\"Munchkin differential\"---not making sense to me

R. Kalia_3

The Munchkin allows the homeowner to set a few parameters. One of them is the "Munchkin differential", described on pg. 36 of the installation manual:

"Simply press in the S3/Program key to get to next value, the Munchkin differential setting will appear in the display ch and alternating value of 30. This allows the installer to adjust the activation of the burner based on the differential temperature setting of the Munchkin. The burner will not start until the outlet water temperature reaches a temperature that is equal to the temperature setting for the Munchkin, minus the differential.
EX: The Munchkin is set to heat at 180° F degrees and the differential is set at 20° F. The burner will not start until the outlet temperature of the system falls below 159° F degrees (180° – 20° F =160° F).
To adjust, press either S1/– (Decrease Value) or S2/+ (Increase Value) Ranges 5° F to 30°F."

Here's the problem. Suppose it is a warm day; the outside temperature is (say) 55F, and the reset process has set the water temperature to be (say) 85F. Suppose the Munchkin differential is 20F.

So the supply water has to be 85-20=65F before the Munchkin will start heating? That isn't likely to happen before the house gets very cold! At 180F, waiting for the return temp to be 160F is reasonable, but it is not reasonable for the differential to be a fixed number.

PS Essentially the same explanation is in the Vision I manual, pg. 13, with an important difference: the temperature being monitored is described there as the temperature of the RETURN water, not the supply water. My problem with it is still the same. In my example, neither the supply nor the return temperature is going to fall below 65 for a very long time after the burner stops. Delta-T may be 20 at 180, bu it is not 20 at lower water temperatures.

Mark Eatherton1

Split differential

If I'm not mistaken, the differential is split. Half high, half low. So in your case, 10 degrees subtracted from the set point would give you a start, and 10 degrees F added to the set point would be the ending temp.

ME

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Bryan_16

Huh?

I was taught that the differential is just as described in the manual(I have attended 2 Vision traing sessions). If the differential is split then wewould see 190 on the high side and 170 onthe low. I have 20 Munchkins out there, some with Vision 1, Vision 3 and straight out of the box , no reset, everyone runs to the set point and then shuts down until the return reaches the differential temp. Makes setting the differential a crap shoot with the reset.

Plumbob

no difference to the problem

> If I'm not mistaken, the differential is split.


This is true in Tekmars although not true in Munchkins. In any case, it makes no difference.

If a reset curve goes to 70 @ 70, the reset temperature will be 75 at some outdoor temperature (say 65). The water temp would then have to cool below 75 - (20/2) = 65 before the Munchkin starts up again, Basically the unit will stop working, because the water will have to cool to the OUTDOOR temperature before the boiler will run again (!).

A fixed differential works fine with an aquastat in a CI boiler, because the temperature range is so small (140-180). It does not work with full-range (down to 70) reset.

I wonder what actually happens in spring and fall, since if Munchkins stopped working in those periods, we would have heard about it before now!

Curiously, the Tekmars have an "auto differential" setting that INCREASES the differential when there is a smaller heating load!! This is claimed to reduce short cycling, but it will cause the same problem I am describing, only worse!! Tekmars are widely used without a problem, so there must be some explanation.

jerry scharf_2

I'm being dense

I keep on seeing these discussions of reset curves and they keep not quite making sense to me. Like I have said before, I see the reset curve as a secondary control for boiler optimization, not a primary control for room heat.

So my answer to your initial concern is 'don't program your reset curve that way.' Just change the reset curve to guarantee that it starts at 70F accounting for the differential. The boiler will burn until it hits the reset target, say 90F. Given the very low heating demand on the days where the reset curve is at it's bottom, there will be a bunch of BTUs stored in the water and the building mass to keep the burner off for a good long time.

In a standard control system you have either thermostat(s) or run constant circulation and TRVs or similar. In either case, having the water hotter than ideal in the extreme shoulder case is not critical, simply suboptimal. It's not clear to me how close outdoor reset is to optimal in estimating heating demand (a subtle difference from heating load.) Things like people want their house a tad warmer on rainy days and the fact that people aren't terribly linear or repeatable.

jerry

R. Kalia_5

so you're saying...

So you are saying that the standard reset curves we all see, the ones starting with 70 @ 70, and ending with (say) 180 @ 0, won't work. One must have something like 90 @ 70.

OK, I can accept that. I understand your arguments. So why don't they tell you that? What if someone believes those example curves (like those shown in Tekmar manuals) and programs a Tekmar starting with 70 @ 70, will their systems then fail to keep the house at 70 in mild weather, because the boiler won't start even though the thermostat is calling for heat?

S Davis

Tekmar

I have been using Tekmar for about ten years and have not had any complaints, but then alot of my systems use indoor temperature feedback wich will adjust the heating curve based on indoor and outdoor temp.
I must have a couple of hundred jobs with standard outdoor reset and no problems.

S Davis


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