System efficiency: forced air furnace vs. water heater/air handler
severaltypesofnerd
| Posted in Energy Efficiency and Durability on
Where would I go to predict the relative efficiency of these setups:
1) An efficient forced air furnace, plus separate efficient water heater.
2) The same class of water heater, but with a water-to-air air handler instead of the furnace.
(In this case for a 1500sf insulated one bath single family home in a mild climate, which has no existing furnace)
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Replies
Bryce,
Furnace efficiencies numbers are reported by manufacturers. You're looking for the AFUE ("Annual fuel utilization efficiency") number. AFUE is a laboratory derived efficiency rating for heating appliances which accounts for chimney losses, jacket losses, and cycling losses, but not distribution losses or fan/pump energy.
Mid-efficiency natural gas furnaces have an AFUE rating in the 78% to 82% range. High efficiency furnaces generally are above 90% AFUE.
For water heaters, look for the EF (energy factor) number. The energy factor is defined as the ratio of the energy delivered to the water over the energy put into the tank, including standby losses. The EF takes into account heat lost up the flue, heat lost from the water sitting in the tank, and how much of the fuel's energy gets captured in the first place -- but not distribution losses.
Gas water heaters have an EF in the 59% to 90% range. Electric storage water heaters have an EF in the 90% to 95% range.
Your system efficiencies can only be estimated. Distribution efficiency is all over the map; many homes have leaky duct systems, and some homes have duct systems that are located outside of the thermal envelope.
In reply to Martin: I can look up the EF/AFUE of all of the parts, but the question is more complex than that. Here the question is the advantage of having the water heat plant operating at lower turndown offset by the pump and fan costs for the air handler?
Here all the variables are held constant. The same water heater. The same house. The same (crawlspace) air ducts. The only variable is air handler vs. gas furnace (e.g. HTP Light Duty vs Trane XR-95 plus HTP Light Duty). Has anyone done a site study for example?