PEX Preheater

[theraven06]

Thoughts on making a preheater out of PEX? I have a steam hood for over my flue pan, and was thinking about lining the inside with PEX to preheat incoming sap. I know it gets fairly flexible when it warms up, but if you anchor it to secure it to the hood it should be ok? The hood does not sit directly on the flue pan, so I don't think it would get hot enough to be a problem.

[Chickenman]

Pex is a poor conductor. Copper would be better.

[maple flats]

Absolutely, Pex, being plastic will do only a small portion of the heating the same amount of copper would do. To get enough heating in a pex pre-heater you would likely need 20 or maybe 30x the length of what a copper one will heat. Very poor thermo conductor.

[Cjadamec]

Pex can be used for hydronic heating (180 F at 15 psi) and can handle temps up to boiling at very low pressure. It gets weak as it gets warmer so you have to keep the pressure down.

As others have said it is a very poor conductor, it's actually marketed as saying pipe insulation isn't needed for cold water pipes because the pex is an insulator. For preheating it would be a poor choice.

[Johnny Yooper]

metal all the way. I have a copper tube preheater around my chimney pipe.....40F sap in, 160F sap out.

[TapTapTap]

I agree. Ditch the pex unless you want radiant heat in the slab.

Hmmmm - Radiant slab heating run off the evaporator! I get credit for that one!

[theraven06]

Thanks for the advice, it confirmed what I was thinking about the heat conducting properties. I just picked up copper to build a preheater out of.

[maple flats]

Use water soluble flux and clean it out good after assembly with pressure and hot water. Don't just rinse, it takes several minutes at full flow to get it cleaned out. Just the pressure of your house is good enough unless that is very poor.
My homemade preheater had 2 levels and 6 passes on each. I flushed for about 10 minutes at a time and repeated 5 or 6 times. The wait between them was to let the water heater get back to full temperature. Then I followed that with about 5 minutes cold rinse.

[Helicopter Seeds]

Thanks for the advice, it confirmed what I was thinking about the heat conducting properties. I just picked up copper to build a preheater out of.

Sounds like you already decided, but for point of discussion, Believe it or not, there is a term called "critical insulation thickness" for pipes and cylinders. The idea is that a thin layer of insulation on a copper pipe can actually improve the overall heat transfer- it does this by increasing the surface area by virtue of being a larger outer diameter. There used to be a 'pipe wrap' insulation available, but if installed single layer on a 1/2 inch copper hot water pipe, it hurt more than helped. Similarly, a PEX pipe, with a thicker wall, and probably a larger OD, will offset the lower thermal conductivity. Without knowing the specs, we can't say a final yes or no. We can say, that the transfer is a function of temperature difference, so the same basic equation works for both directions. The fact that PEX is used for radiant heating systems tells you that it is suitable, so if economics made sense here, my guess before math is that it would work.

[Woody77]

For the cost difference and the time difference to build a preheater and the fact that you don't get the condensate as badly on the PEX tubing I would say anybody else that was thinking this and his reading this maybe it's worth a try I have had Pex in my Sugar House and it works just fine. It may not generate as much heat transfer as copper but it still works and saves a ton of money.

[tcross]

I have PEX that connects my preheater to my float box and i'm changing that out this year. Mainly due to the fact that the contraction and shrinking with the heat changes from when in use and not makes the fittings not work very well. i have to constantly tighten and change the clamps that hold it in place. many times about 30 minutes into my boil i have leaky fittings. i would not recommend using Pex as a preheater just because of that! its a real PITA

[theraven06]

In the end the cost difference isn't that bad for a project this small. Copper isn't bad right now, I was able to get 10' sticks of 1/2" type M (thin wall) copper for under $10 each. The PEX would have run less than $5 per 10' but the fittings cost more than copper sweat fittings. I'll have some more time into the copper to sweat everything together, but overall cost out of pocket is similar to PEX in this instance - plus it will be mostly self supporting with only brackets on the corners.

[n8hutch]

My leader made pre heater has 5 lengths of 5' copper and it easily gets the sap up to 170°. It's a parallel flow . I was very happy with it. The preheater is pitched from the back of the sap pan to the front. The sap enters the bottom of the high end of the preheater and exits on the bottom of the low end of the preheater this allows the preheater to drain completely. I do have a valve where the cold sap enters the preheater yo drain that short section of copper. Also the preheater is vented on the high end .

[bill m]

As others have said PEX is not a good option for a preheater. As for radiant heating using condensate from the evaporator, that is not a good option either. Water temp for radiant heating needs to be between 90 and 135 MAX. To hot and it will crack the concrete and adjusting the temperature of the condensate is very difficult.