Munchkin controls question

Wayco Wayne_2

on how the munchkin modulates. Someone at Wetstock was saying that at the lower latitudes, like MD. that outdoor re-set was not necessary because the Munchkin would self regulate by delta T between supply and return. He said that as the house warmed up the delta T would get lower until when it reached 2 degrees or less it would modulate down and lower the set point automatically. My question is how does the delta T diminish as the house warms up? What about the sizing of the heat emitters? Does that play a role in the behavior of the water temps. I would like to really understand since I have been a proponent of outdoor re-set and Munchkins to date. WW

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Plumbob

If the radiators are at 180 and the house is at 50, the water will lose more heat than if the radiators are at 180 and the house is at 70. So the rate of loss of heat depends on the temperature indoors. And yes, the Munchkin modulates the flame by looking at the delta-T. However, it still heats the water to the set point (say 180); it's only a question of what flame is needed to hold the set point. Obviously, if delta-T is 20F and you need full flame to heat the water back up by 20F, then if delta-T is 10F you only need half flame.

But outdoor reset is concerned with the supply T (set point), NOT the delta-T. Without outdoor reset your supply T is always set at a fixed temperature, say 180. With reset, the suppply T varies with the outdoor T, so that the radiators/baseboard don't get too hot unnecessarily. Modulation in the Munchkin can't achieve that.

If your source says that the Munchkin varies the set point (supply T) based on the delta-T, that is incorrect. In principle one can design a boiler to run that way, since the steady-state delta-T depends on the heat loss, but there are many practical difficulties, based on the fact that delta-T varies a lot. For example, delta-T is very high at the beginning of a heat call, but this does not mean the house is freezing, just that the return water is cold from sitting in the pipes. It also depends on the house in which the boiler is installed, and the water flow rate. So it is difficult to implement a reliable reset strategy based on delta-T, but it is still perfectly reasonable to base the modulation strategy on delta-T.