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 · September 10

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 · September 10

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…

dandub1960 · September 10

https://forum.heatinghelp.com/discussion/comment/1864377#Comment_1864377

I appreciate the offer Ed. Just realize this is a 100 year-old 1 1/2 story home near Chicago, built of hollow tile block, some walls 12 inches thick (3 courses), some 16 inches (4 courses) covered by concrete stucco on the exterior. It has original steel casement windows, most with interior storms. Oh and it's a rather complicated shape (think a number of smaller boxes smooshed together). If you think you have factors for something like this beast I'm happy to gather up room and window sizes.

I also have prior year gas use, though that of course is average.

I feel like I've been living with a permanent blower door experiment (negative pressure anyway) without the benefit of IR camera. Crawling around feeling for infiltration on cold nights has helped to find some odd leaks. Attic was insulated with R19 fiberglass on top of cast in place gypsum in main house. Living room had R21 foamboard added on top of sheathing on open truss ceiling in the 90s. Windows have tightened up some.

I did have a timer measuring runtime for a while. On the worst polar vortex day it clocked 16 hours (at lower firing rate) so have never lacked for heat. Balance between older standing rads upstairs and a bunch of cast iron baseboards downstairs (1950s "improvement") gets a bit challenging then but we're never opening windows.

Also the distribution system is two pipe steam. Was likely a vapor system initially, converted to higher pressure and pumped return in 50s, then back to gravity return and lower pressure in the late 90s Even before downfiring the system ran happily on less than 5 inches WC, until very end of heat call when I set it to cut out at 1.5 psi (that kept the dry returns from flooding and making all kinds of weird surging.) With the downfiring the heat call is generally satisfied without excess pressure building.

dandub1960 · September 10

BTW I found some notes from last season when I was trying to estimate dilution air from temperature measurements just before the draft hood and then 5 feet downstream. I can't say for sure these gas streams are well mixed but boiler outlet was about 350F and post dilution air was just 140F (I didn't log outside air temp u fortunately). That corresponds to about 250% dilution air by my calc. With the blade damper dialed in, the flue got up to 240F, which would be 60% dilution air.

If I got this right that would mean about 153 cfm make-up air wide open vs 66 cfm throttled. Since I didn't account for excess air in the boiler (which surely is higher in the downfired state) true numbers could be a bit higher.

EdTheHeaterMan · September 10

Since a ModCon replacement boiler would offer you a sliding range of Gas Input based on actual need, you can oversize a bit and still get the proper size boiler. If you find that your heat loss on any given day is 84,260 BTUh, the boiler will ramp up or down to that actual input to provide the 84,260 NET output required. If the weather is milder, and you only need 41,987 BTUh, then the boiler input will change to produce 41,987 BTUh at that particular time.

With this in mind and the fact that on the coldest of days your boiler input of 225,000 BTUh was only used for about 66% of the time, it would be fare to say that a boiler rated at 199,000, BTUh input with a combustion efficiency over 95% will produce more than enough heat for your home. Of course you need to be comfortable with the accuracy of your testing procedure. Was that 16 hours of burner operation accurate? When the timer test was completed, was that the actual lowest temperature you encounter during the season?

Now you need to make a decision if you want only one boiler or you want two smaller boilers. Two 120,000 boilers will be more than enough and only one boiler will operate for 85% to 90% of the heating season. The second boiler may only be needed when the outdoor temperatures are below 10°F. But if you have two boilers and one boiler fails, you have a backup that will do the job for most of the year and protect your pipes from freezing when the temperature drops below the capacity of only one boiler. The pipes will be at maximum temperature for the one boiler's capacity, and the burner will operate 100% of the time, so they will be no where close to freezing.

You might even try the one boiler with a provision to add another boiler if needed. I did that when I removed a 450,000 BTU Boiler from a home that clearly only needed 200,000 BTU boiler. I could not convince the powers to be that all they needed was a 200,000 BTU boiler so I gave them a price for two 200,000 BTU Boilers and said that I would put one in right away and add the second one when they called to say there was not enough heat. I would stop what I was doing and install the second boiler the day after they called with "not enough heat".

Eight years went by, and I never got the call for the second boiler. And their oil usage dropped significantly. So the boiler room has a smaller boiler on one side of the room and a big open space for another boiler that will never be installed. Go Figure!

Jamie Hall · September 10

WAIT a second here. Right near the end of @dandub1960 's 11:04 comment above I read "distribution system is two pipe stream" .

That's fine, but makes all the comments above about mod/cons and so on quite irrelevant.

Now. Let's get back to some basics. Our OP's complaint appears to be that the house is draughty. Quite possible, given the age and the mention of metal framed windows — which were junk when they were installed and haven't improved with time.

There seems to be an impression that the problem is the oversize chimney. That probably isn't helping much, but is unlikely to be the major problem.

Assuming it is a significant problem, though, the only approach which will do much to address that problem is to restrict the chimney itself. Since there is a draught hood on the boiler, messing with the boiler air supply isn't going to address the problem, unless the boiler and chimney were placed in a sealed room. Difficult to do, but not impossible. Then you could add an outdoor air supply to that otherwise sealed room and let it go.

A cheaper approach will be to insert an appropriate sized liner in the flue, connected to the boiler draught hood, and seal between that and the flue (airtight seal at the bottom, screened at the top , or alternatively fill between the liner and the existing flue. Which I don't really recommend…

dandub1960 · September 10

https://forum.heatinghelp.com/discussion/comment/1864414#Comment_1864414

Ed, does ModCon offer a steam boiler? I was under the impression theirs were all hot water. I'm also constrained somewhat by the "connected load" requirement for steam. If I go too low some rads just seem to shut down. That being said the standing rads upstairs are pretty pinched back on the valves to keep from overheating those rooms. So maybe the effective connected load is less than estimated? They could probably stand to be derated a tad more but those old valves are pretty crunchy and hard to fine tune.

dandub1960 · September 10

https://forum.heatinghelp.com/discussion/comment/1864415#Comment_1864415

The draftiness is really only noticeable on the main floor, probably since leaks are outward upstairs, but i think that also speaks to some efforts to seal windows and block some sneaky flows into the attic. Once the boiler has shutdown the air movement is damn near still. It's almost eerie, but I kinda like it. It's only when the boiler kicks on that the breeze begins again across the living room floor. Maybe some of this is just perception when the room is cool enough to call for heat but I think the stack temps measured give credence to excessive dilution air.

Anyway thank you for you time here.

EdTheHeaterMan · September 10

OMG. One of my rare mistakes…. on September 7 @dandub1960 said: "I own an older home with an atmospheric vented gas steam boiler". I'm sorry about that, I was thinking of another discussion when I answered this one, with heat loss and ModCon as a solution.

Your replacement boiler should be a Cast Iron steamer based on the EDR of the connected radiation. Your heat loss is irrelevant to sizing a replacement steamer.

@Jamie Hall can tell everyone that he found a rare error in one of my posts. Mark this day in history!

🤣🙃🤫🤣🙃🤫🤣🙃🤫🤣🙃🤫🤣🙃🤫🤣🙃🤫🤣🙃🤫🤣🙃🤫🤣🙃🤫

Bob Harper · September 11

If your primary concern is cold air infiltration, then seal the upper envelope and reduce stack losses by installing a barometric damper so it's not exhausting at standby. The Neutral Pressure Plane follows the leaks. If it's high on the first floor or higher, you really don't have an upper seal and your house is just one big chimney. Put a cork in it then provide some relief as low as possible below the NPP to move it downwards and thus move the infiltration to the basement- not living space.

A high mass boiler is going to have some stack losses at standby, but you can minimize those losses with a baro. and an interlocked vent damper with a spill switch on the baro.

If your boiler was sized properly for the EDR and you down-fire the boiler, you are shooting yourself in the foot.

dandub1960 · September 11

https://forum.heatinghelp.com/discussion/comment/1864440#Comment_1864440

Thanks Bob,

This whole exercise started as an experiment to decide on a "proper" sized replacement boiler for my house. If I replaced it on failure, i figured all i would know (from my own observation and comments from a couple heating guys who looked at it) was that it was too big. Admittedly the testing went on over a long period, as this old beast has been raised from the dead too many times. My wife is terrified that its next death will be its last and of course occur during a week of polar vortex, which is a fair concern.

So i am currently bidding it out for replacement. Still likely an atmospheric gas unit with draft hood, as the inducer driven models don't quite go big enough for me. Looking at 250 - 280 input to the connected load of 660 EDR.

What this exercise did underscore is the significance of the flue venting. Just tying into the existing chimney was going to be a problem. Nobody I've talked to previously even mentioned a liner, which surprised me.

It seems clear to me that even with a liner (and rain cap) this tall chimney has the potential to draw like a freight train on the coldest days. I have a feeling trying to eliminate the draft hood for a barometric control might not sit well with manufacturers warranty? But actually controlling the draft is pretty tempting all told.

Thanks everybody for the stimulating conversation. It's certainly given me food for thought.

Jamie Hall · September 11

Y have two quite separate problems here. The first is controlling the overfire draught on the boiler. That is what controls how well it operates, and that is done with the boiler. If there is a draught hood — or a big enough barometric (which you might have real trouble installing) the operation of the boiler is completely separate from the chimney.

Now the chimney draught is as I say a completely separate problem, and the best way to reduce problems from that is to seal all the draughts in the rest of the structure as best you can first. Then if there sill is a problem, consider a liner in the chimney.

dandub1960 · September 11

Thanks Jamie,

It is easy to forget that there are really two "appliances" operating in series here, first the boiler itself, burner compartment and exchanger, that is designed more than controlled, for atm gas, at least. Then there is the "venting unit", the draft hood, flue connections, and chimney, liner and cap, if any.

I also find myself saying draft when truly I mean draw. Kind of a voltage vs current analogy. The only way to control the draft is to cool the chimney contents presumably with dilution air. Draw rate is probably what could stand to be managed, to exhaust whatever the boiler producing but without going nuts.

As Bob mentioned, I need to push the neutral pressure plane down below the floor level as best i can. Then at least leaks go out and don't annoy occupants. I do think it's interesting that my living room floor is the prime culprit for draftiness and it is 10 inches lower than the remaining rooms on that level. So when my boiler fires up and chimney is drawing, that neutral pressure plane rises in response to air demand, and I feel those drafts again. Presumably sealing any leaks above NPP will help, as will providing combustion air close by the boiler, and I think throttling the dilution air being hoovered up by an oversized and overtall chimney. I'm thinking flue liner is a definite must have here. If there was an active draw rate management I'd be tempted to give it a go too.

Oversized overtall chimney?

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