[RE-wrenches] Solar thermal covering

Wed Jun 18 04:11:43 PDT 2025

I have not read all of the comments on this thread so please excuse any repeats.

The valley that I live in has so many drain back systems it’s amazing. I have inherited the service on them not that I wanted to.  The trick seems to be that you need a pump that gets the Evac tube manifold full of water & pushes the air out very quickly in order to eliminate the possibility of steam lockup. The taco 2699 series of pumps were used here for a lot of the systems .
There are many up sides to employing evac tubes in general & many down sides.
Upsides include:
Higher temperatures at colder air temps (if not covered with snow).
Lighter weight at installation .
Smaller footprint on roof or area of installation .
Production of hot water in less than desirable weather conditions.
Production of higher water temps.

The down sides of evac tube systems with drain back are:
When the tank gets hot & the circ pump to the collector gets turned off as long as it’s sunny the manifold is too hot to restart & resume to make additional hot water as it steam locks.  This Of course limits the overall efficiency of the system & comprises this the systems production, not good as you get 1 tank of hot water/ day unless very cloudy conditions for a bit & the manifold cools.This requires a controller that will not restart the pump or you burn out pumps.
Avoiding shut down requires having a very large tank(s).The systems here have oversized atmospheric tanks 300-800+ gallons each with dual heat exchangers one for solar & one for DHW  before the backup source.
Pollution during the production of the tubes
All are Made in china due too pollution during tube production pollutionp
Steam lockup
Tubes Still covered in snow long after the sun has returned after the snow storm has cleared out

Evac tubes have their uses though I tend to not install them In residential systems. I think that high water/ higher temperature requirements usage applications all day like schools, jails, commercial production facilities, dairies, etc .

That all said. I tend to install flat plate  collectors appropriately sized for system needs, with some kind of way to address overheat when people go on vacation midsummer so I don’t have to go rescue the system when I am on a vacation!

Dana Orzel - dana at solarwork.com - 208.721.7003

On Jun 17, 2025, at 6:23 PM, Luke Christy via RE-wrenches <re-wrenches at lists.re-wrenches.org> wrote:

 Hi Jay,
I agree with Brad that Evacuated tubes are not a good (or even a reasonably possible) candidate for a drainback retrofit. If they are pass-through tubes, then they cannot fully drain as piping connections typically come through header connections at the tops of the tubes. If they are sealed heat-pipe tubes, then not only is the header unlikely to drain properly, but the tubes themselves will stagnate at a very high temperature without heat transfer fluid flow.

I recommend a diversion heat- dump arrangement such as Todd suggests.Use paralleled runs of conventional hot water baseboard fin tube for the heat dissipator. This stuff is readily available at almost every plumbing supply house and it works well.

Diversion can be activated by either an electric 3-way valve and a set point controller, or the passive wax-thermostat 3-way valve that you mention. I have used both methods extensively with good results. The wax thermostat will probably be significantly harder to find. Some years ago I ordered them through Low Energy Systems in Denver. Reach out to me off list and and I can supply some contact information.

-Luke

Luke Christy
Renewable energy consultant

NABCEP Certified PV Installation Professional™:

Solar Gain Services, LLC
PO Box 531
Monte Vista, CO 81144
719.588.3044
sgsrenewables at gmail.com

On Jun 17, 2025, at 3:46 PM, jay via RE-wrenches <re-wrenches at lists.re-wrenches.org> wrote:

In doing research today I found the following, not sure where I can buy them at this point.

1. solar dissipation tubes.  that will do about 12,000 btu or at least this one does.

2. 3 way thermostatic valve which uses a type of wax that opens and closes it at a specific temp and routes the heat through the dissipation tube vs the heat exchanger down below.

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