Ground source wells being drilled and installed this week!

April 24th, 2010 - Posted By: Brad Liljequist

We started drilling the ground source heat pump wells last week. We’ve had a couple bumps which I thought I’d share about.

But first maybe I should give a little explanation of what I’m actually talking about.

Ground source heat pumps in homes are used for heating both hot water and the home interior. Generally, they are a water based system – they heat water, rather than air, like your typical forced air furnace.

Ground source heat pumps essentially combine two very efficient technologies to result in an extremely efficient technology. They include two parts: a ground well system for preheating the fluid to the average ground temperature (in Issaquah, 50 degrees or so). Once the fluid has run through the ground well field and been prewarmed, it is then run through the heat pump, which then heats it further up to 125 degrees or thereabouts.

The radical thing about using the preheating loop is that effectively, the heat pump only has to heat from the 50 degree baseline, year round. That means that in the dead of winter, when it’s 30 degrees outside, the heating system is effectively only having to heat from 50 degrees, since it’s using the well field water. The heat pump has its own cool efficiency which I’ll explain another time.

So, back to our well field. To create our well field, our mechanical engineer (Stantec in Seattle) determined the appropriate sizing based on the total heating load for our ten units. They determined we needed 3,000 feet of vertical wells. Our initial design configured that 3,000 feet into ten 300 foot deep wells connected by a set of pipes (called a manifold) just under the surface.

Into each well, a very heavy duty double U shaped one inch diameter pipe is run down the bore hole. The pipe is made out of polyethylene, and is the same pipe used to carry natural gas to homes. The pipe is then grouted in place with a material called bentonite, which is a sort of flexible inert concrete. The grouting ensures that the pipe and the fluid inside the pipe is in full contact with the surrounding ground, to maximize the thermal conductivity.

The problem we’ve run into is that we’ve had some issues boring holes that deep. Portions of Issaquah Highlands are an old lateral moraine from the big Puget Sound glaciations of the last Ice Age. There are lenses of gravel which tend to collapse when we’ve drilled through them. In particular, there is a lens at around 200 feet.  As a result, we’ve had to modify our well field, and increase the number of wells to fifteen or thereabouts. It’s added cost and taken more time.

I was in the field yesterday watching them place one of the pipes and grout it in place. It is tough work for the drillers – sledgehammers, rubber boots, hard hats, a big drill rig and a lot of sweat. It was a very good reminder to me that this is where the rubber hits the road. After years of planning, evaluating the options, and weighing the pro’s and con’s, here was a crew of three guys working hard all day, drilling a hole, muscling a tube down the hole, mixing a big batch of grout, pumping it down the hole, and moving on to the next one. When zHome is done it will be a mix of brains and guts and stamina.

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