KNOW-HOW: GEOTHER­MAL SYS­TEMS UP­DATE

En­ergy from the ground: it’s catch­ing on with old-house own­ers on smaller lots.

Old House Journal - - Ohj - By Mary Ellen Pol­son

Per­haps your trea­sured hot-wa­ter or steam sys­tem suf­fered a cat­a­strophic freeze, or the forced-air HVAC sys­tem is reach­ing the end of its use­ful life. Or maybe heat­ing and cool­ing bills in that ram­bling old pile you call home are cut­ting into re­tire­ment sav­ings. If any of these cir­cum­stances sound fa­mil­iar, your home may be a can­di­date for a geother­mal HVAC sys­tem—even if the house is on a com­pact lot in a his­toric street­car sub­urb.

Geother­mal sys­tems make use of the free, re­new­able en­ergy stored in the earth. A geother­mal heat pump uses the con­stant tem­per­a­ture un­der­ground as the ex­change medium for heat­ing and cool­ing, in­stead of out­side air. Just a few feet be­low the sur­face, ground tem­per­a­tures are sig­nif­i­cantly warmer than the air above dur­ing win­ter, and much cooler in sum­mer, usu­ally av­er­ag­ing around 50° F year round. For this rea­son, a geother­mal heat pump uses less en­ergy to warm in­door air when it’s cold, and to cool the air when it’s hot out­side—all with­out di­rectly burn­ing any fos­sil fuel.

While the up­front costs of geother­mal are about twice that of con­ven­tional sys­tems, the en­ergy sav­ings are so sig­nif­i­cant that most home­own­ers will re­coup their costs in five to 10 years— sooner, fac­tor­ing in the state and fed­eral tax in­cen­tives now avail­able. (Sev­eral of the larger geother­mal com­pa­nies also of­fer fi­nanc­ing on fa­vor­able terms.) The whis­per-quiet sys­tems make a lot of sense for older houses with gen­er­ous floor plans and min­i­mal in­su­la­tion, too.

The key com­po­nent of any geother­mal sys­tem is the ground loop sys­tem. Com­posed of high-den­sity poly­eth­yl­ene pipes that con­tain a wa­ter–ethanol mix to pre­vent freez­ing, the loop sys­tem is buried be­low the freeze line. Dur­ing cold weather, the tem­per­a­ture-sen­si­tive fluid cir­cu­lates and picks up heat from the steady tem­per­a­tures un­der­ground and de­liv­ers it to the house. The heat pump con­cen­trates the ther­mal en­ergy and trans­fers it to con­ven­tional duct­work or an un­der-floor ra­di­ant sys­tem, which cir­cu­lates the heat. In warm months, the process is re­versed and the cooler fluid ex­hausts heat out of the house. As a bonus, the unit can as­sist with hot-wa­ter heat­ing.

There are sev­eral types of ground loop sys­tems, and all re­quire quite a bit of earth mov­ing and lawn re­pair. The most com­mon and cost-ef­fec­tive is the hor­i­zon­tal loop sys­tem. Pip­ing is laid in trenches from 100' to 400' in length. In­stalling the trenches re­quires enough land, of course—at least .25 of an acre, and of­ten up to .75 acre. In a vari­a­tion called the “Slinky,” loop­ing the pipe in over­lap­ping cir­cles al­lows the use of more pipe in a shorter trench, cut­ting down on in­stal­la­tion costs and per­mit­ting hor­i­zon­tal in­stal­la­tion on smaller sites.

Ver­ti­cal loop sys­tems are used in lo­ca­tions where there isn’t enough land to lay a hor­i­zon­tal sys­tem, like small lots in his­toric neigh­bor­hoods. In a ver­ti­cal sys­tem, holes are bored with a drilling rig, and a pair of pipes with spe­cial U-bend fit­tings in­serted into the holes. A typ­i­cal home re­quires three to five bores with about a 15' sep­a­ra­tion be­tween the holes.

If you’re lucky enough to have a pond a half-acre or larger on your prop­erty, your home may be a can­di­date for a pond loop

sys­tem. A se­ries of coiled, closed loops are sunk to the bot­tom of

the wa­ter body. The loop is con­nected to the house via more pipe laid in a hor­i­zon­tal trench. Wa­ter-to-wa­ter heat trans­fer is much more ef­fi­cient than wa­ter-to-air heat trans­fer, so this type of geother­mal sys­tem is highly eco­nom­i­cal and ef­fi­cient.

Which­ever loop type is re­quired, bear in mind that the life of a typ­i­cal geother­mal heat pump is 15 to 20 years; loop sys­tems can last 20 years or more. Forced-air sys­tems will re­quire a wa­ter-to-air heat pump, while hy­dronic ra­di­ant heat­ing sys­tems re­quire wa­ter-to-wa­ter heat pumps.

Although geother­mal heat pumps can be in­te­grated eas­ily with ex­ist­ing sys­tems such as forced-air or ra­di­ant floor heat­ing, you’ll need space in a util­ity room or base­ment for all nec­es­sary com­po­nents. Where there is no duct­work in place, a delivery sys­tem will have to be de­signed and in­stalled, prefer­ably one that dis­turbs as few orig­i­nal walls and ceil­ings as pos­si­ble. For a two-storey house, for ex­am­ple, it may make sense to in­stall a split sys­tem, plac­ing duct­work and an air han­dler for the first floor in the base­ment, with a sim­i­lar set-up in the at­tic to serve the up­per floor.

De­sign­ing and in­stalling a geother­mal HVAC sys­tem is a com­plex process. It goes with­out say­ing that this is not a do-it-your­self project. Look for a con­trac­tor/en­gi­neer­ing firm with sig­nif­i­cant ex­pe­ri­ence in geother­mal in­stal­la­tions, prefer­ably one cer­ti­fied by the In­ter­na­tional Ground Source Heat Pump As­so­ci­a­tion (igshpa.org).

As part of a ver­ti­cal-loop geother­mal in­stal­la­tion, a drilling rig pre­pares to bore deep holes in the front yard of a Queen Anne house in a his­toric Syra­cuse neigh­bor­hood.

LEFT Ex­pect to do a lit­tle land­scap­ing af­ter the sys­tem is in, as shown in this his­toric geother­mal in­stall in the Hud­son Val­ley. ABOVE Curl­ing the poly­eth­yl­ene pipes in over­lap­ping loops gives this vari­a­tion on the hor­i­zon­tal loop method the nick­name “Slinky.”

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