I've just spend the last 2 hours with our backhoe operator digging test pits across the back yard looking to see what the depth is to bedrock.

Just to give everyone the sense of the site I'm hoping to install in...

We live a the base of a mountain in western Massachusetts and based on the test pits the water table is about 5 feet down in most places.  This would make sense given that just to the south of my property there's an intermittent stream coming off the hill, and even my basement has an intermittent streaming flowing through it!   I'm looking to swing a series of 4 800' 3/4" pipe slinky coils across the backyard and wrapping around the north side of the house (I'm installing a 4 ton Tranquility 27). The test pits we just dug show that bedrock for most of the southern end of the property is about 5.5 to 6 feet down. As you move north it becomes progressively deeper, but this part of the site is complicated by the fact that there are 100' white pines covering this part of the property.

So, the question is, in Massachusetts does it even make sense to install the pipes in the region with the 5.5 to 6 deep bedrock?  From what I've been told 6-7 feet is ideal in my region.  Do I get any benefit from the fact that the soil at that depth is saturated for most of the year? Or does it make sense to bite the bullet and start cutting down trees to the north to take advantage of the ability to dig deeper. I'm not a tree hugger (not that there is anything wrong with that!), it's just the fact I don't want to spend the extra $1000's to take down some very beautiful 100 year old trees.

  Suggestions appreciated.

Thanks

David

Hi Dave,

Your frost depth in your area is about 4' correct?

I do not see you having any problems. It sounds like you have an excellent situation: Trench depth, high soil moisture, bedrock thermal mass.

Most trenches are dug 5' to 7' deep. Plus with the high moisture content of the soil, you should have excellent thermal conductivity. You may want to dig your trenches down to the bedrock, bed the trench with a 3" layer of soil and lay the horizontal slinky on this bed. You can also added more soil on top of the loop field surface to increase its depth. How long are your trenches going to be?

My situation was after digging the first trench, the depth at the end was only 2' down to bedrock. Had to relocated the loop field which had some of the pipe laying on bedrock at 4' depth. Measuring each loop temperature, I can not see any major difference to the one trench at 7' depth.

You should have a glycol or methanol antifreeze solution in the loop to prevent freezing.

Bob

I agree. No sense cutting down the trees. If you can add extra dirt on top of the slinkies, it will improve your loop performance.

Frost line is the key question. In MI, 5 to 6 feet is fine. Did wonder about trench length, the first thing that jumped out at me in your question was size of your slinkies. We can go as short as 110'/ton here with a 20 degree minimum design temp for a 4' of 3/4"pipe /foot of trench. Soil obviously matters, but you are approaching twice my minimum design length. perhaps depth gets easier if runs are shorter. Check the design.
Joe