Geothermal Retrofit

Q: Is it feasible to do a geothermal retrofit?

A: Sometimes it is…sometimes not.

If you’re gutting the house, a geo retrofit is as simple as a new installation. If you’re not, read on…

The primary goal is to reduce the load. This means upgrading windows and insulation first. These costs will translate into a significantly smaller geo system which will then pay for them and save more energy than geo alone. An unimproved house with a geo system will cause the geo system to be overloaded. Remember, the primary goal is to reduce the load.

The subject should then be broken down into 2 parts.
1) Do you have forced air heating with ductwork?
2) Do you have hot water heating with baseboards, radiators or radiant floors? This is called hydronic heating.

If you have forced air heating and it currently uses electricity or gas or oil, then generally the ductwork is too small and needs to be upgraded. The reason being is that the “dead dinosaur” burning furnace would deliver heat at 140 deg F and the new heat pump system would deliver only 100 deg F. (all approx values) This requires bigger ductwork to deliver the same amount of total heat. Simple, right?

An exception might be a really old house that had a big heating system designed for single pane glass and no insulation. If the glass and insulation were upgraded then the originally large ductwork would now be adequately sized. However this house would likely have been renovated at some point and the heating system could be inadequate or dysfunctional. If the basement is unfinished then ductwork upgrades are simpler than if the basement is fully finished, blocking access to the ductwork.

If you are replacing a tired air-source heat pump then the ductwork will likely be adequate.

If you have hydronic heating and it currently uses baseboard radiators that have copper tubes and aluminum fins (they’re not radiators at all, they’re convectors) then you’re out of luck unless you replace them and probably the distribution piping too.

If you have baseboard radiators that have cast-iron fins (rare) you may be in luck except you’d probably have to add more of them to make up for the previously mentioned lower output temperatures. If you have cast iron radiators (really old school) they may work but you may need to add more of them. They’ll need to be flushed and maybe some piping upgraded too.

If you have radiant floors, they are the easiest to convert, except some older floors (and still today) were/are badly designed and installed. They required high operating temps and were not that comfortable to begin with. If the radiant floors have enough tubing and loops installed the conversion should be relatively simple.

The 2 best parts of hydronic heating conversions is that you can rip out the mixing valve that used to mix down the high “dead dinosaur” burning boiler temps to what the radiant floor requires. You can also rip out the evil chimney and the CO detector. This was so inefficient due to the high temp standby losses in the mechanical room. Geo systems operate at low temps and have little or no standby losses.

The other best thing with hydronic heating conversions is that you can install outdoor reset control. http://www.tekmarcontrols.com/literature/acrobat/p022.pdf This is a further energy saving control that has been around for more than 40 years but only works on hydronic heating. At exchangenergy we install it on every hydronic system we design.

Summing up, geo retrofits can be feasible and even have a shorter payback than a new installation if you are replacing oil or propane as the fuel source. If your home meets the above criteria then a site visit is the next step.

Helpful Tip: Beware the Oversized Kitchen Exhaust Fan

Homeowners often select oversized kitchen exhaust fans not realizing that they require makeup air - adding considerably to the cost of their house project.

A sales representative may not even mention the “make-up air” issue at all and the the homeowner can subsequently experience some serious sticker-shock at the cost of make-up air.

For example a 600cfm fan requires 10kW of heat to temper the air.

This amount of makeup air represents the capacity of an entire electric furnace in a small home.

Slinky Loops and the Laws of Physics

Q: I have limited yard space for a horizontal loop. I’ve seen “slinky” loops in photos. How do they work?

A: The premise that slinky loops will let you get more heat capacity from a smaller loop footprint is inherently flawed. When applying the laws of thermodynamics to ground loop design, it’s clear that there is a limited amount of energy available from a given amount of earth, and a limited amount of energy that the earth can absorb. Cramming in more pipe by using a slinky pit will have a number of ramifications:

1. In heating mode, the loops compete with each other for the same thermal energy, interfering with each others ability to do the intended work. This results in very low temperatures in the loops in the centre of the pit, and lower overall loop temperatures. This results in what we call the toilet effect. The loop temperatures start out OK, but spiral downwards until they are “flushed” and can’t recover. We’ve seen these loops running at 20F, abysmal performance. The earth is frozen and the moisture is crystallized, air pockets appear and the thermal conductivity nose dives. The system is forced to run on back-up heat, your operating costs go through the roof, and the high efficiency system you invested in becomes a black mark on our industry.
2. In cooling mode, the loop can’t dissipate the heat, so the loop heats up far above design temperatures. The slinky loop has hundreds of potential pinch points. When the pipe is softened by the high temps, and with 5 or 6 feet of earth on top of it, it tends to flatten at each pinch point, causing flow restrictions that decrease system performance even further. We use slinky loops in surface water loops (pond, lake, ocean), and in wet, swampy earth with high ground water flow. We don’t recommend this layout unless the conditions are ideally suited.

Q: So if I don’t use a slinky loop, what do I do now?

A: The tried and true horizontal loop system uses single straight pipe in trenches and requires way more space. Other established horizontal designs can use two, four, or six pipes in a trench. Each time you add a pair of pipes, your trench length per nominal ton of capacity goes down, but your pipe length and antifreeze volume goes up.
Another method is the “parking lot” loop – a pit loop using straight runs of pipe on 2’ centres. This will require a space typically more than twice the square footage of the building depending on the load and the ground conditions. If the loop field is large, temperatures in the centre may be low and impact the overall average temperature. This will be considered when GeoExchange experts are evaluating design options.
The ideal design for your property depends on the space available, the ground conditions, and the cost of the system making the system site specific – one of the things that makes GeoExchange so much fun and the reason we don’t get bored doing this over and over again!
If your lot doesn’t allow a properly designed horizontal loop, vertical boreholes are the order of the day: more upfront cost but well worth the investment.

Do-It-Yourself DIY Geothermal Projects

Q: Can I install a geothermal system myself? It doesn’t seem that complicated.

A: While it might not “seem” that complicated, Geoexchange / geothermal systems require multi-disciplinary skill sets and the most important part of the system is the design of it. A heat loss analysis of the building should be done. The thermal conductivity of the ground must be calculated or estimated from established data. The ground loop must be sized to meet the building heat load given the type of earth at the site. The flow rate pumped through the ground loop piping must be calculated to ensure effective heat transfer. The piping must be sized to minimize head pressure, pump size and pumping costs. The heat transfer fluid needs to be selected and quantity needs to be calculated. The ground loop must be flushed to ensure that no airlocks occur that can render sections of the loop ineffective, and to remove debris that could foul heat exchangers. The distribution system in the house must be designed to operate at system temperatures and take to maximize the potential efficiencies. Knowledge of control strategies and installation will also help maximize efficiency. Fusion welding of the piping requires experience and a ticket.
Manufacturers will not honour warranties if the system is not installed by a trained and qualified contractor. The system may require service or maintenance, who will perform that? Since the most important part of the system is buried underground, making repairs is near impossible.
If you’re a determined Do It Yourselfer go ahead and do other parts of homebuilding yourself. If you put in your own kitchen cabinets and don’t like them later they are easy to replace. You can even do your own framing since it will be inspected by people who know framing and corrections can be made in time.
For geo systems (and radiant floors too) please save yourself the grief and find a qualified designer and installer, check their references carefully and enjoy doing it yourself in a different part of your homebuilding.

Geothermal to Heat a Greenhouse

Q: I have a dry water well borehole that I’d like to use for geothermal / geoexchange to heat my greenhouse.

A: Your borehole can heat and cool a greenhouse of 1,000 to 2,000 sq ft depending on what temperatures you want to maintain. With the geothermal / geoexchange system you can keep the greenhouse mostly “closed”, meaning when it’s sunny, instead of opening the vents and dumping the precious heat, you will cool the plant zone instead and move the heat into the borehole. That night when you want to heat, the warmed borehole will return the day’s heat into the greenhouse. This is where you save lots of money because you’re using the day’s earlier solar gain instead of buying the heat energy from your favourite utility.
This system can be taken a giant step forward yet by changing the typical outdated heating methods to heating the plant zone instead of the whole space. You’ll need an air handler (a box with a fan and some coils in it) and some plastic ducting that will blow the conditioned air into the plant zone instead of the ceiling space. This will give you much improved plant yields and further energy savings. In essence you are abandoning the upper area of the greenhouse that is normally heated (uselessly) and you’re only heating/cooling/(de)humidifying the place that matters: the plant zone.

We can design and build the geothermal / geoexchange system for greenhouse heating in-house and we have partners who specialize in this new air handling technology.

Hoping this sheds some “warmth” on the subject!

DHW Domestic Hot Water with Geothermal

When adding domestic hot water to your geoexchange system, be careful. Most clients opt for the desuperheater method which works well enough with a forced air system but remember it only works when the heat pump is running.

It can be made better by adding a preheat tank that has no other heat going to it.

That way the desuperheater can usually be useful. What I mean is, what if the system is off for a few hours (mild weather) and someone has a bath? The hot water coming into to the DHW tank is cold and the heat pump won’t do a thing. In a few hours the DHW will be hot from the gas or electric in the tank.

The heat pump will start at some point that day and the desuperheater will shut off because the DHW is already hot. Great! If you have a preheat tank, then at least when the heat pump starts the desuperheater will have something to do.

The only good thing about desuperheater systems is that they are inexpensive compared to DHW on demand.

DHW Priority is more complex and expensive but gives you the entire heat pump capacity when you need it (instead of the 10-15% from desuperheater systems). The heat pump can either be ordered with it built-in, or it can be field-installed much like a typical “indirect fired” tank in a boiler system would be. The indirect tank becomes the preheat tank (up to max 120 degrees Fahrenheit) and a regular DHW tank is still needed to bring the dhw up to 140 degrees Fahrenheit.

The pitfall in an indirect tank system is that you must use the proper type of tank or use an external heat exchanger. The typical tank used for an indirect boiler application will lead to failure. These tanks use stainless steel coils inside. The coils are typically schedule 40 pipe. We must look up our thermal conductivity charts and see that stainless steel has abysmal heat transfer properties.

Because of the low temperatures that geoexchange systems supply the heat transfer will not be enough to get the heat out of the heat pump. Therefore we must use special tanks. The European tanks are good because they have figured this out long ago. They use exotic S.S. alloys and near paper-thin coils.

There are some North American tanks that may be good such as the ones with internal copper-finned coils or some models with tank in tank construction. However these manufacturers don’t care enough about our small segment of the tank industry and can’t be bothered to supply us with lower temperature specifications. We can’t afford to test them all ourselves.

An alternative is to use a brazed plate heat exchanger. This uses more piping, an extra pump as well as more labour.

The Controls: this can be as simple as an aquastat in the preheat tank that starts the heat pump and its circulators and drops out the heating system. If you have hydronic heating you should have some form of outdoor reset control which will likely have provisions for DHW priority.

If you don’t have outdoor reset on your hydronic heating system you should have it installed before the cost of heating goes any higher. But that’s a subject for another blog…

Geothermal with Well Water

HERE IS A RECENT Q & A WITH AN INQUISITIVE GEOEXCHANGE CLIENT REGARDING GEOTHERMAL USING WELL-WATER:

Q: What would be the approximate electric charge per month for the geothermal heating in a house that is 3600 square feet using wellwater?
A: It will cost you ¼ to 1/5 of regular electric heating because you have a radiant floor and relatively warm wellwater compared to a closed loop. This is the most ideal geo scenario for efficiency. We have a heat sheet which shows efficiencies of different systems. As regular fuel prices escalate you will be immune to that and even if electricity prices rise much, you’ll still be paying only ¼ or 1/5 as much as everyone else. Rising energy prices is where geoexchange stands out.

Q: Could the run-off be used to irrigate the vegetable gardens?
A: Yes, although you’ll get more runoff in the winter than in the summer. How much extra would it cost to set this system up? In one proposal we run the runoff pipe back to near the well which can be extended to the pond or allowed to seep back into the aquifer. You can extend it at low cost with a lower grade of pipe from what we use down the well. You can extend it later by yourself.

Q: What is the anti freeze solution that is used in the geo thermal process?
A: There is no antifreeze used in this scenario. Antifreeze is used in closed loop installations only.

Q: What kind of maintenance and upgrading could we expect to do over the next fifteen years on the system?
A: None, unless your water is of poor quality in which case the heat pump heat exchanger may have to be cleaned. Speaking of which, it is best to send us a water test report. Our main concern would be sulfur in the wellwater. I’ve had a well pump has run since 1990 without a problem. We can do an annual inspection as with a conventional heating system, but it would typically be to check on operating parameters and can safely be extended to every 2 years or more after the first year.

Q: How much wear and tear does the system cause the pump?
A: The pump in question is sized to run the irrigation system as well. Here, we spec’d 22gallons per minute but may be able to reduce that depending on the irrigation requirements. The heat pump would use 6 gallons per minute (gpm) only when it’s running.

Q: How much water per day does the process use?
A: In a brutal cold snap, here in Vancouver, it would run 6 x 60 minutes x 24 hrs = 8,640 gallons per day but in reality it will run 3hrs avg/day or 1080gallons. By comparison at my house I irrigate 8,000 gallons/day every day in the summer for 2 acres.

Q: I don’t quite understand the geothermal process. Could you give a simple explanation?
A: The heat pump absorbs low level heat from the groundwater which is then returned and the heat is upgraded to a higher temp via the same refrigerant phase change process as in your fridge.