Conversation with Alan Rushforth on January 13, 2011

From Green Garage Detroit
Jump to: navigation, search

return to Solar Thermal Collectors

return to Mass Thermal Storage

Green Garage questions are numbered, responses below each are from Alan Rushforth from Rushforth LLC:

1) What size and type of pex do you run from the tanks to the panels?

(I thought you ran 1" pex-al-pex. Also I remember you recommending a copper lead right near the panel)

The collector loop pipe sizing depends on the flow. Often about .02 gallons /minute/sq.ft. flow is recommended. That translates to about .8 gal/min for a 4’x10’ collector or about 8 gpm for 10 collectors. If you are avoiding the efficiency loss of using a heat exchanger in the drainback loop and are taking the water directly from the bulk drainback tank and pushing it directly through the collectors and dumping it directly back in the bulk tank, it is my opinion that you can successfully get away with a bit less flow (like 6 or 7 gpm). I feel this flow is in a borderline range where you could probably get by with ¾” pipe, or certainly 1” pipe would be plenty big. Personally I am not one to over sizing just for the sake of it. Larger pipes have more surface area, more heat loss, more materials, more cost… If it were just 8 or 9 collectors I would say use ¾” (I just did a system that is working great with 9 of these exact same 4x10 panels per row fed by ¾” per row). If it were 11 or more collectors I would advise 1”. If your other designers or plumbers have no strong opinions, I would recommend 1”, and that way no one can accuse anyone of undersizing.

Regarding Pex/al/pex vs. copper, the conventional wisdom for solar hot water systems is to always use copper and never use pex. IF (and only if) you are doing a non-pressurized bulk drainback AND IF the last 3’ of the feed and return (where connecting to the collector array) are plumbed in copper (to keep stagnation temps away from the pex/al/pex), I believe one can successfully use pex/al/pex in such a situation, and have done so repeatedly with good results. However in your case, this is a 1 story building. The collector pipe runs will hopefully be relatively short. It might just be simplest (and even less expensive) to have your plumber just run these short runs in 1” copper. You save tracking down pex/al/pex and special adapters, the plumber is happy using copper he is very familiar with, and no one will ever challenge an all 1” copper collector loop as substandard.

2) About 5 - 10ft of this pex ewill be xposed on the roof, what do you use to insulate it with?

Regardless of whether the piping is pex/al/pex or copper, it should be insulated with at least ¾” Armaflex type insulation (the better quality black stuff available from refrigeration supply houses – not the cheap grey stuff from Home Depot.). This stuff must be protected from sunlight. This can be done with PVC drain pipe or PVC conduit for straight runs. On bends and odd connections one can wrap with 6” wide aluminum flashing wrap (sold at Home Depot – roofing department). There is a newer (more expensive) version of the black foam insulation that is rated for exterior exposure to sunlight, where you can skip the wrapping. is one source for this and they will ship small quantities. I have used it and it saves a lot of time, but I cannot say from experience if it will hold up for 20 years.

3) What brand / model temperature gauge do you use to sense the temp of the panels to start the flow? You recommended starting flow at 90F, is that right?

I normally use the basic Stecca TR0301 controller ($150ish range). It is prewired to plug into the wall and has an outlet to plug the pump into (if you have a plug cord wired to your pump). Often one can avoid the need for an electrical permit (for the solar system) with the prewired plug-in controller and pump. One source to order this controller from is You can get it with 2 or 3 sensors depending on what you want.

Regarding the 90F start temp, normally I would NOT have such a start temp restriction. Our systems are normally controlled solely by the differential controller. In our solar tanks (used just for domestic preheating) temps can get quite low in the winter. On poor solar weeks (dark and freezing) with incoming street water as low as 38F, and high apartment building water use, the big solar tank temp can on occasion get drawn down quite low – 60 F or even lower. If the collectors are 12F hotter (or whatever the differential is set at) the pump will turn on and start harvesting BTUs even if the collectors are only 75F or 80F. However I read through most of your write-up and you have some extra complications with the heat pump, dual tanks, plus you are looking for some redundancy in the controls to assure no freeze problems. So the shut down feature below 90F may make sense. To be honest, I would have to dig much deeper to weigh in more definitively on this. Hopefully your other experts have this worked out reasonably well.

4) What is the size and type of drainback tank you recommend?

(We have 10 Aurora 4x10 panels. Or give us the way you calculate it.)

I would recommend skipping the small drainback tank and using your big 2000 or 3000 gallon tank as the drainback tank. I will do my best to relay a concept from a very talented engineer I met: “It is easy to design something complicated, and hard to design it simple.” Unless you have some unusual situation like a high rise building with a very long high pipe run, or a situation where the array is below the bulk storage tank, I would aim to use the bulk storage tank AS the drainback tank. I think you should be able to do that here. If this does not make sense, we can talk about this on Friday.

5) What brand and type of pump do you typically use? What is the normal range for the size of pump?

(We are lifting the water about 15 feet. We'll have our Mechanical contractor do the final sizing...just want to make sure we're thinking about it right.)

On an atmospheric vented bulk drainback (where you will have some oxygen in the water) we will use bronze or stainless steel circulating pumps. We have used Taco, B&G, Grundfos, Armstrong, and Laing. Sometimes 2 will be needed in series. I can’t really say that any brand is a whole lot better than the others (although Laing just makes small ones). For your purposes you could probably do a lot worse than to use one bronze Taco 009. One of these hould have plenty of head (about 32’) for your application. My guess is it will pump about 7-8 gallons/minute through your pipe circuit (once air is pushed out and return flow stabilizes), which would be about right.

6) Are there any other controls/sensors you use in your design for flow rates, leak detection, heat build up, BTU's generated, etc.?

You may not need this, but for data monitoring, charting, BTU generation for domestic hot water and internet data transfer, I use an $800ish unit made by a small local (to me) company, Emerald Electronics plus a $200 - $300 (depending on size) pulse meter from With you/your people being on site regularly, and with you having lots of other monitoring and control systems, you may not need an extra system and extra complication like this.

7) Who is the supplier of your heat exchangers for your tanks?

To get the maximum performance for the least money, I/we make our own submersed heat exchangers typically from soft ¾” x 100’ L copper coil. The single line diagram I sent earlier showed an example of how that is plumbed. The hard part with those copper coil heat exchangers is figuring out clean simple efficient ways to support the copper inside the big tank. We have a few approaches plus a new variation I plan to adopt. None are rocket science – just a little ecommon sense and ingenuity.

That's it. With this info we can have a local mechanical engineer design it.

Alan Rushforth's 2 additional comments are:

I don’t see the need for the pressure alarm on the return line. If the return line is dumping into the top of an atmospheric vented bulk drainback tank, I would predict no positive pressure in this line (maybe even a slight negative pressure) while there is flow.

I think you will find the PV covers for the thermal panels as a nifty idea, but not a practical idea. If you can afford PV, I would put it in its own proper permanent location.

  • Alan Rushforth
  • Rushforth Solar LLC
  • 3700 Darby Road
  • Bryn Mawr, PA 19010
  • P: 610 520 1968
  • F: 610 520 1969
  • C: 215 620 6345