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Going Geothermal Part 3: Calculating Residential Heating and Cooling Loads

As I mentioned in my previous post using accurate heating and cooling loads to size your geothermal system is very important.  In this post I’ll show you how to calculate them yourself so you can compare your results with what your potential contractors come up with.  I’m going to try to make this as concise as possible but you have to understand it takes over 100 pages to explain this in ASHRAE’s (American Society of Heating, Refrigeration, and Air-Conditioning Engineers) Fundamental book.  The calculations are very easy though once you have all of the information gathered.


Step 1: Determine the U-values of Your Home

Today home owners are more aware than ever about energy efficiency.  I’m sure you’ve all heard of R-values on things like windows & insulation.  Well in the HVAC (heating, ventilating, and air-conditioning) industry we use U-values which are just the inverse of an R-value.

U-value = 1/R-value

R-values are the thermal resistance of a material so the higher the R-value the better.  Looking at the equation above you can see that means the lower the U-value the better.  Collect as much information about your home’s R-values.  If you don’t have any info on a certain material google it and just use an average value.  Or better yet go get your hands on an ASHRAE Fundamentals book.  Check your local library or buy an older version for cheap online (they are updated every 4 years but anything from 1990 on will work).

For exterior walls you’ll have to calculate an overall thermal resistance using the R-values for each material that makes up the wall.  But don’t just go ahead and add everything up you have to account for the studs in the wall!  Here’s an example of a typical 2×4 exterior wall:

Stud R-value Cavity R-value
Outside Air Film, 15 mph wind 0.17 0.17
Vinyl Siding, un-insulated 0.61 0.61
Rigid Foam Insulation, 2” 10.0 10.0
Plywood, 0.5” 0.62 0.62
Wood Stud, 2×4 nominal 4.38
Batt Insulation, 3.5” 13.0
Gypsum, 0.5” 0.45 0.45
Inside Air Film, still air 0.68 0.68
Total: 16.91 25.53

If the studs are 16” on center then the stud R-value accounts for roughly 25% of the wall and the cavity R-value accounts for the other 75%.  So to get the overall thermal resistance:

Wall R-value = (0.25 x 16.91) + (0.75 x 25.53) = 23.375

Wall U-value = 1/23.375 = 0.0428 [Btu/h*sf*degF]

Other common building material R-values:

  • Lapped cement board, 0.25” = 0.21
  • Hardboard siding, 0.44” = 0.67
  • Insulated vinyl siding, 0.375” = 1.82
  • Particleboard, medium density, 0.5” = 0.53
  • Batt insulation, 3.5” = 13-15
  • Batt insulation, 5.5” = 19-21
  • Expanded polystyrene = 5/inch
  • Loose fill insulation, 3.5-5” = 11
  • Spray applied polyurethane foam = 5.9/inch
  • Spray applied cellulosic fiber = 3.2/inch

To find your window’s U-values your best bet is to get them from the manufacturer but if that isn’t an option you can use general values.  Here are a few average U-values for vertical oriented operable windows:

  • Aluminum frame with thermal break, single pane = 1.2
  • Aluminum frame with thermal break, insulated, double pane = 0.88
  • Aluminum clad wood, single pane = 0.60
  • Aluminum clad wood, insulated, double pane = 0.55
  • Wood/vinyl, single pane = 0.55
  • Wood/vinyl, insulated, double pane = 0.49
  • Insulated fiberglass, single pane = 0.37
  • Insulated fiberglass, insulated, double pane = 0.32

A few door U-values:

  • Solid wood door = 0.40
  • Steel door with urethane foam without thermal break = 0.38
  • Steel door with polystyrene core without thermal break = 0.29
  • Steel door with polystyrene core with thermal break = 0.20
  • Steel door with urethane foam with thermal break = 0.20

Next you’ll need to calculate your attic/roof U-value.  You could go through and try to calculate an overall R-value of the drywall, trusses, and insulation.  But because in most homes there is a significant amount of air beyond those layers you can skip that step and just use the insulation and drywall values.

Useful attic R-values:

  • Gypsum, 0.5” = 0.45
  • Gypsum, 0.625” = 0.56
  • Batt insulation, 3.5” = 13-15
  • Batt insulation, 5.5” = 19-21
  • Batt insulation, 6-7.5” = 22
  • Batt insulation, 8.25-10” = 30
  • Batt insulation, 10-13” = 38
  • Loose fill cellulosic insulation = 3.4 per inch
  • Loose fill mineral fiber insulation, 3.75-5” = 11
  • Loose fill mineral fiber insulation, 6.5-8.75” = 19
  • Loose fill mineral fiber insulation, 7.5-10” = 22
  • Loose fill mineral fiber insulation, 10.25-13.75” = 30

*Remember to convert them to a U-value.*

Partitions also need to be considered.  Partitions are any vertical wall between a conditioned space and an unconditioned space (or less conditioned space).  Interior walls between garages, crawl spaces, or unconditioned mechanical rooms are all good examples.  To calculate the U-value of a partitioned wall use the same method I showed you for calculating exterior wall U-values.

Finally, you need to calculate the U-value for your basement walls and floor.  Below grade walls transfer heat differently than exterior walls.  Heat transfer occurs in a circular motion between your walls and the surface of the ground.  The farther down the wall you go the less heat loss you have.

Different soils will have different conductivities but an average soil is 9.6 Btu*in/h*sf*degF.  Using that conductivity heat loss for below grade concrete walls is as follows:

Feet below ground Un-insulated R-4.2 R-8.3 R-12.5
0-1 0.41 0.15 0.09 0.07
1-2 0.22 0.12 0.08 0.06
2-3 0.16 0.09 0.07 0.05
3-4 0.12 0.08 0.06 0.05
4-5 0.10 0.07 0.05 0.04
5-6 0.08 0.06 0.05 0.04
6-7 0.07 0.05 0.04 0.04
Total 1.15 0.624 0.445 0.348

So for an un-insulated basement wall that is 7 feet below the ground each linear foot of wall will have a U-value of 1.15.  For basement floors use the following table to determine your U-value:

Depth of Floor Below Grade [ft] Shortest Width of House [ft]
20’ 26’ 32’
5 0.032 0.0275 0.023
6 0.030 0.026 0.022
7 0.029 0.0245 0.021

If your house is wider than 32 feet just interpolate the U-values to the size you need.


Step 2: Calculate your areas

Now that you have the hard part out of the way you need to go around and measure your house.  If you can create a spreadsheet for this it will make your future calculations easier.   Add a row for each room in your home and put the area type (ie. wall, window, door, etc.) in separate columns at the top.  You will need the areas of your walls, windows, doors, & partitions as well as the floor area of each room and the linear foot of your basement walls.  Keep track of which direction (north, northeast, east, etc.) your windows and above-grade exterior walls are facing by creating separate columns for each direction.  Make sure that you subtract your window areas from the overall wall area so you aren’t accounting for that space twice.  When you have the overall floor area calculated (in square feet) multiply that by your wall height to determine the volume of air each room can hold (in cubic feet).  This will be used later in the air changes per hour calculation.


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Step 3: Determine your design temperatures

To do a heating or cooling load calculation you need t o figure out what your “design day” is.  A design day is basically a worst case scenario.  For your heating calculation it is the coldest day of the year and for your cooling calculation it is the hottest day of the year.  Your local contractor should be able to tell you what design temperatures they use for your area otherwise get your hands on an ASHRAE Fundamentals book like I mentioned above.  You also need to determine what temperatures you want to keep your house at in the summer and in the winter.  I’m going to show you how to calculate the heating load first so you can just enter the winter temperatures for now.  Next you will need to come up with a temperature for the spaces on the other side of your partitions.  The only partition we have in our home is our garage wall.  Typically a garage temperature would be close to the outside design day temperature but half of our garage was converted to a workshop so they have supply air registers in the space.  Even though we have them closed completely they leak air into the space and keep it milder than outside (which we plan to change in some of our future renovations).  So I used a partition temperature of 30 degF.  Finally, you need to find out what the average ground temperature is in your area.  ASHRAE uses this diagram of the ground temperature:

For Wisconsin I used a ground temperature of 23 degF.  Insert these temperatures at the top of your spreadsheet and use them to calculate three delta Ts.  Equations as follow:

  • Heating Delta T = Room Design Temp – Outside Heating Design Day Temp
  • Partition Delta T = Room Design Temp – Partition Temp
  • Ground Delta T = Room Design Temp – Ground Temp

Keep all of these temperatures at the top of your spreadsheet so we can reference them in our calculations.


Step 4: Calculate your heating loads

In your spreadsheet create columns for each of the following: wall, window, door, roof, partition, basement wall, basement floor, infiltration, and total.  Above each column name put the corresponding U-value you calculated earlier for each of the construction types.  Above the infiltration column put your air changes per hour (ACH) rate from the table below:

Use your design day temperatures for the winter and summer outdoor temperatures and take a good guess on your homes tightness.  I used a medium tightness for our home so our winter air changes per hour is 0.91 and our summer air changes per hour is 0.48.  For now just enter the winter air changes per hour over the infiltration since we’re starting with the heating calculations.

Now onto the actual heating load calculations.  For each room enter the corresponding equations:

  • Window = (Sum of window areas) x Window U-value x Heating Delta T
  • Wall = (Sum of wall areas) x Wall U-value x Heating Delta T
  • Door = Door area x Door U-value x Heating Delta T
  • Roof = Floor area for rooms with roofs x Roof U-value x Heating Delta T
  • Partition = Partition area x Partition U-value x Partition Heating Delta T
  • Basement Wall = Basement wall linear feet x Basement wall U-value sum x Ground Delta T
  • Basement Floor = Floor area for basement rooms x Basement floor U-value x Ground Delta T
  • Infiltration = (Room volume x winter air changes per hour/60) x 0.018 x Heating Delta T
  • Total = Sum of the equations above for each room

Now all you need to do is add up the total column to find out the overall heating load of your house.  You finally have your answer!  I do need to not e that this total is a sum of each individual room peak so it might be slightly higher than it will actually be.  But for our house my hand calculated heating load was 71,652 Btuh whereas the computer generated heating load was 70,720 Btuh.  So that’s pretty accurate if you ask me!


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In cold climates like we have here in Wisconsin residential units are sized to handle the heating loads but in warmer climates where the cooling loads are higher they are sized base on cooling loads.  So let’s go on to calculate your cooling load.


Step 5: Calculate your cooling loads

Cooling loads are more difficult to calculate because you have to take into account your latent load (humidity) too.  For residential calculations ASHRAE has come up with some generalized numbers to do cooling calculations by hand.  The accuracy of the calculations is reduced but there isn’t many other alternatives.

For walls ASHRAE uses cooling load temperature differences (CLTDs) and for windows they use glass load factors (GLFs).  Both are dependent on the wall/window direction.  Use the tables below to find a CLTD for your walls, roof, and partitions and a GLF for your windows :

Put these CLTDs and GLFs at the top of your spreadsheet.  Then add the summer temperature (room design, cooling design day, & summer partition) as well as the delta T’s calculated from them (cooling delta T & summer partition delta T).  You can assume the ground temperature is the same but you will need a new ground delta T using the cooling design day temp.  Next you’ll need to determine your summer air changes per hour using the table below:

Now all you need to do us enter in the following equations for each room:

  • Window = (Sum of north window areas) x North GLF + (Sum of east window areas) x East GLF + etc.
  • Wall = (Sum of north wall areas) x North CLTD x Wall U-value + (Sum of east wall areas) x East CLTD + etc.
  • Door = Same equation and CLTD as walls
  • Roof =  Floor area x Roof CLTD x Roof U-value
  • Partition = Partition area x Partition CLTD x Partition U-value
  • Basement Wall = Basement wall linear feet x Basement wall U-value sum x Summer Ground Delta T
  • Basement Floor = Floor area for basement rooms x Basement floor U-value x Summer Ground Delta T
  • Infiltration = (Room volume x summer air changes per hour/60) x 1.1 x Cooling Delta T
  • Total = Sum of the equations above for each room

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For my cooling loads I came up with 41,000 Btuh but my computer generated calculations came up with 26,000 Btuh.  So those generalized numbers obviously overestimate the cooling loads!


And that ends the longest post I’ve ever written! Wasn’t that fun?  I hope that was helpful.  Leave a comment if you have any questions.

To see the entire Going Geothermal mini series click here:
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[Figure & Table Source: ASHRAE Fundamentals Handbook 1997]




Going Geothermal Part 2: Finding the right geothermal contractor

To understand what to look for in a geothermal contractor (or what some might call an installer) you first need to understand how many residential HVAC (Heating, Ventilating, & Air Conditioning) contractors work.  As a residential contractor you would spend a good part of your time visiting homes and giving free estimates for your services (which typically include replacing furnaces & air conditioners).  To make the process more efficient most contractors use rules of thumb to determine the heating and cooling loads of your house and therefore size your furnace and/or air conditioner.  And why wouldn’t they when for +30 years all they’ve had to choose from is a handful of unit sizes anyway.  As long as you’re in the ballpark that’s all that matters.  If you’re unsure pick the larger size.  Well that works fine for furnaces & air conditioners where the difference in cost between unit sizes is minimal.  (Granted it’s not good to have things oversized for your future energy bills but that’s another discussion.)  But when it comes to geothermal systems where the cost difference between a 3 ton system and a 4 ton system can be anywhere from $2,000-$5,000 that makes a big difference!

So what is a homeowner to do?  Well first try to do your research and find the most throual HVAC contractor you can.  Ask them how they calculate your homes heating and cooling loads.  Do they use rules of thumb or do they actually calculate them out?  Keep in mind that “rules of thumb” come in many shapes and sizes.  Some rules of thumb are based solely on the square feet of your home while others take into account how much roof and window area your house has.  So as a homeowner your best bet is to just make sure your contractor does a Manual J of your house.  A Manual J is the widely accepted calculation used to determine residential heating and cooling loads.

Side Note: As a mechanical engineer that designs HVAC system for commercial buildings I’m not a big fan of the Manual J since it is less accurate than what we do commercially.  But since it is the standard residentially it’s probably all you’re find contractors use.  Don’t worry in Part 3 I’ll show you how to accurately calculate your own heating and cooling loads.

A good starting point to look for contractors in your areas is to see what contractors are members of your local and national geothermal associations.  The International Ground Source Heat Pump Association is a great source.  Members of this group have taken one or more classes through the association.  Many states also have state groups.  For my fellow Wisconsin readers here’s our state’s association.  Search around!  You may find local groups that do more than recommend contractors to you.  Some organize tours of homes or building that have installed geothermal systems while others have elaborate displays for people willing to learn.  Along the same lines ask around and see if anyone has any recommendations for a mechanical contractor.  But keep in mind that a company that came high recommended by someone with a typical split system may not be good at installing geothermal systems.  And don’t forget to also check out the Better Business Bureau to see how potential contractors compare.

Like most quotes people get make sure you get more than one.  In our experience we found the prices we were quoted to vary a lot from contractor to contractor.  This is due in part to how relatively uncommon residential geothermal systems have been for the last 20 years.  But when tax credit limit of $2000 was removed geothermal systems became a hot ticket item.  What resulted, at least in my area anyway, is that a lot of mechanical contractors have dabbled in installing geothermal systems but few have a lot of experience with them.  Of course ideally you would like to hire someone with a lot of geothermal experience but there may not be anyone in your area.  Instead you might have a number of people with moderate experience levels so how do you choose?  Well the best I can say is do your research and ask a lot of questions.

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Finally, I thought I’d share with you our contractor picking experience.  We had 5 contractors give us quotes.

Contractor 1 has been working on expanding their geothermal department.  They’ve investing lots of money on radio advertisements, fancy websites, custom designed hybrid cars, and even flashy embroidered shirts stating they are the industry leader in geothermal for our area.  As a result they have installed many geothermal systems particularly in the new construction McMansions that were built before the economy turned.  Most people don’t even interview anyone else when they are looking to install their geothermal systems.  They sound great don’t they?  Well when they came to our house to survey we weren’t too pleased with their cocky attitude and the way they treat their clients as if they are stupid.  I could tell in my grilling interview that they weren’t used to dealing with clients who actually knew what they were talking about.  Then we got their quote and were blown away.  They were $9,000 more than the other 4 contractors we interviewed (who were all in very close price range)!  Turns out they used some moderate level rules of thumb to determine our heating & cooling loads.  They said we needed an 8 ton unit where we really only need somewhere between a 4-5 ton.  Classic HVAC oversizing!  On top of that I didn’t like the brand of heat pump they used (I’ll go through these in Part 4) so we crossed them off our list.

Contractor 2 wasn’t a part of any of the geothermal associations I mentioned above but came high recommended to us from someone with a conventional system.  In meeting them Flannel Man was not impressed with their lack of knowledge and when I questioned them about how they came up with our heating & cooling loads they admitted they used rules of thumb.  One more off the list.

Contractor 3 had only 4 geothermal projects under their belt and their quote was a few grand more than Contractor 4 & 5’s bid so we came off our list too.

Contractor 4 specializes in radiant floor systems but also do heat pumps.  They seemed very educated about geothermal systems.  They had installed between 10-12 geothermal systems.  Both Contractor 4 & 5 used one of the brand of heat pumps I like.

Contractor 5 is a family based company that installs more traditional furnace/air side systems than Contractor4.  They had installed a dozen geothermal systems but most of them were horizontal loops (we needed a vertical loop).  They also had a leg up on the competition because they used a software in addition to their hand heating/cooling calculations to size the geothermal loop.  The software was able to tell me our expected electric bills, savings with a desuperheater, and savings versus our current system.  A very handy tool!

It was between Contractor 4 & 5 but in the end we went with Contractor 5 because we felt they were better able to answer my questions and were fast to respond.

Moral of the story: Shop around or you could end up with the pricey Contractor 1.

In fact a few days after we signed our contract with Contractor 5 we saw a house in town was installing a geothermal system with Contractor 1 (we knew this because they put up flashy signs of course).  It took all I had to not go knock on their door and tell them our experience with them but they were already in the middle of drilling so it wouldn’t have done much good.  We found out later that the couple had in fact not gotten quotes from anyone but Contractor 1!

For those of you who made it to the end of this post you have the reward of seeing a sneak peak of our heat pump introduced by our rescue dog Sophie:

Going Geothermal Part 1: How A Geothermal System Works

I’m not going to go into too much detail about this because there are so many websites that cover this better than I ever could (which I have listed below).

Basically a geothermal system is made up of a heat pump, ground loop(s), and either ductwork or a radiant system to heat/cool your house.  A heat pump is just a refrigerator that can reverse between heating and cooling.  If the heat pump is connected to ductwork like a furnace would be it will be able to both heat and cool the air as it is blown over the heat pump’s coil.  If the heat pump is connected to a radiant floor or baseboard system it acts as a boiler by creating hot water but it can’t provide cooling or dehumidify the air.

The heat pump is connected to an underground loop filled with water or a water-refrigerant mixture via a heat exchanger.  A common misconception is that the water/refrigerant mixture in the ground loop is what actually runs through the heat pump but that is not the case.  They are completely separate loops that don’t mix.  Ground loops can come in many styles (which I’ll go through in Part 4) but the most common style today is a vertical bore closed loop system.  The whole system works off the fact that below the surface the Earth’s temperature stays relatively constant.  Because of this the ground loop is able to absorb or reject heat to the Earth.  Even though the temperature difference between the ground and the water/refrigerant mixture might be small heat will still transfer.



Below is a list of websites & a helpful video you can go to for more information on how a geothermal system works:

Going Geothermal: An Eight Part Mini-Series

As I mentioned in my last post we’re converting our house from a traditional fuel oil furnace to a sustainable geothermal ground source heat pump. But instead of just telling you about our system I want to go into depth and do a series of posts about everything you need to know as a home owner if you’re considering going geothermal. When I was researching residential geothermal systems I was disappointed in the quality of info out there about geothermal systems. Yes, there is lots of info about geothermal systems but almost all of it is describing how ground loops work. Go ahead and Google “geothermal systems” you’ll find pretty graphics of cold water/refrigerant mixtures going into the ground and hot water/refrigerant coming out. Once you wrap your head around the concept of getting heating and cooling from the Earth is that all you need to know as a home owner? The answer is NO.
In an effort to help inform others I’m going to write an 8 part mini-series from my point of view as a homeowner and HVAC engineer. I’ll give you every question you need to ask your contractor and calculations you can do to determine the true cost of going geothermal. If you’ve ever considered installing a geothermal system you’ll want to read this!

To follow the series just follow my blog or click on the new “going geothermal” button I created on the right menu bar.

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Going Geothermal!

After weeks of getting quotes, tons of research, doing my own personal heating and cooling loads for our house, and checking with our tax adviser we’re signing a geothermal contract this week!!! As an HVAC engineer who specializes in energy efficient design and sustainability I’m so excited to apply my large scale commercial knowledge to my very own home. I could go on and on (and I probably will anyway) but here is the very basics:

  • We need a new furnace and condensing unit and though they might last us one more year we’d rather start saving money now
  • We really want to get rid of the huge oil tank in our basement and stop sucking on the teet of the couple fuel oil providers in our area (it’s very uncommon in our area)
  • We live in the country with no natural gas access
  • Previous home owner spent roughly $6,000 per year for heating and $600 for cooling
  • After re-insulating the attic, replacing a patio door that was rotted into an open position, installing plastic shrink wrap over every window, and keeping the temperature low last year we spent $2200 on heating and $225 on cooling
  • Projected cost for heating & cooling our house with geothermal is $1300 for heating and $51 for cooling
Vertical Loop System from

Vertical Loop System from

This is the story of two twenty something newlyweds who are learning to adjust to life in their first house, a 1973 fixer-upper.
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