, depending on the nature of the house…

Not that tightening up the envelope is a bad idea. But that, too, has limits depending on the are and construction of the house. It can also be done very poorly; some knowledge of the age and construction of the structure can go a long way towards avoiding "fixes" which are not cost effective — and may actually make things worse (a very common example is replacing old (pre 1920 or so) double hung windows with modern replacements — they will not be as good as adding quality storm windows and doing necessary minor repairs on the sash, and will cost a LOT more and not last). Misapplied or incorrect insulation is another, which can cause real disasters. There are others…

Oversized overtall chimney?

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