Your definition of "Marginal Days"

[Starting Small]

This is my first year going to high vacuum, I always hear people say that high vacuum will allow you to gather sap on "marginal days." I am curious as to what you consider a marginal day. I ask because I want to know what the bottom temperature range is for turning on the pump. Is it 30 with full sun? 32? 35? Thanks for the advice.
-Dave

[GeneralStark]

This is why I generally keep it simple and leave the pump on 24/7 unless it is going to freeze hard for several days. I have found that there are all sorts of "marginal" situations where I likely would not have gotten sap if the pump were not on. Cold but sunny days are certainly one common situation, but other weather factors seem to be at play as well. Obviously, stretches without a freeze will be the time to run the pump constantly.

Using temp. as a rule for turning on the pump can be inaccurate as your temp. sensor may be in a cool location, or your woods or trees may be warmer than the location of your pump and sensor.

[unc23win]

I would agree with GeneralStark although I do not run my pump 24/7 temperature reading and forecasts can be wrong and sometimes it's more about what the barometer reads than the thermometer. It is surprising every season as to the days it runs when you didn't think it would and the opposite days where you expect a good run and nothing happens.

[ennismaple]

Marginal days are generally the ones where it doesn't get very much above zero or when you have multiple days in a row without a freeze. Guys on buckets will complain they have nothing to boil but we'll get a half gallon of sap per tap.

If you can keep your releasers from freezing up keep the pump running at all times - unless the only sap that's running is off the end of your nose!

[Starting Small]

Good advice, Thanks, I have used diaphragm pumps in the past and have to get it out of my mind that I can run things below freezing without risk of causing damage to the equipment. If the releaser does freeze are there any ramifications aside from lost sap? Thanks,
-Dave

[BreezyHill]

My system is set to turn on at 28" as long as the photo cell is getting enough light to allow it to power the unit. Cloudy days she wont trip, full sun yes.

at 28 I have seen the frozen sap in drops and lines start to thaw from the suns warmth. Marginal day for us is 33-34 and cloudy or a breeze chilling the trees.

Parts of the main bush will be running at these temps and will sometimes even be warmer as it is in a bowl and protected from the wind.

My $3.50 photo switch has paid for itself many times over in electricity not used. And I cant count the times she was running when somebody went down to check on the temp for me while I was on the road.

[maple flats]

I did have 1 time that freezing caused an issue with the releaser. A seal developed a small air leak into the manifold (one manifold end cover O ring got a tiny leak). As the air rushed in, it made a snow like fluff. This was on a double releaser. That "snow" fell or was drawn into the releaser tank on that side which in turn held the flapper open between the manifold (where all the mains enter the releaser) and the tank on that side. At a point when the open flapper side was full and ready to dump, the change over valve switched the vac. and sap flow to the other side. The "snow" then continued to fill the first side because the flapper was held open by the "snow". When that tank filled enough , since the vacuum was near zero, the "snow pushed that canister flapper open, as if to dump. The inrush of air froze it all to a hard chunk. Meanwhile, the second canister , with near zero vacuum was filling, but it never tried to dump. The float continued to rise until it was against the top, where it eventually had enough push upward to unseat the bushing thru the lid that helps hold the float rods in alignment and keep a seal. Once that was pushed out the top, I then had another air leak and everything froze in the second canister from the vacuum pulling air in as fast as it could. When I found it, I had no vacuum, because in this process sap had also filled the moisture trap and plugged the vacuum line to the pump. It took me almost an hour to get everything back running properly again, but no permanent damage was done. I did have to pull the releaser apart and get the "frozen "snow" out.