How to feed preheater pan?

[MarquisVII]

Smoky Lake preheater pan over 2x4 divided. How can I keep the preheater reliably full? I tried rigging a float valve but they won't feed fast enough off my gravity head tank, which is basically 1-3 psi. So I'm back to gravity feed with a 1/2" valve for metering. But without being able to consistently maintain the preheater level, every now and then I'll end up low, or I overfill and spill sap off the side of the evap. Ideas?

Second, I'll be upgrading to a 2x6 hybrid with float box in the next year or two. Once I step up to that is the preheater pan basically obsolete and I can just gravity feed cold sap into the float box without worry of hurting boil?

[maple flats]

You can feed cold sap into the float box, but at the slight loss in evaporation rate. Back when I first got my previous evaporator, a 3x8 wood fired I made a hood for it but had no preheater in the hood. The next season I had built a preheater, my evaporation rate went up by 12-15%. A few yrs later I added air over fire (AOF), with a high pressure blower, that gave me another roughly 15% increase in evaporation rate, along with using less wood (1 arm load every 9 minutes vs 1 arm load every 7 minutes before the AOF).
My preheater had a thermometer in line before the sap went into the float box. I use an auto draw, between draws my inflow to the float box was 160-180F, when the auto draw opened it slowly fell to about 120-135, than after it closed it took a few minutes and it was back up to 160-180. The average in the temperatures was likely about 150F, because I was getting between 6-7 gal per hour draw of syrup with the preheater and concentrate averging about 10%.
My preheater was made using 4 copper manifolds each a 1" header with 6 out of 1/2" each. The cold entered low on the first header, then sloped up about 2" in about 55" distance , into the next header, then straight up to header # 3 and again up about 2 more inches in the 55" length, then to the 4th header, directly above header #1 and out of the hood. I had a sloped drip tray catching the condensate drips which ran into the hood gutter and out to a bucket on the floor. Over the preheater I also had a large funnel, sort of similar to an upside down chinamans hat, it also drained condensate from the 15" steam stack to the hood gutter and out. On a second steam stack, also 15" diameter, I had another funnel under that stack, over the syrup pan. I don't know if copper preheaters are still allowed, mine was made with lead free solder, check it out, regulations might have changed.

[maple flats]

If you check old threads on this forum there is a post by a former member "Johnny Cuervo". I don't think he's stillactive, but, he describes hoe he made a pre-heater using the heat in the stack. He made it with several loops going almost all the way around thestack then up and repeat. each loop was not touching the stack until he attached a spring to pull it against the stack. That way he could control the temperature, more loops in contact, hotter, and vise versa. If I recall he has pictures and described how he did it.
He visited my sugarhouse once along with the man who 2 or 3 yrs later formed Next Generation Maple Products, a fine gentleman named Steve.
If you don't find the thread, call NGMP, and askif he would have Johnny Cuervo contact you. That was his online name, I forget his actual name, but knowing him my guess is that he still has the info from when he posted it. He (Johnny) works or worked at Syracuse University.

[maple flats]

I searched it for you, try this link https://flickr.com/photos/57535094@N00/albums/72157601539320490/with/1168551955

Enjoy. It looks like he posted it in 2006 or joined then and posted a while after.
Dave