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Yes, all of the mechanical energy of the pump does end up as heat in the fluid. There's a really good article in HPAC Engineering by Gerald Williams which explains pump/fan heat called "Fan Heat and Pump Heat: Sources and Significance." You might be able to find a pdf copy floating around on the internet.

You can think of hot water pumps as in-line electrical pre/post heaters for the boilers. By reducing the speed of the pumps, you're reducing the "supplemental heat" from the pumps, and this must be overcome by the boilers. There are still good reasons for implementing variable hot water flow systems:

• Electricity can be on the order of 3 times more expensive than natural gas. So even though you're total energy consumption has risen, you're still going to save money. Natural gas is also usually perceived as greener than electricity due to the inefficiencies in the electric grid. This is the same reason people tend to opt for natural gas boilers over electric boilers even though electric boilers are more efficient on a Btu basis. Using the previous analogy, you can think of variable flow hot water systems as removing an in-line supplemental electric heater.
• The delta-T in a variable flow system will tend to be greater than those in a constant volume. If you have condensing boilers, this allows for lower return temperatures which increases the efficiency of the boiler. Larger delta-Ts also will result in lower environmental/radiant losses in the piping because the average loop temperature will be lower.

Yes, all of the mechanical energy of the pump does end up as heat in the fluid. There's a really good article in HPAC Engineering by Gerald Williams which explains pump/fan heat called "Fan Heat and Pump Heat: Sources and Significance." You might be able to find a pdf copy floating around on the internet.

You can think of hot water pumps as in-line electrical pre/post heaters for the boilers. By reducing the speed of the pumps, you're reducing the "supplemental heat" from the pumps, and this must be overcome by the boilers. There are still good reasons for implementing variable hot water flow systems:

• Electricity can be on the order of 3 times more expensive than natural gas. gas (per Btu). So even though you're total energy consumption has risen, you're still going to save money. Natural gas is also usually perceived as greener than electricity due to the inefficiencies in the electric grid. This is the same reason people tend to opt for natural gas boilers over electric boilers even though electric boilers are more efficient on a Btu basis. Using the previous analogy, you can think of variable flow hot water systems as removing an in-line supplemental electric heater.
• The delta-T in a variable flow system will tend to be greater than those in a constant volume. If you have condensing boilers, this allows for lower return temperatures which increases the efficiency of the boiler. Larger delta-Ts also will result in lower environmental/radiant losses in the piping because the average loop temperature will be lower.