Hey all, My company is going to be installing a few ductless heat pumps very soon and I'm going to need a load calculation software. Any suggestions?

Thanks, Andrew

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I've specified quite a few of the mini split systems, and for the load calcualtions I use both Right-Suite Universal (by Wrightsoft) and Energy Pro (by Energysoft), and both work very well in coming up with accurate loads and designs. Energy Pro is the only one that will allow you to model the energy savings of variable refrigerant flow systems (mini or multi-splits).

Both softwares have a big learning curve, but are probably the most accurate and recognized as such. Another one I have not use as much of is Elite (by Elitesoft).

Right-Suite Universal - http://www.wrightsoft.com/

Energy Pro - http://www.energysoft.com/main/page_energypro_ep_information.html

Elite - http://www.elitesoft.com/

I am partial to the Elite software line. I use their CHVAC for my commercial and institutional work but have not used their RHVAC residential/Manual J based product. The interface takes getting used to, not as "clean" as the types that have a hierarchy tree (where houses own rooms, rooms own walls, walls own windows and doors, etc.). There is always an element of "preference because you are used to it", with any system of course.

Full disclosure, I am not an HVAC installer/contractor but on the engineering and design side and with a focus on energy conservation. So my approach is more towards saving energy versus that of a contractor where you have to have that balance of appropriate size (the right increment), not spending too much time getting to that point (time IS money), and having a satisfied customer good for repeat word of mouth business. My angle is different, not better, to be clear!

Regarding calculation inputs (with a nod to Phil Jeffers in this thread), I agree to a point, some inputs are made up, or at least empirical. The limitation here is, if you go with what worked in the past, you never really know if your number is as low as it might be, how close it is to the edge of "not being enough".

Because I take the energy approach and have more time but not too much time to dwell on such things, here is a short-hand way I get some confidence in my inputs for an existing structure. What follows below is for heat losses only, not AC:

1. Use an IR camera and thermistor set to measure delta-T, air and surface temperatures. Determine R value as closely as you can for all surfaces.

2. Calculate transmission of these surfaces for a design day (u x A x DT). About as good as you get for new and existing structures at this point.

3. Obtain fuel bills. Separate the three lowest months of gas use, subtract from all months to get a net heating number. If oil and domestic HW is gas, all the better. In any case, I like to get three years worth of energy input to even-out the years average weather.

4. Using degree-day data   http://www.degreedays.net/  specific to the area, I can back-calculate an approximate heat loss. One has to "tune" the annual efficiency and the Cd factor which accounts for internal gains, night setback, solar gains, etc. but have a go anyway.

5. From the above, I can subtract my transmission losses done in step 2. These have the highest confidence level of any of my numbers at this point. What remains is infiltration and that can be back-calculated to obtain a reasonable -and annually averaged- number.

6. I will also calculate the infiltration number using the empirical methods of ACH by exposure, the crack method, CFM/SF of wall method, and a blower door test if available, etc. and triangulate these. Compare to those numbers derived from item 5. The real number is elastic, highly variable, but my transmission number, being the most solid, gives an anchor to the infiltration and fuel use numbers.

I do not recommend this approach when you have to size a system on a cold night in a cold house though :)

One should keep in mind that there is an appreciable difference between the best "design day calculated heat loss" and actual energy used (or projections). The difference can be as high as 30 percent. Infiltration is easily the most elastic variable and most difficult to correct.

Part of me wants to side with Phil because he's my friend.  Part of me wants to side with Phil because the idea there is a correct load is absurd.  

Worst case load is incredibly dependent upon things most HVAC guys don't measure (infiltration and duct leakage), and "superstitious behavior and expectations" of the homeowners (that they can heat or not heat their home).  A 3500 sf home that can maintain 70 at 0 with 45,000 btu needs 3x the btu to "warm up" from 60 within an arbitrarily acceptable time period.  How do you calculate to arbitrary? 

Meeting superstitious behavior and expectations of the homeowner is much more important to the typical contractor, who has absolutely no skin in the game of energy reduction.  Homeowners want to jerk their thermostats all over the place and receive instant gratification, and I don't want to field a slew of zero pay complaint phone calls during record cold snaps.  

Re-education is a lot harder than education.   Changing perceptions is much harder than creating perceptions.  Getting homeowners to understand that designing for efficient operation vs fast recovery are two diametrically opposed propositions takes a lot of re- education.  I usually have to go over things 3, 4, and 5 times.   A home is like a freight train, if they want it to perform like a corvette they will pay for it in energy and comfort. 

For example:  My clients have dramatically downsized equipment.  Their equipment would take weeks to recover on the coldest day.  They have abandoned aggressive setback, setting back only for comfort.   Their equipment runs nearly continuously, they are incredibly comfortable, and they typically save 29-70% on their energy costs. 

I recently installed a GREENSPEED heat pump.  My first Propane hybrid, so instead of locking out at 20-40, I want that baby to run until it can't.  I showed up after completion to find the thermostat programmed for a 6f setback, so of a fair number of BTU were being provided by Propane for recovery.  (Smell of peanut oil burning off the heat exchanger was a dead giveaway.  I reprogrammed and told them if they smell peanut oil again, please call me.)

While pinpointing home base can never be done, we need to get to the ballpark because designing for the broadest load matching operation across the season is where huge efficiencies are gained.   

So I agree with Phil, we need to recognize that the assumption of a "correct" load calc is flawed.  But we still need to do them.  Load calc's are not the end point of good design, they are the starting point of good design.   They tell us where the ball park is, and we need to get to the ball park if we want to play the game. 

To determine heating capacity required for existing building one may simply extrapolate design load by graphing metered use to outdoor temperature for a series (preferably a minimum of 12) of billing periods.  See HeatingHelp dot com > search Therm_lag (that's me) and scroll to  "Graphical Load Estimating Method."   The guide is attached to this reply.  I use Etracker software (freeware) developed by Kissock at U Dayton. 


Awesome John!  Thanks!!!

Hi John,

    Very good Graphic Method.  With your permit, I would like to download your PDF to my website heatExch.com. Please leave me a reply here if I could do that.


    Perry Ning, owner of heatExch.com



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