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Posted by HeatPro on August 12, 2005 at 10:38:12:
In Reply to: Plumbing Configuration for Tankless DHW Coil in Boiler posted by Jeff Gray on August 12, 2005 at 10:14:02:
The electric heater plumbed as you plan after the boiler coil is OK. It is a bit amusing though, because the coil itself is the restricting factor, so piping the coil to the electric heater gives the same amount of heated water with just the electric heater in the way. It is unusual to get a tankless coil boiler with less than a 1.15 gpm nozzle which would give you 2-1/2 gallons per minute (150 gallons per hour) of direct hot water through the coil. A 40-gallon electric heater can give you 25 gallons of hot water in an hour or 40 gallons if piped from the tankless coil. So all the extra electric tank is going to do is act as a storage mass in the way of the water flow. People shut off wter heaters because they are notroious for losing heat from the jacket, so you will be losing heat from the jacket ALL YEAR 24 HRS PER DAY from the 40 gallon electric heater jacket in order to prevent losing heat from the 5 gallons of water in the boiler in the same time, and 2/3 of the year the boiler is going to send the heat to the rooms instead of losing it BUT NOT the water heater.
If you place a bypass through the coil, you are going to heat the water in the electric heater all summer. This seems like a good idea; but do you know why people use boilers to heat hot water? - Because the oil is still almost half the price of electricity in urban areas per gallon, even at $2.20 per gallon. a gallon of oil at 80% efficiency makes 112,000 btuh or 150 gallons of water (1.5 cents per gallon. A water heater making 150 gallons of water at 12 Cents per kw cost $3, except it loses heat from the jacket making it about 80% efficient or costing actually $3.60 to make the same amount of water s the boiler. THAT'S why boiler coils still exist. If it was cheaper to make hot water with electricity there WOULDN'T BE boiler coils.
So you are going to shut off the boiler water making function in the summer to make a MORE EXPENSIVE water heater do the job because it LOOKs like the boiler is bigger and more complicated, that is the difference between math and wall painting.
The tempering valve is always a good idea as while a boiler that size makes 2-1/2 gallons of direct hot water, the tempering valve will mix in cold water to bring the water temperature down to 110F or less for showers. The mixed water will then be able to supply 4 gpm of hot water for two showers. The insistence on doing the wash at the same time is just pure family one-up-man-ship instead of cooperation.
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