Hello.

What a great forum.

I have about 3 acres of land behind my home and wanted to know if it is possible to lay a ground loop either via trencher or if I had an area cleared to what the required depth of burial would be? 

I am in the Mid Atlantic region near DC and the frost depth is supposedly about 18 to 24" around these parts. 

Home is a 2 zone split system with an electric heat pump/AC upstairs and a condensing high efficiency propane furnace/AC in the basement that feeds the basement and main floor. 

Basement and main floor is about 4,000 square feet with the upstairs being about 1,700. 

I didn't want to incur the well drilling expense if I can do ground loop (although well might be cheaper), but I wanted to see what my options were.

Thanks.

As a general rule for horizontal loops, 6 to 8 feet deep is recommended. Not many chain type trenchers will reach that deep. With a wider trench (24" backhoe bucket or 36" or wider with a trackhoe) you can lay more than one pipe in the bottom of the trench, i.e. with a 24" wide trench you can lay the loop going out on one side of the trench and return the loop on the opposite side of the trench. In general you would need 600' to 800' of pipe per ton of HVAC which would equate to 300' to 400' of 2' wide trench. With 36" and wider trench you do a slinky and put more loops of pipe in the trench and that requires less length of trench. But note that the limiting factor for heat transfer is not normally the amount of pipe, but the thermal conductivity of the earth, so less trench with more pipe may not perform as well as more trench with less pipe. It is an installed cost and space required vs. benefit gained that you have to weigh.

If you have the space and good soil (not rocky), then a horizontal loop is generally less expensive to install than a vertical bore loop.

The rocky soil part is probably an issue around these parts. I have a couple of friends of friends that have heavy equipment dozers and excavators.

I guess I will have to see where the property lines are and where the set back would accommodate that. Sounds like a vertical boring is going to be easier the more I talk about this.

I am in Southern MD, Calvert County. My loops are in 6 trenches, 24" wide, 6' deep, 200' out and back.
This is to support a 4 ton heat load, so right at 600' per ton in generally moist soil.
Performs great 3 years in.
If your soil is not rocky, it is the way to go.

Thank you.

How much tonage would you estimate that my primary system (based on above - roughly 4,000 sq/ft of insulated home, 2x6 walls, insulated glass windows, getting ready for a full foam exterior remodel) would need?

Obviously I want to make the shell as efficient as possible to be able to drop the size on the HVAC to as small as possible. I am going to have a wood burning stove installed (more ambiance for the wife than anything) but that will be providing some heat obviously.

Are any folks integrating Solar water heating (i.e. via evacuated tube) with their geo systems?

I'd take a look at adding an air source heat pump to the furnace, eliminating most of the use of propane. Geo is efficient but considering the initial expense and DC weather, another air source heat pump might be more cost effective.

I will check with my friend to see what equipment rental and trenching would be like.

Based on the square footage, safe to say about 6 tons if I was very efficient? The trenching seems pretty darn hard given the depths needed. Would have been much, much easier if done when the site was being prepped I suppose.

So with 6 tons, I am looking at about 3600 linear feet of loops? Ouch.

So...how about solar and going with a heat pump? Are there systems that combine radiant water heating with either floor heating or heat pumps?

If you want to size a heating/cooling system, you need either a Manual J or analysis of your current usage. Solar PV is worth considering if you have reasonable net metering rules in your area.

While I am highly opposed to rule of thumb sizing for AC, below is a blurb I found elsewhere. It sounds like your home will be better insulated than most and exceed code. If that is truly the case, investing in a detailed load calc will help reduce upfront costs on your HVAC system. I'd suggest someone who is in the energy audit business and understands an efficient stackup. A starting point for an efficient home might be 4 tons, but maybe as low as 3 if you are all in on windows, foam and air sealing.

At least two well-known energy consultants, Michael Blasnik and Allison Bailes, have proposed a new rule of thumb for sizing air conditioners in homes with insulation that meets minimum code requirements: namely, one ton of cooling per 1,000 square feet. According to Blasnik, “Sizing an air conditioner using tons per square foot actually works pretty well, as long as you choose the right rule of thumb.”

Read more: http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-cooling-loads#ixzz40ztL1TeI
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I will check on the metering in this area.

Sound like going with a super high efficiency envelope, air sealing, and heat pump might be the way to go if I can sprinkle in some solar.

2' wide trench that is 6' deep who the hell goes in the trench and if nobody is in the trench how are you stapling to the bottom.
Im sorry but this IS a dangerous possible life ending trench to be in. If you have ever been in a cave in you know how much dirt weighs and when you bend over to staple down pipe in a 6' deep trench you can die if there is a cave in.
We only do 5-6' wide trenchs that are a max of 5' deep. We always keep a minimum of two people in the trench with each in a line of sight to each other.
If you plan on dropping pipe in from the top you still have the possibility of the wall caving in with you standing/laying on it and still have a possible lethal combination
Pipe layer beware.

Who did say stapling down pipe in a 6' trench? It is a safety requirement to have trenches banked at 4' if anyone goes in the trench.
If you stay on top and the wall caves in,, at least you are unlikely to get buried.

The the OP:

I guess I don't understand why you don't put in a single heat pump to serve both upstairs and downstairs? Instead of geo, propane, and air sourced. That way geo becomes much more economical.
And the price is not significantly more than a smaller heat pump just for upstairs.

You are probably looking at 4 +/- 1 ton, but get a load analysis done. If in doubt, get a larger size, but try to make it variable speed.

Thank you for the updates posters.

I was semi-scraping the idea given my unfamiliarity with the volume and mass of piping that would have to go in. I was not aware of what it was given the tonage, but I am still open to the idea.

As far as going with a single system, the home is zoned currently and while I probably could create some sort of mechanical chase (scratching head and looking at my floor plan now), it would take some doing to hook up to the ducting drops that run through the attic.

Are there any hybrid systems that use solar hot water systems for heating? I have a huge roof surface that is South Facing so plenty of square footage in that regard.

Yes, there are hybrid heat pump/solar hot water systems. They tend to be a little more experimental than GSHPs. One approach is to have a water-source heat pump, a solar collector, and a tank buried in the ground. You can use various combinations of running the heat pump off the water in the tank, using the solar to heat the tank, or using the solar hot water for heating more directly. But by the time you get that all configured, it can also be quite expensive, unless it's all DIY.

So it generally tend to work out that the most cost effective solar/heat pump system these days is a well insulated house with PV on the roof, grid connected, and then a very efficient air-source heat pump, usually mini-splits, but there are starting to be larger systems with variable speed compressors and ECM fans that are as efficient as mini-splits.

Thanks chrs.

I have a pretty large basement and could drop a super insulated tank in there. What sort of required volume would I be looking at could it therefore be used in conjunction with the water heating efforts?

Do you design these systems?