Desuperheater Retrofit and Performance Question

snax · Tar Sands Joined: Jan 20, 2007 Posts: 75

Lately I've been intrigued by the idea of a heat pump operated water heater, but it has occurred to me that already having a heat pump would make installing a second one just for the water heater a bit superfluous if a desuperheater could be retrofitted to the existing tank and heat pump.

So I'm asking if anybody here has any direct experience with a desuperheater retrofit?

I'm wondering what purchase and install costs were, and what difference it really made on your power consumption?

Reading through company brochures and other sources, it suggests that one might save as much as 30-40% on water heating and another 15% or so on AC in the cooling season. Do these things really bear that much fruit?

Any problems?

What brands are good and which should be avoided?

Thanks!

Aimrehtopyh

I'm no expert, but it seems to me that refrigeration systems need to be pretty well tuned. Plugging that extra load into your heat pump system might screw everything up.

The efficiency advantage from sucessfully mating your heat pump with a water heater would be pretty small anyway. If you have the cash and like the idea I'd say go ahead and get a stand-alone heat pump water heater. It'll be less of a headache to install and will be less failure-prone. Isolate your systems whenever possible, integrate them only when the efficiency/convenience gains will be LARGE.
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pup55 · Expert Joined: May 26, 2004 Posts: 3681

I think it's brilliant. Both a heat pump and your fridge have hot coils on the outside waiting to be exploited.

Out in Suburbia, where I am, the problem is that the hot water heater is stupidly located in the basement, about 50 feet away from the showers. Every time you turn on the shower, it takes about 45 seconds for warm water from the water heater to reach the shower head, thus wasting that much water while you wait for the shower to warm up. Then after you take your shower, the hot water, which cost you something to heat, sits in the pipes and cools off, I suppose in theory radiating this heat out into the house.

All the idiot designer of this place would have had to do was put the HWH in the attic, five feet away from the showers, which are on the second floor. No problem to put a containment system in (plastic tub drained to the outside somehow) for the once every 20 years that the water heater wears out and starts to leak.

But that would have cost them an extra $50 for the plastic tub, so they did not do it.

There are just a few little things that the designers could have done to make this place much better to heat and cool, plus made it easy to retrofit a passive solar system, or other methods of heating. Example: make a conduit shaft that runs from the basement up through the house somewhere to the attic/ceiling to run circulating pipes.

But they did not, because when they built this place they assumed energy would be cheap and plentiful for the life of the house. Unfortuntely, they might have been right. The house might not last as long as the mortgage anyway.

snax · Tar Sands Joined: Jan 20, 2007 Posts: 75

I get your point about seperation of systems, but I think costs would about double for a stand-alone heat pump water heater. Adding a desuperheater supposedly reduces the load however. They are designed to pick up waste heat from the cooling cycle, dropping the load for AC.

Aimrehtopyh

Wow, half price. If this is a product that you can get from the company that built the heat pump for your house, you're set. Mainly because you get a warranty and decent prospects for competent service.

http://acforsale.com/online/product_info.php?products_id=1158

"You call that a desuperheater? This

snax · Tar Sands Joined: Jan 20, 2007 Posts: 75

Competent service is the one thing I'm a bit concerned about. If you have ever frequented the HVAC forum, you'll note that they are a bit protectionistic with respect to telling the layman anything. It gives me the impression that anybody I contact in the industry is going to try to upsell me on a whole system for umpteen thousand dollars of installation cost alone.

Perhaps that's an unfair characterization however. I haven't actually called anybody yet. Wink

Retsel · Tar Sands Joined: Oct 27, 2006 Posts: 24

We just had a ground sourced heat pump installed and it included a desuperheater to heat the water. Adding the desuperheater option cost us an additional $1600. This included adding two hot water tanks, one is a 30 gallon and the other a 40 gallon and a hot water pump and heat exchanger plus the copper piping to hook it all up.

The system works by using the hot refrigerant to heat the water in the 30 gallon hot water tank. The hot refrigerant is moved through a heat exchanger to heat the water from the first tank. While the refrigerant is pumped through this heat exhanger by the compressor, a pump is needed to pump the water from the first hot water tank through this heat exhanger. This heated water is a reservior tank for the second 40 gallon tank which has an electric heating element which ensures that the water is at the desired temperature (the desuperheated water may not be high enough in temperature).

To retrofit your unit, you would need to contact the manufacturer of your heat pump to see if this retrofit could occur. Some of the heat pump manufacturers design their units to heat the hot water first. Thus, it turns on whenever hot water is needed so that a back up heater is not needed.

Keep in mind that while waste heat is used during the summer, during the winter (I live in the north) heating the hot water is competing with heating the house.

Retsel

snax · Tar Sands Joined: Jan 20, 2007 Posts: 75

Given the age of our system at 12 years, I doubt the retrofit option is viable.

I have also decided that the benefit of such a system is really going to be limited to cooling days anyway, which around here is a relatively small chunk of days, making the payback period ridiculously long.

I would not rule out desuperheating if we were replacing the entire system anyway, but the efficiency payback of doing that simply does not compute until we have a major failure of the current system.

As such, we are looking for lower hanging fruit like on-demand gas water heat and installing a heat recovery ventilator instead of the current direct air exchange system we have now.