Hi,
I'm thinking of installing a tank to pre-warm the groundwater before it enters the hot water tank.  Does anyone know if the Temper Tank actually does what it claims?

http://www.integritywatersolutions.com/

Have to wonder how much more efficient it would be than using an old hot water tank stripped of it's insulation.

There's very little magic to tempering tanks- they basically work, and yes, an old hot tank stripped of insulation works about as well. Their "tempering vein" is a simple enough heat exchanger to speed up the process a bit, but it' wouldn't be plumbing rocket science to plumb something similar between the tank drain and a T on the output port of an old tank. An old gas-fired tank with the burner removed will do something similar for you as long as it's flue/heat exchanger is unobstructed both top & bottom.

In heating dominated climates they work at the same efficiency as your heating system during the heating season, since they're taking on heat from the surrounding space. The benefit is greater in cooling dominated climates. In humid climates there will be periods of significant condensation- installing it with a drip-tolerant floor (or drip pan) might be a good idea in hot-humid climate zones.

Be careful about installing it where it might get as warm as 85F or more, since that's about where microbe-growth starts to take off big-time. (google "legionella temperature") 70-75F is fine though.

How about using a wall mounted 200 lf coil of 2" pipe, mounted in the mechanical/laundry room (usually the warmest room in the house). Copper would be ideal but PEX much more affordable. Much more surface area than a uninsulated tank per gallon. Understand that it is just using heat from the house HVAC, but in a super insulated, passive solar home the house heating system is rarely running and even then the DHW preheat is stealing BTU's from the mechanical room, not a conditioned, thermostat controlled space. Bumping incoming water from 45deg to 70deg is one third the heat load of DHW. Perfect for a tankless steup.

A similar length of 3/4" PEX embedded in a passive solar floor is probably even more ideal. Using the thermal mass of the floor rather than the mass of a tank of water as the thermal buffer/storage, you can let is stagnate at any temp you like, since 200' of 3/4" PEX contains only ~5 gallons of water, but has enough contact area with the concrete to do a fairly good heat transfer of the stored energy. (Radiant heating/cooling in slabs works!) With only 5 gallons of water in the tempering system it's guaranteed to purge itself every day, several TIMES a day, dramatically reducing any health risks should it ever warm into the legionella-zone. It should work about as-well or better in a masonry or concrete wall.

Heat exchangers fabricated from tubing tends to be more effective using the smallest practical diameters, since the ratio of surface area to water-volume is better. I've heard of people preheating the DHW feed from radiant heated slabs with as little as 50' of half-inch (but go 3/4" for longer loops- it gives you better flow.)

In any pre-heat or tempering system, plumbing the tempered water to both the bathing cold feed as well as to the HW heater reduces the amount of fully heated water required to mix to showering temps. This is good for a ~20% performance improvement when using drainwater heat exchangers- it has to be good for way more than 10%, maybe even 15% in tempered-water systems.

This may/may-not be ideal for a tankless, depending on the tankless: With warmer incoming water, at low flow propane & gas fired tankless units have a difficult time modulating LOW enough to regulate the temperature well (or even keep from going over temp and shutting down.) A gas/propane tankless needs to modulate below 15KBTU/hr in to handle low flow really well in warm water areas. Some (eg. the lower end Bosch units) don't go under 30KBTU/hr, and have notorious flame out issue mid-shower when low-flow shower heads are used. But with an electric tankless this shouldn't be an issue.

I've wondered about this issue when people use solar preheat with a tankless booster. So either use electric tankless or a better quality gas? Other question about tankless booster with solar preheat: Wouldn't the small amount of warm/cool water in the pipe between tank and tankless cause the gas tankless to fire even if not needed?

I like the coiled pipe idea. I happen to have a 250 ft. loop under my wood floor that I'm no longer using. I can just hook that up to reroute the water before it reaches the tank.

I would like to verify the temp. difference a buffer makes. Does anyone know of a line thermometer that works well with pex? As in connects without installing a metal T.