I did a inspection on a home with a waste heat recovery system attached to a residential air compressor unit. I read some info on these systems but none could clarify if this is worthwhile on a residential system. Any info on these systems would be greatly appreciated. Thanks!
I am assuming you mean a heat recovery system that feeds water to the water heater. I see these systems quite often. From what I am told, they work quite well and help to reduce energy costs.
FYI - most AC companies do not re-hook them back up when they replace the condenser unit.
Boys! Boys!
If this unit heats domestic hot water using a refrigeration compressor with ambient air as its heat source, then it is a heat pump water heater. Probably, in more temperate to hot /and or humid sections of the country, the unit is worth it if you heat water electrically now. If gas is cheap in your area, it may not make sense. You must do an economic analysis for each case; you cannot make blanket statements.
It is worth it in ANY case.
You run the A/C at the same cost with or without the device.
You can dump the heat in the out door air or you can pre-heat the water to your water heater. The water heater will operate cheaper (regardless of fuel source) when the water temp differential is less.
Very, very common here, and as David said, very much worth it.
Since we use the air conditioner about 8 months a year in this climate, and we’re almost 100% electric water heaters, why not utilize the heat from the A/C. Depending upon the size of the water heater, it can save the homeowner a significant amount on the electric bill.
Oh, a heat pump water heater is something different. I have one on my pool! 
Now if I could just figure out a way to get the cold air thrown off by my pool heat pump into my house!
Thank you gentlemen! I thought as much I just didn’t know if it was worthwhile. But as you said if the compressor is going to be running anyhow it might as well heat some water too.
“a waste heat recovery system attached to a residential air compressor unit.”
Was this in fact an air compressor or an AC, HP/AC or a HPWH??
I feel there is still some confusion as to what hmcbride actually saw- a home made waste heat recovery unit or a manufactured add-on or ??? Does’t sound like anything that I’ve seen and I’ve been working/training/regulating in the energy industry for 30 years. Some web sites, gents:
http://www.toolbase.org/Technology-Inventory/Plumbing/heat-pump-water-heaters
http://www.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12840
http://www.energy.wsu.edu/documents/building/res/ht_pmp_water_htrs.pdf
**From the last site:
**
**Applications: **
Properly applied, HPWHs save energy in almost every situation. Initial
investment is recouped fastest if electric rates and hot water usage are high and there
is a steady need for the cool air generated as a byproduct. HPWHs have difficulty
being cost effective when low-cost natural gas is available.
Or if your live in a northern climate where heating costs surpass cooling costs.
"Or if your live in a northern climate where heating costs surpass cooling costs."
Yes! I live in Canada. In the last 14-15 years, I have poured water on the entrepreneurial flames of at least 2 people who wanted to import such units to make their fortunes.
There is no confusion, Brian, because we have heat recovery units attached to probably 30% of our HVAC systems here. Remember, we use our air conditioners 8 months or so a year, so we save a TON of money on the water heating by using these systems.
Here is a pdf file from Florida Power that contains information about Heat Recovery Systems.
Blaine:
De-superheaters for partial supply of hot water are integral to many ground source heat pumps up here in the great white north. Is the heat recovery unit a de-superheater??
I was confused by the terminology used. We’ve had to speciically label certain appliances with changed generic names to prevent confusion. e.g.- the air-to-air heat exchanger was confused with the air-to-air heat pump so now it’s called a heat recovery ventilator (HRV).
It’s funny that these add-on units are so popular. Carrier had an integral unit (maybe a desuperheater) for domestic hot water in its Weathermaker series (I believe). About 6-7 years ago they discontinued the line for lack of interest.