TCaldwell said:
joe, being that the garn is a burn and store, with a widely fluctuating supply water temp as the day progresses, and not a relatively constant temp boiler that can controlled with a setpoint high limit. would the outdoor reset be beneficial. with the garn burned 12 months a year would it be better realizing the fluctuating supply temp to use a variable speed circ based on a delta t between the supply and return, basically only if there is demand will the circ be pumping supply water based on the delta t need, if no need the hot supply water will stay in storage, thanks tom
"Beneficial" will depend upon what you're trying to accomplish, and what your potential inefficiencies are.
For example, dropping the pump speed based upon outdoor temp will save electricity, and will reduce heat loss from piping (particularly important for buried pipe, where heat is lost to the outdoors, versus indoor pipes where the heat is just lost to the wrong part of the building). However, while
heat loss will be lower,
temperature loss will be higher, so if the heat delivery system requires higher water temps (eg, a fan coil, or baseboard), the heat loss may not be as big of a factor as the temperature loss - heat is only usable if it is at the correct temperature.
Controlling based upon the delta-T will give you a very controllable delivery of heat, as you will be in an essentially-linear relationship at your heat exchanger (with a fixed delta-T, double the flow means double the heat transfer). Higher flow rates than are actually necessary for conditions will result in greater heat loss from underground pipe (but lower temperature loss), so the issues above apply, but reversed.
Another option is to control for a fixed return water temperature. This is frequently done to keep a condensing boiler in condensing operation, by slowing down flow when the return temp gets too high. However, you can speed up flow to a zone pump in order to keep the return temperature
up, which can be very important with things like fan coils - by keeping the average coil temperature over a certain setpoint, you never have the issue of blowing air that feels cool. You can even operate based upon discharge-air-temperature, if you are dealing with a process that requires a certain temp (eg, an incubator). Or based upon discharge-water-temp from the other side of a plate heat exchanger (or other heat exchanger) when that is the critical value (as in heating domestic water, or water for an aquarium, pool, or hot tub).
Variable-speed technology allows for a wide variety of control possibilities. Adding "smart" controllers can allow for control based upon multiple functions at the same time, or establish certain limits. For example, you could operate based upon outdoor temperature, but also have a hard limit programmed into the controller that will monitor the discharge temperature of the heat exchanger, and control based upon that if the outdoor reset curve tries to push the temperature too low (standard outdoor reset controllers provide that in a rudimentary way).
Joe