. Instant water heaters do NOT reduce the amount of energy needed to heat water. They reduce the STORAGE losses to pretty much zero. Know also that they do have some quirks; they do not work below a certain flow level; a trickle will be cold forever. They also have max flow rates ( at a given temp change ) above which they will produce warm instead of hot water . . . if you fill a tub; there is NO flow restrictor . . . if you have several sinks in use, wash is running, dishwasher etc . . . the instant heaters may or may not be able to keep up with the TOTAL flow at an acceptable temperature. Also know that they do use GREAT amounts of power ( electric OR gas ) when they are running as they are basically heating the water instantaneously . . . this can mean some pretty hefty wiring or perhaps a larger gas line to support the high demand when they are running. I've also heard that some are quite prone to mineral build-up over time; which eventually makes them perform poorly. Not sure how easily "serviceable" they are as compared to conventional water heaters if you need a part. As far as environmental impact; geothermal will be your best bet . . . it's using the earth's heat as the source of heat for the water . . and some electricity to achieve that. Gas or electric heating means there's fossil fuels getting burned somewhere; either directly in your place, or somewhere else to create electricity. Oh yeah . . nukes . . . let's not bring that one up. Unless you're somewhere that has ( and will have ) cheap electricity; a strict electric tank will be more costly to operate. The idea of getting much of your heat geothermally, and supplanting it with electric sounds like the best way to go to me. You can either buy a very well insulated tank; or a pretty good one and add a blanket to it for additional insulation. Believe Marathon makes some pretty well insulated, well made electric water heaters. There's my two cents . . . . . Bob

Re: mathteacherva, First of all, HAPPY BIRTHDAY!! If you air-conditioned a lot during the summer, geothermal might make sense. On the other hand, if you throw the windows open most of the time you will not benefit from Âfree hot water. As for the next familyÂwho knows? Geothermal certainly would add value to your home, and could provide free hot water if you air-condition during the summer. It would provide hot water at a reduced price in the winter and you could also have radiant in-floor heating at a reduced cost. I guess youÂll have to crunch the numbers. It might be a Âproject of conscience though. God bless those that go ahead - even when the numbers donÂt add up. You certainly have my respect. BTW: I prefer evacuated tubes (but thatÂs not my field of expertise). SR Here is a link that might be useful: how-efficient-is-it-magazine.com

First, what do you mean by "verified to be right-sized for my house?" If that means the unit sizes were selected based on a Manual J or similar calculation to calculate the expected maximum heating/cooling loads, then there may well be enough extra capacity in the existing system. If you mean that the system has been found to be running close to flat out to keep up with the heating or cooling demand at the design minimum outside temperature, then there is no extra capacity in what you have now. Since you have two units, then even if the size of each was based on good calculations, those sizes were rounded up to whole tons for selecting the units. If each involved just a half ton round-up, you already have the extra ton you are assuming you need. Adding 600 sqft of living area doesn't necessarily mean the extra load will be 18% of your present capacity. You need to calculate the difference based on the shell of the addition. Presumably you will have some added roof/attic area over the new space, but that could (probably ought to) be better insulated than over the existing house, so that the addition of that part of the new heat load may be minimal. Similarly, the addition will add some new exterior wall area, hopefully better insulated and with better windows than the existing structure has, and you need to subtract from the current maximum load for any existing walls that become interior walls. You need good numbers, not guesswork and rules of thumb. You may be able to calculate a decent approximation of the aggregate U*area of the existing house, then multiply by design maximum temperature difference to see how the result matches up with the current capacity, then correct for the proposed changes to get a properly calculated load for the new configuration. For this you need to have good information on power cost for just the heating system, from the electric bills over the heating system without the base load for hot water, appliances, lighting, etc (from the shoulder season bills). You also need a good estimate of overall COP (coefficient of performance) of the equipment, which ought to be available from the specs provided by the vendor at the measured EWT (entering water temperature) and air flow, with an allowance for pumping power. Finally, for the season for which you have the power cost you need the heating season total heating degree days for your location (assuming you are in a heating climate, zone 5 or higher). The calculations aren't rocket science, but if it is outside your comfort zone then you ought to have someone knowledgable do it for you.

The answer to your question all depends on the comparative energy rates in your area. Your contractor may best answer this. Some things to keep in mind are that natural gas could be a very good choice for backup, particularly if you plan to install gas for other household applications (stove, dryer, fireplace, BBQ). Keep in mind that ground source heat pumps have no defrost cycle and the gas backup will only activate to assist the GSHP - not instead of the GSHP. So in all likelihood very little gas will be used, comparatively speaking. The geothermal heat pump can be used in conjunction with a gas instantaneous water heater such that the hot water generated by the desuperheater is stored in a buffer tank, which then feeds the gas instantaneous water heater. This way the gas instantaneous water heater only has to make up the difference in temperature between its set point and the output of the buffer tank (about 110ðF), again resulting in reduced usage of gas. If I were choosing this type of configuration, I would install a geothermal split heat pump coupled to a Carrier Infinity gas furnace, appropriately sized. You would of course require the Carrier NIM module to interface the geo unit to the Carrier Infinity system. SR