What Do You Use to Size HVAC Equipment?
Hopefully your answer is not a rule of thumb, but instead relies on a careful room by room Heat Loss/Gain Calculation. Many Contractors tell me they rely on the Plumbing Supply to run the calculation.The important thing is to complete a Heat Loss and do it accurately, so you can avoid an unsatisfied customer, call backs, and possibly a change of equipment with restocking charges and extra time. Just do it right the first time, and often that involves having someone in your Company double check calculations before the quote goes out.
Install 100% Secondary Back Up
Let's say you have a home with a 40,000 BTU Design Heat Loss representing the worst hour of the year averaged over 30 years, within 2% of record temperatures. Using any type of Heat Pump will have lower performance in freezing weather at the same time the heating requirements approach the Design Temperature. In other words, a 36,000 BTU heat pump may only put out 50% at 17 F of the "Nominal Value" at 47 F. When the owner needs 40,000 BTU they are only getting 20,000 BTU in the example. The answer is not to over-size the Heat Pump by double, instead use a 40,000 BTU electric backup for those unexpected days that need more.
Why Full Backup?
Why not just 1/2? By providing a modulating electric inline heater, it gives an extra overlap of coverage to add nothing, a little, or do it all. For Heat Pumps, there will be days that the weather is so bad that full backup is a wise investment. Full backup also covers the owner should the Heat Pump stop for a few days because of a part replacement. Most people would rather pay a little more to know they have that extra margin, and it keeps the Contractor from getting calls during Christmas that the Heater is not keeping up. The typical fix is to add a variable output Electric Tankless on demand heater, if your panel has the room. Other popular solutions are a wood stove, pellet boiler, or propane on demand heater. The concept is low initial cost, low fuel cost, and if it works when the power goes off.
NOTE: Consider the 6 kW Electric in a SunPump as just for the Hot Water. Provide an additional 100% secondary backup inline for the space heating.
Planning Requirements
Use a CSA F-280 compliant orhttp://www.hrai.ca/skilltechtraining.html equivalent spreadsheet to do the Heat Loss calculation, or get it from your Wholesaler. It is wise to review it carefully, because it is the Installer who is responsible for the accuracy, so double check free calculations. Your local Inspector may require a mainstream software calculation (like HOT2000, or HeatCAD), to be submitted in order to get a permit, this is becoming the expected norm. Cities look for the TECA or HRAI approved format, and often proof that the person performing the calculation has passed their course, but certainly that it is completed accurately. Attached below is a TECA form required by many BC Cities, that SunPump has made into a fillable PDF for easy use.
Get Some Training
Have your staff take a TECA or HRAI course this year to keep customers happy, improve professionalism, and get extra incentives for qualified Trade Tickets.