charlz said:
Right my question was more about 'how fast'. I realize that if you let it sit the tank will eventually reach the same or close to temp of the boiler hot water loop. But how long does it take to bring up a cold tank? Can you turn your hot water heater off altogether or at least way down?
Well with a bit more information, you can calculate this pretty accurately, but making some assumptions, I can get a pretty good idea of performance. 1/2' copper pipe has a OD of .625", so 5' of it will have a surface area of 117.81 SQ/IN. A copper water to water heat exchanger has a heat transfer co-efficient of between 60 and 80. That is 60-80 Btu per SQ/FT per hour per degree F of temp difference. So with a heatex with .818 SQ/Ft of surface area, say enough 180F water to have very little drop on the primary side and a stone cold 50 gallon water tank(55F), the exchange would start out at around 8180 BTU/HR at the best case coefficient of 80(80 X .818 X 125F temp difference/per hour). 50 gallons is 400LB of water. 1 BTU will raise the temperature of 1 pound of water 1 degree F. So 8180 BTU in the first hour divided by the 400 pounds of water equals a temp rise of 20F or from 55F to 75F. This of course is not strictly accurate as you raise the water temp in the tank, you lower the temp difference and decrease the thermal transfer rate.
If you split the temp difference between 180F and a cold 55F tank(125F) and 180F and a hot 140F tank(40F), you get a mean average temp difference of 82.5F. At that figure, best case your transfer will be 80 X .818 X 82.5F delta or around 5400 BTU/HR. 400LB of water raised 85F(55f to 140F) requires 34,000 BTU. At a rate of 5400 BTU/HR average, it will take this heatex around 6.3 hours to get that HW tank from 55F to 140F best case, and assuming basically unlimited BTU input from the boiler(no temp drop across the primary side). I would probably rate this smooth single wall heatex on the low side toward 60 BTU/SQFT/F/HR(add 25% to the time to temp figure or around 8 HR) as it has no flow control to break up the laminar flow that will develop along the smooth pipe wall.
Someone said it earlier, these things usually have coils and coils. To transfer heat, you need surface area and temp difference. If you don't have much temp difference, you need more surface area to compensate and vis-versa. To transfer heat efficiently you need turbulence to break up the laminar flow along the transfer surface so the actual temp of the fluid along the transfer surface is always at it's maximum difference. With laminar flow, it actually insulates the transfer surface the same way a layer of stagnant water insulates a diver in a wet suit. This thing obviously transfers heat and can maintain the tank, but it will have one heck of a recovery time if any significant ammount of hot water is used from the tank.
If you want to optimize this system and get more use out of the boiler heat for domestic water, you need many times the surface area you have now and something to turbulate the water such as bare copper wire wrapped around the inner pipe and a strand or two set down the middle of the inner pipe to force the fluids to tumble. Or you could go to E-bay and search for "brazed flat plate heat exchanger" and probably get exactly what you need for around $100-200. These exchangers are small yet have a huge internal surface area. The plates have a herringbone pattern stamped into them that makes for very turbulent flow with low restriction. You will still be able to thermosiphon thru it, mine thermosiphons off my generator great. I transfer 16,000+ BTU/HR from my generator to domestic water with a 70F final temp difference with no problems. I am using a 10 plate 5" X 12" flat plate heatex that cost me $98.
Goto
FlatPlate Heat Exchangers and they have some descriptions of how they are put together and a free online calculator. Just type in your temps and flows and it will make size reccomendations for you, then look for that size on E-bay or thru your boiler supplier.