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Vitodens with Solar Boost

Can it be this simple?

Does Viessmann make an appropriate control for the Vitodens?

It will need to regulate the 3-way mixing valve to produce the supply temp required by the reset curve as measured AFTER the flow through the tank has merged back into the stream.

Unsure of 2nd temp sensor location or need. It may need to be in the tank or perhaps not needed at all. Something tells me that it will need to know the temp available from the solar.

Some things are omitted, but there is NO LOW-LOSS HEADER. Fully TRV'd system. No other mixing valves--just a differential pressure bypass. ONLY the internal circulator. 141° supply temp at outdoor design.

Also presume I need some way to prevent overheating of the tank during periods of non-use, but such shouldn't be too difficult.

ANY suggestions greatly appreciated!


  • Constantin
    Constantin Member Posts: 3,796
    Allow me to wonder...

    ...after all I'm a mere homeowner...

    If I read your JPG correctly, you aim to install the water temp sensor for the Vitodens upstream of it. Otherwise, I imagine it's going to get confused about it's role (i.e. it's going to see very
  • Constantin
    Constantin Member Posts: 3,796
    How about this?

    I think the issue always revolves around whether you can move the temp sensor on the Vitodens. If the sensor can be moved, then your implementation might work. Otherwise, the Vitodens is going to get confused. Plus, the location of the solar heater will not maximize the amount of BTUs you can extract from it (because the supply temp is already up).

    I have come up with a simple diagram of my own for you to look at. Here, a mixing valve diverts water to the tank whenever the tank temp is higher than the return temp. This maximizes the amount of heat you can extract from the solar tank (by going in with the lowest temp possible).

    The Vitodens then gets the warmed water and boosts it further as needed. On really cold days with a lot of sun, you may lose some efficiency due to the "return temp" being very high. On the other hand, I suppose that the ΔBTU is more than made up by the fact that most of your heat will be coming from solar.
  • Kal Row
    Kal Row Member Posts: 1,520
    being that you have not indicated...

    how you are going to control the flow from the solar?, you might loose more heat than you getting, siggi did a great articals about his own system, you will find it educational



  • Am quite certain that it wouldn't work by just adding the temp sensor as such changes the circulator from "system circuit" to "boiler circuit". It's probably going to be an automated manipulation of the desired setpoint during an "external call for heat".

    Unless the boiler knows what's going on (whether you're boosting return or supply) it would very likely mess up the modulation logic.

    Was toying with the idea of boosting supply temperature to ensure lowest possible return temp to the boiler.

    The way I can visualize it working finds the solar heating the water in the tank until it slightly exceeds the current reset temperature. Supply water starts to divert through the tank and the boiler uses the sensor at the mixed supply point to reduce the burner output.

    Concurrently the 3-way valve modulates to keep temperature in the tank constant. The "new" supply temperature produced by the boiler becomes the minimum return temperature from the solar with the collected energy producing a boost. As long as sufficient energy is being collected to overcome the transmission loss in the solar system, it [should] be able to find the ideal rate of flow diversion and burner output to keep the system supply temp right on target.

    Once the minimum rate of diversion through the solar can no longer maintain temp in the tank, the 3-way valve closes off the solar component completely and the system waits for the temp in the tank to again exceed the current supply temp.

  • Thanks Kal. If you can follow my operation description above (AND if such will work), the system won't be able to loose heat through the solar portion as flow diversion will stop before this happens.

    It's trying to use just the amount of solar heat being collected at any given instant of time (reduced by transmission loss of course). Was hoping for gravity circulation in the solar component as it's generally slow and steady.

    The efficiency "hit" will come from the elevated return temperature in the solar panels. Wouldn't the evactuated tube models help with this considerably?
  • Kal Row
    Kal Row Member Posts: 1,520
    maybe i am missing something..

    but you solar panels are above the tank, and when the sun is "off", the cooler denser water in the panel, will drop into the tank, and cool it off, would do that, even if there was just one pipe between the solar and the tank - “so” you have a mixing valve, two pumps and a vie$$mann, you are still throwing away btu’s – like I said, maybe I am stupid and am missing something obvious, it wont be the first time ;) – need more input
  • Quite possibly ME with the problem

    But if no water from the boiler is flowing through the tank when the water in there is cool, I don't see the loss (provided the diversion valve has truly positive shutoff).

    With the tank lower than the boiler and rest of the system and being cooler in temp that the supply above, I'm not seeing any way for a "ghost" flow either.

    Again, only ONE pump in the Viessmann system (the one built in). No primary secondary, no low-loss header. If a pump is required for the solar component, it will be completely isolated from the rest of the system via the HX coil in the tank.

    Not trying to store the solar heat--am trying to use only when available and in the quantity available at the time. Very small tank and extremely low volume in the solar portion of the system.

    Will work on diagram for complete system to include excess heat "dump" and/or a DHW boost (DHW not coming from the "V" in this case). System itself really is nearly as simple as drawn--only component missing is the differential pressure bypass.
  • Kal Row
    Kal Row Member Posts: 1,520
    as long a you dont mind that after a full sunny day..

    with the tank nice and hot, you will be throwing it away, if you don’t use it relatively soon, the collector becomes an emitter (tough maybe condensing tube style collectors don’t make very good re-emitters - cause there is very little water vapor at the top of the tube when the sun is "off")

    just try to be the water - take the easy flow path - and cooler more dense water displaces warmer less dense - and heat trap U's and loops don’t help if the temp difference is enough - which can very well be in the winter
  • Still Thinking...

    The gravity circulation in the solar component may be a pipe dream, but for some reason I seem to recall a number of solar DHW boosting systems that achieve a non-electric thermosyphon with the collector above the tank. Might have been one of the really strange dreams I've been having lately...

    With a small, dual-coil tank it would be easy to provide a boost to DHW should the space heating system not require the input.

    I'm trying here for a modest boost with the simplest possible system. Solar is "easy" when initial cost is no object, but for the light of pocketbook and simplistic minded it can get really tough.

    Idea is useless unless appropriate controls are produced by Viessmann.

    Am also considering my (continental) climate. In the dead of winter our coldest weather often has the clearest sky and the warmest the cloudiest. In the shoulder seasons all bets are off. The reason I want to supply the space heating system FIRST is that DHW generally requires a higher supply temperature in my system. And IF such can be incorporated into the Vitodens control system, I'll literally be using only the boost available at any given moment.

    The "orange bug" bites me, but only because it's an adjunct to the "proportional bug".
  • Brad_9
    Brad_9 Member Posts: 29
    Thermosyphon solar

    Thermosyphon circulation in a solar loop works fine, if a little slow and a little loss in efficiency. But absolutely the best for reliability. With antifreeze it's slower still but still works. BUT, the solar collector has to be lower than the tank. The hot water in the collector rises to the tank where it is cooled and sinks again to the collector. Just insulate the lines well and that part will work fine.

    If a collector above the tank is needed, you can use a small PV panel, about 10 watts, and an El Cid pump. Also very reliable. To keep from cooling the tank off in the late afternoon, tilt the PV panel slightly to the east. Though it really isn't much of a problem in my experience in a mild climate.

    You will have to address overheat in a big way. Traditionally this is done by making the tank at least 1 1/2 gal per sqft of collector, and I prefer 2 gal per sqft. Let me know what you come up with.
  • David Woycio
    David Woycio Member Posts: 107
    in line solar

    Why not use the solar as an injection loop? Make sure you use a check valve or pump w/check. What your going to spend on a buffer tank you could even run an in line heat exchanger to separate the loops.

    Just a quick thought.....good luck!

  • Thanks again Constantin.

    The Thermomax suggestion you made [seems] to be leading to a way to make this work efficiently since they can even be used for process heating and steam production.

    In cold weather the temp range I need is in the area where covered flat collectors begin to rapidly loose efficiency.

    I can't seem to find an appropriate Viessmann control. Perhaps something is made for another purpose but I can't find such yet. By the time I can afford the evacuated tubes perhaps the control will be around as well ;)

  • This IS a proportionate injection loop operating though a single proportionate circulator--that's the whole point.
  • Kal Row
    Kal Row Member Posts: 1,520
    at the the end of the day...

    i recomend "drain back" solar - and while not the "coolest", that type is the safest - dont have to worry about overheat or freezing
This discussion has been closed.