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This Old CFD House: Part II

John Wilson

John Wilson

Posted Mar 29, 2011

Welcome to Part II of a multi-part series where, in this installment, Travis takes a look at a typical chimney.

tm_bioFor this next installment of “this old CFD house”, I’d like to investigate another gremlin that was discovered during the inspection period of the house we are purchasing. This one related to the chimney. The current damper is rusted and hard to operate. According to the chimney expert we had out to estimate the repair, the damper doesn’t fully close due to all the corrosion. He stated that at a minimum, 8% of our heat would be wasted, going up the chimney which he said for the average home works out to be about $200 a year. I instantly had a flashback of myself as a kid leaving the door open and having my dad yell about “not paying to heat the neighborhood”. I as well don’t want to “pay to heat the neighborhood”, but I also don’t want to pay the chimney guy. So I needed to decide what was worse, leaving it as is and wasting some heating energy, or paying the $600 to have this fixed.

His statements gave me a lot of questions. Where were these “average” houses? I’m sure the heating losses would be worse in Minnesota then California, where we had only a couple months that need heat and the outside temperature is rarely below 32 degF. He said the damper doesn’t close completely, but how close to being closed is it? I would think a ¼ inch gap would leak a lot less than 8% of my total heat, but I don’t know. I also don’t know how much worse the heat leakage is as the opening increases.

I knew I needed to use FloVENT, our HVAC CFD tool to solve this problem, but I had to do some research. What does a typical damper system look like in a chimney, and what does the inner workings of a chimney look like? Well I found this picture on a chimney company website. It seems to exhibit some of those dreaded “airflow arrows”, which I hate because usually the airflow doesn’t do anything like that. For a future blog, I may look into the air flow of the fix, the “Strong Draft” side of the picture below to see if FloVENT matches the image and if the airflow is any better between the old and new. I already have questions about whether a strong draft is better, as if it’s winter and I’m heating my house, do I really want MORE air leaving my chimney? I likely want the minimum amount of airflow needed to remove the smoke from the house. But let’s get back on the current topic.

With an idea on the geometry, I needed to research one other piece of information. I needed to find out what pressure was realistic for my house. I found this image and article from my former employer, the National Research Council of Canada, which showed that about the worst I could expect for my home was for it to be pressurized to about 10 Pa. I didn’t use the open chimney, as I don’t know how “open” the criteria is for an open chimney system.

With that, I created the FloVENT CFD simulation model. I ran it with a 68 degF inside temperature (72 degF? that’s what sweaters and blankets are for), pressurized to 10 Pa, with 32 degF outside air temperature. The living room was assumed perfectly insulated so I could focus on the heat loss due to the faulty chimney damper. I used our Command Center application to automatically run through a sweep of damper gaps at 0.25 inch increments to investigate how the heat loss changes as the gap changes.


So from the simulation I found how much energy is leaving my house, but I need to associate a cost to this. As I haven’t gotten a utility bill yet, I don’t know what the typical energy usage is at this place, or what the cost of natural gas is. I researched and found that $4/CCF (100 cubic feet) seems to be reasonable for the price of natural gas, but I need to figure that cost out in energy units. According to one site, 1 cubic foot of natural gas is equal to 1000 BTU’s, so 100 cubic feet is 100,000 BTU’s and costs roughly $4. Using this, I obtained the result table below.

chimney_tbl_01After starting to do the cost calculation, I realized that using 32 degF for 24 hours a day for 6 months was way too conservative for California.  If I ever move anywhere with a cold winter, these numbers in red will be seared into my brain if I have any type of chimney.  I adjusted my cost calculation to just the overnight hours for 2 months, as that is seemed more reasonable for California.  Still, it’s neglecting the day time heat loss, which I still need to address.  I can see that a mere 2 inch increase in the gap results in over a hundred dollars more in heat loss and is approaching the quoted $200 from the chimney repair man.       

To address the issue about the heat loss during the day, I ran another Command Center sweep on the different gap sizes, but with a 50 degF outdoor temperature.  This temperature just seems right to me, as on cold days here the temperature highs are around 50-60 degF.  I looked at the cost 2 ways, one, using 24 hours a day and 6 months, mainly to compare to the 32 degF and see the magnitude of the buoyancy forces on my heat loss.  Secondly, I calculated the cost for 16 hours for 2 months and added that to the overnight 32 degF heat loss cost to obtain the total 2 month winter costs.



The 2 major things to take away from the above findings is that with more realistic temperature estimates, the chimney inspector estimation for heat loss cost was very reasonable.  I’m not sure how long it took him to run his CFD analysis though Cheerful.   The other main point that’s illustrated in the graph and the red table row is how much of an effect an 18 degF temperature change makes on the heat loss.  Partly due to the higher buoyancy forces driving air flow up the chimney and partly due to the simple fact that larger temperature differences drives more heat transfer.  It’s clear to see though, that the slopes of the graph lines are different, with the low temperature line having a steeper slope.   So in colder climates, the heat loss of a house is more sensitive to damper gap size.

Looking at those results, and thinking about buoyancy forces, made me think about my pressurization effects.  Right now, the simulation had 10 Pa for my house pressure.  Is this driving all heated air out of my house, or is it a non-factor?  It’s hard to have a feel for Pascal’s, but just thinking that atmospheric pressure is 101.3 kPa, makes me think 10 Pa isn’t a big factor.  I could likely have calculated the cross section area of the chimney opening to get an idea of the real forces involved.  But, I took the easy route, changed the 10 Pa to 0 Pa and hit “GO” on my 32 degF FloVENT simulations again and let it run during my lunch break to see if having an unpressurized house would have substantially less heat loss through the chimney.



I didn’t expect this.  Reducing the pressure, which seemed pretty low, down to 0 Pa reduced our heat loss by over half.  This effect is as important as the outside temperature and the damper gap.  While an unpressurized house likely isn’t feasible or desirable, reducing the pressure of the house could yield some cost savings.

So after looking at the results, I think it’s clear that fixing the damper is a sound investment.  For a gap of anymore than 1 inch, it will only take about 3 winters to get a return on investment on the $600 repair if our home is pressurized to 10 Pa.  Of course my calculations assumed that every day/night beyond 2 months was the 72 degF weather that makes California so expensive to live.  Any additional inclement weather will reduce that payback period.

Join me next time when I look at moisture in my crawlspace and see if my under floor ventilation is up to the task.

Travis Mikjaniec

Chimney, Computational Fluid Dynamics, CFD, HVAC, efficiency, FloVENT

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About John Wilson

John WilsonJohn Wilson joined Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd) in 1999. John has worked on or managed more than 100 thermal and airflow design projects. His modeling and design knowledge range from Electronics Cooling IC packaging level to Data Centers and Clean Rooms. He has extensive experience in IC package level test and analysis correlation through his work at Mentor Graphics' San Jose based Thermal Test Facility. He is currently the Consulting Engineering Manager, WRO in the Mechanical Analysis Division. Visit John R Wilson's Blog

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