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Drainback enabled, but no drainback tank?

vilordvilord Member Posts: 34
edited June 2019 in Solar
I'm trying to understand how our solar water heater is set up, and whether it was setup wrong.
It has been going for 7 years, has not frozen, and seems to help, though our boiler contractor said he was surprised how often we were calling for backup DHW heat when we have solar.

I looked at the settings on the control (sungo us plus) and it is set for drainback.

We have an Intertek SGL-2 350 tank with two heat exchanger loops. Top exchanger is 9 sq ft for the boiler, bottom is 12 sq ft for the solar.
The proposal lists "propylene glycol head exchange loop"

Are drainback settings ever supposed to be used with a heat exchanger?

If not, should I set the controller to the default values?

How often do we need to flush and replace the coolant? It has never been done.



  • vilordvilord Member Posts: 34
    Hmm, it appears I may have answered some of my own questions... this is a Wagner Secusol, which is apparently a single-tank drainback heat exchanger hybrid thing.
    Time to do some research!
  • hot_rodhot_rod Member Posts: 13,495
    Those dual coil tanks do limit your solar harvest as the top of the tank is always maintained by the boiler. Sometimes you can lock out the boiler, or element, until the end of the day to allow the collectors to heat the tank first. Boiler only backs up when solar cannot cover the load. If your schedule allows for that type of use.

    Any sound when the solar pump shuts off, like water draining back? There were some solar drainback/ glycol systems that allowed fluid to drainback into to a large capacity coil in the tank. The collectors were small tube serpentine type so very low system volume.

    How many collectors, what size tank?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • vilordvilord Member Posts: 34
    Tank is 93 gallons
    2 Collectors, Wagner Euro C20 AR, 28 sq ft each
  • Jamie HallJamie Hall Member Posts: 12,947
    On the use of the "backup" heat when you have solar -- those two collectors have an absolute maximum theoretical output of 2 kilowatts, taken together. That's when the sun is shining on them, perpendicular to the plane of the collector, and the collector glass is clean. Subtract a solid 50% for various inevitable inefficiencies. So -- one kilowatt or so. When the sun is shining. 3,400 BTUh. That will heat a whole 6 gallons of water from 50 to 120 every hour. Is it any wonder that you need more heat to heat your hot water to something reasonable?
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • vilordvilord Member Posts: 34
    Amazing what you can get "sold" on. Even if the installation was free, if we were paying for the annual maintenance (which has never been done), it might still be more than the energy savings.
    Guess I'll just maintain it as long as we can so we don't have to replace it...
  • Larry WeingartenLarry Weingarten Member Posts: 1,821
    Hi, Going with Jamie's numbers, if you get five hours of sun, you would have 30 gallons of water daily. That could supply two non-wasteful people. Hopefully the maintenance isn't too big a deal. :|

    Yours, Larry
  • vilordvilord Member Posts: 34
    My partner insists that it significantly cut her oil bills, so I won't knock it too much. I slowed down the pump speed a lot, it wasn't heating the water much on part-sun days to get a big enough differential to actually do much.
  • hot_rodhot_rod Member Posts: 13,495
    Where are you located? Any idea what the incoming water temperature is? It's fairly easy to get a 50% solar fraction, SF in most of the US or Canada with a properly sized and oriented system. I think California requires you prove, via a simulation run, that 50% of your DHW will be provided to get the incentives.

    The system needs to be sized to your specific water demands. Over-sizing is worse than undersizing as too large of an array can overheat and breakdown the glycol. Drainback systems eliminate that concern.

    1.5 - 2 gallons of tank per square foot of collector is the rule of thumb, again depending on your location.

    Payback really depends on what your cost of heating the DHW with fossil fuel/ the cost of the system. If for example you have a tankless coil in an oil fired boiler for DHW, the solar should make a bit of difference on the cost of DHW.

    With State and Federal rebates the SDHW systems can pencil out.. Without incentives, residential SDHW is more of energy conservation symbol :)

    The system really should not need a lot of maintenance if properly installed and controlled, check the glycol ph every few years to show if it is stagnates often. With DB systems check the pump operation yearly.

    I get about 3- 4 months of DHW, "solar showers" with no LP backup here in SW Missouri.

    Your control should have a record of hours run, HP. That would give you an idea of how much sun it has harvested.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • vilordvilord Member Posts: 34
    We're in Southborough, Massachusetts, well water temp changes a little winter vs summer, but not a whole lot. Probably low to mid 40's incoming temp, but I haven't metered it.

    It is a drainback system with a 30% glycol mix. It has been installed for 7 years with no maintenance whatsoever.
    - The glycol is probably not what it used to be
    - The tank probably has some scaling/sediment
    - The collector probably could use a scrub
    - The tank's anode rod likely is shot, depending on the makeup of the water. Hopefully the tank hasn't started to rust out.

    Pump is working, not sure if there's much more to it than just 'working' vs 'not working'.

    There is a little visible corrosion in a couple of spots near solder fittings at the tank. Can't tell if it is from an old or active leak, so I'm going to clean them and set a reminder to check in a couple months.
  • hot_rodhot_rod Member Posts: 13,495
    Get a ph meter and test the glycol. new glycol is around 9- 10.5. As the glycol breakdown the ph drops. If it drops int the low 7's it is time to flush and replace.

    If it has a factory built pump station they generally have flowmeters to indicate pump operation and flow rate.

    The control will also have a pump running indicator. This could indicate the pump is powered, not necessarily spinning.

    No harm in checking the anode on any water heater to tank, really.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
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