Thursday, March 19, 2009

Geothermal - Part 3 (Concentric Fitting for Domestic Hot Water)

Most Geothermal heat pumps available today have the option of providing free hot water in the summer time. Although it sounds too good to be true, it really does work. Here's how: A geothermal heat pump cools your house in the summer by removing "heat" from the inside air and transferring that heat to the ground outside. Instead of transferring all of this excess heat to the ground, the heat pump can transfer some of this waste heat (about 10%) to your hot water heater. The only extra energy required to get this free heat is the electricity required to run a small pump (50 watts) to circulate water from your heat pump to the hot water tank. In this picture of one of our heat pumps, you can see four insulated lines - two large ones are for the underground loop and two small ones are for the hot water tank.

The system works best if you are willing to install two hot water tanks - one heated exclusively by the heat pump, and one heated by conventional means (gas or electric) for periods of high demand when your heat pump does not provide enough free hot water. In actuality, most installations probably use only one hot water tank for both purposes, with admirable, but not optimal, results.

Like everything else in our house, our hot water supply is set up a little bit differently. We have something called an indirect hot water heater, connected to our wood boiler. In the winter, the wood boiler heats the domestic water in this stainless steel tank via an integral heat exchanger. In the summer, the water will be heated by the heat pump. Downstream of the indirect hot water heater will be an on-demand heater to provide hot water in the spring, fall, and during periods of high demand. Typical hot water tanks have three ports on them... "cold in" "hot out" and "drain". So typical geothermal heat pump installations take advantage of the drain port to inject the "free" hot water into the tank. Our indirect hot water tank has no such 3rd port, so our installation required something custom-built. It's called a concentric fitting. (aka radial fitting)

The purpose of the fitting is to allow water to flow "in to" and "out of" the same port of the hot water tank. In fact, this was the original installation method employed by Water Furnace several years ago, until they came up with a more reliable method of using the drain port. I searched the internet for concentric or radial fittings, but came up empty handed, so I set out to build my own. I thought I'd post the pictures of it here in case anyone else needs to build their own some day. It turned out to be relatively simple. The ports on our hot water tank are 1" ports instead of the typical 3/4" ports, so my fitting will hopefully be more reliable than the original water furnace fittings, which (I'm told) had a tendency to clog up with mineral deposits.

The pictures should be self explanatory to anyone with the modest plumbing skills required to replicate one of these fittings. If not, post a question here and I'll answer questions if I am able.