How to prevent cold water from going into storage at night

[ddiabetes]

Hi. My first post.
I have a solar water heater system with the 4x10 collector panel on the roof. The storage tank is inside the kitchen, about eight feet below the collector. There is a circulator pump that activates when the collector heats up. I have two one way dielectric fittings to prevent backflow. My problem: at night, the hot water rises from the storage tank and forces the cold water from the collector to flow down to storage. The hot water goes up the "down" pipe and the cold water flows down the "up" pipe. In the morning, the water in the storage is luke-warm. I am thinking about putting an electric valve in-line to open when the circulator starts up. I haven't seen such a jury rigged system, in any of my searches, and I haven't seen anyone else posting about this same problem.
Does anyone have any suggestions?
Thanks for any help you can offer.

[Ampster]

Have you tried to loop one of the pipes to break the convection upflow?

[peakbagger]

Normally there is a spring loaded check valve on the discharge of the pump to stop thermocycling. A weighted check will do the same thing. Make sure you put in isolation valves and drain on the check as they do wear out and need to be inspected annually. That reverse thermosyphoning is the end of many SHW systems in cold weather. I have had one fail in 25 years. Years ago I was at a solar garage sale at long term solar installers operation in VT. They had a stack of identical heat exchanger bundles made out of 1/2" copper pipe. They were damaged from freezing on the fresh water side from failed check valves. Usually the basement would get flooded when the heat exchangers failed on the fresh water side. I have a friend that got a free system with a similar heat exchanger with split elbows.

Your powered valve would work, just make sure its a fail closed valve with spring return. The check valve is just a KISS version that works most of the time.

Either method stops actual fluid flow through the system, In addition to actual system flow, there can be convective flow in vertical pipes. The heat traps can work on hot water tanks but I am fan of the loop in the pipes as there are no moving parts. Just make sure the pump can handle the extra friction loss. More than few folks are inadvertently electrically heating their basements from thermosyphoning off a hot water storage tank.

[J.P.M.]

Hi. My first post.
I have a solar water heater system with the 4x10 collector panel on the roof. The storage tank is inside the kitchen, about eight feet below the collector. There is a circulator pump that activates when the collector heats up. I have two one way dielectric fittings to prevent backflow. My problem: at night, the hot water rises from the storage tank and forces the cold water from the collector to flow down to storage. The hot water goes up the "down" pipe and the cold water flows down the "up" pipe. In the morning, the water in the storage is luke-warm. I am thinking about putting an electric valve in-line to open when the circulator starts up. I haven't seen such a jury rigged system, in any of my searches, and I haven't seen anyone else posting about this same problem.
Does anyone have any suggestions?
Thanks for any help you can offer.

For small and physically local driving temp. differences, thermosyphon breaking loops will work, but for most larger system temp. differences like from the bottom of a collector to the bottom of a tank, a spring loaded check valve at or near the tank outlet to the collector (often, but not always integral with the pump at the pump inlet) will work. There are also weighted gravity driven valves but for larger height and/or large temp. differences they seem to be impractical for several reasons.

As Peakbagger writes, put isolation valves on either side of any check valve (or the pump if the check valve is integral with the pump) and expect to change the valve out yearly or more often as a matter of routine maint., especially if you have hard water that will gunk up the o-ring area of the valve and make it not do its job, or if the tank outlet water is consistently > 60 C or so. Reason: Most spring loaded check valves including most of the integral to pump check valves common for solar applications use cheap o-ring material that has a service temp. of ~ 60 C. and gets brittle, splits, and then fails.

However, before I did that, I'd check to see if you have the type of pump that has an integral check valve. As Peakbagger also writes, pumps with integral check valves are pretty common for residential solar applications. You may have one of that type and the valve may be gunked up or deteriorated, especially if it's not been serviced in a while. Worth a snoop before getting too involved.

If you do wind up getting a separate (spring loaded) check valve, try to get one designed to use o-rings made of something called viton. Also, do not simply replace a standard (made of something called buna or buna- n) o-ring with a viton o-ring (or material other than what the valve comes with for that matter). Reason: the "stiffness" of the o-ring materials is different. The spring in a spring loaded check valve is designed to work with the stiffness constant of the specific o-ring installed in the valve. Most available check valves use buna o-ring material because it's cheap and usually pretty good for most solar thermal applications provided the operating temps. are consistently < 60 C and/or the water quality is pretty good, or if the fluid circulating through the collector is a H2O/glycol mixture (but still < 60C operating temp.). But a direct system of the type where potable H2O circulates directly through the collector(s) such as you seem to be describing will do better if the check valve is cleaned and the o-ring replaced at least 1X/yr or more often.

As a practical matter, I scrapped out the pump integral check valve and added a motorized ball valve (with isolation valves) to the line at the tank outlet to the collectors. that line valve is normally closed but opens when the pump starts and closes when the pump stops. Not cheap, but easy to get at, pretty reliable and doesn't need to be cleaned/changed out.

Take what you want of the above. scrap the rest.

[ddiabetes]

Thank you for your help. I have a check valve in the system and it is not working. To be honest, I didn't know what it was. . . until you talked about it. . .but it obviously isn't working. I will replace it this week. Thanks!

[J.P.M.]

Thank you for your help. I have a check valve in the system and it is not working. To be honest, I didn't know what it was. . . until you talked about it. . .but it obviously isn't working. I will replace it this week. Thanks!

For anything I might have written that results in progress, you're most welcome. I'd be interested to read about any results or outcomes from your labors. While you're at it, your system probably has, or ought to have, an air bleed valve at the highest point it the line (at the collector outlet. That valve may well need replacing also.

If you need to drain all or part of the system to change out the check valve, be sure to refill the system in such a way that you don't wind up with an air lock (trapped air) in the system. That will cause problems with the flow, and likely stop it altogether.

[peakbagger]

The top auto air vent is definitely controversial. Many installers omitted them due to call backs. Mine is still up there after 20 plus years but I have no idea if it works. IMO if you have a proper charge pump, you really do not need an air vent on a sealed system with an expansion tank with an intact bladder. When I run my charge pump, I let it run awhile and eventually it runs clear.

[J.P.M.]

The top auto air vent is definitely controversial. Many installers omitted them due to call backs. Mine is still up there after 20 plus years but I have no idea if it works. IMO if you have a proper charge pump, you really do not need an air vent on a sealed system with an expansion tank with an intact bladder. When I run my charge pump, I let it run awhile and eventually it runs clear.

The way I learned it, air trapped in a system is only controversial in the ways to deal with the problems it can cause in hydronic systems including solar thermal systems. I'd hazard a guess the callbacks are often due to poor system design and/or poor water quality. The vents are pretty dummy proof but they need replacement on a regular basis in applications with hard water and or where the vent is simply stuck at the top of a collectotr piping system exposed to the elements.

Air remaining in a hydronic heating system and solar thermal systems of the direct type can cause flowrate reductions from something called air binding. Because most well designed solar DHW systems have their high point - and quite possibly other local high points - at or near the top of the highest collector(s), and because flowrates and fluid velocities in piping are often relatively low, air bubbles from air that is either trapped/not removed from the system at startup/refilling or as dissolved air in the water is removed from the water when/as the water is heated. Such accumulation of (un)dissolved air will, over time, cause air blockages which will reduce flowrates and if large enough, stop flow altogether. If the pump can develop enough head (in the form of velocity head) to push those high point (including local high point) air blockages through the system, the air not removed can cause corrosion problems for tanks and ferrous piping components.

Depending on pump location and elevation, for direct systems that have high enough flowrates to overcome the air buoyancy of the trapped air at the top of a system, the bubbles can often sound and act like pump cavitation with similar attendant problems.

The common way to get air out of a system is using air vents at all high spots, including all local high spots such as where piping needs to be routed over a rafter or such. Such high point vents are readily available, easy to incorporate and work. However, they do not last forever. The usual types with a float and spring usually wear out in a year or two if the potable water is hard and either get stuck shut or will bleed a bit. Obviously, neither is good. With good water quality, they can last a long time, but regular inspection is essential. I have a direct system and as part of maintenance, I inspect the vent often and replace it every 2 years or more often if it appears fouled or otherwise malfunctioning. I also keep an empty bush's bean can over the vent to prevent the elements from raising too much hell with the vent. It does not in interfere with the vent operation. Being mostly designed with (indoor) hydronic heating use in mind, I expect they are not designed for exposure to the elements. The can seems to help w/valve longevity. I also wrap the vent body w/1" Armaflex insulation.