Quote:
Originally Posted by Xringer
...professionally tested... you needed 20,000 BTUh..
But, the brand you wanted only came in 18,000 and 24,000 BTUs..
What would most people consider to be the wise choice??
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This is exactly the right question, because it illustrates the difference between sizing for HVAC and sizing for fossil fuel.
(* Regarding "professionally tested", it is a reasonable task for a person to do heat loss calcs themselves if they have the knowledge and tools. Build it Solar has a good on-line heat loss calculator that is based on Manual-J, and as mentioned before other integrated packages (some are free) also have heat loss calcs. *)
In the example you cite, most people would go for the 24,000 BTU heat pump.
But
the most efficient heat pump would be 18,000 BTU, with the proviso that secondary heat be available to fill in the heating shortfall on the unusual days (2% of the heating season) when required.
This is the method I have seen on pro HVAC blogs, free on-line design literature, proprietary HVAC design books, etc.
As I have been learning this stuff, and applying it, and since I tend to over-research everything, I used several different pieces of software and compared the results from each and finally I used the real-time testing method of having a bunch of electric heaters with Kill-a-Watts attached to each one (recording the readings every four hours) when the temp got really, unusually cold here a few seasons back. It was pretty handy, because the actual temp outside was the same temp as the design temp in the computer programs. I found that there was some difference between software packages, and I have greatest confidence in the real-time testing method, rather than the calculation methods. But they were all in the ball park.
* * *
Fossil fuel furnace & boiler sizing is not done this way. The correct way for fossil fuel is to go a bit over the minimum required heating. In fact, I have seen some of the older software for fossil fuel boilers that has a +50% sizing factor built in to the software! The basis of this thinking is that the installers want to avoid call-backs.
Regards,
-AC_Hacker