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Solar Bypass Loop

My drainback hot water plan calls for bypassing the hot water boiler when sufficient solar heat is available for my radiant floor. (see attached)

What would be wrong with NOT bypassing the boiler? I would allow the radiant floor loop to circulate through the (well insulated) external HX when solar is not sufficient (and the boiler is working) and the solar heated water to circulate through the boiler when solar is sufficient (and my control setup has disabnled the boiler burner). My plumbing would be greatly simplified if I could set it up this way, and I could eliminate one 3-way motorized valve.

Any thoughts on this?


Thanks,

Martin Romero

Comments

  • Radman
    Radman Member Posts: 70
    planning to direct charge the radiant?

    Hi Martin,
    Do you intend to use the radiant zones as storage? It would be better to charge the DHW tank first, then have the 3 way divert valve send solar to alternate storage for use with the radiant. Also, it depends on what type of boiler you are using. It would be best to supply the return side or the system with of a modulating/condensing boiler. The current config bypassing the boiler is acceptable for an atmospheric boiler, but direct charging the floor would not be advised.
    How big is the array, tank, etc? If the array is 4 panels as you show, assuming the size is approx. 4x8 or 4x10, your storage should be about 2/1. If that is a Rheem 120g ext. HX tank shown in the diagram, you should consider more storage. You can't consider the existing water heater as storage unless you pipe the two tanks to recirculate, which in this config I strongly recommend.
    As a DB system you have the option to forgo utilizing storage should the radiant be satisfied. In this case you don't have to use the solar energy that is available, but should.
    "If it was easy, they would have called it PV."
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    Precious BTU's...

    Should NOT be wasted in raising the temperature of a non used cast iron boiler, hence the need to NOT send solar water through the OFF appliance. Even if it has a damper, it will still lose heat unnecessarily.

    But it's YOUR btu's. Spend them however you want :-)

    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Radman
    Radman Member Posts: 70
    Thanks Mark!

    When he puts it that way, the cost of that diverting valve seems a lot lower now doesn't it?
    "If it was easy, they would have called it PV."
  • Mad Dog_2
    Mad Dog_2 Member Posts: 3,471
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • hot_rod
    hot_rod Member Posts: 15,048
    a dual coil solar tank

    could make that a bit easier to harness. But I to question how much solar you will have from a 4 panel system to do much heating? Depends on the sunshine you recieve, DHW load, building load, etc.

    Check the SRCC rating for your panels to get an idea of how much energy you can expect in various conditions and in different areas of the country.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Martin Romero
    Martin Romero Member Posts: 40
    Charging Radiant Zones with Solar

    Thanks to all of you that have responded to my questions.

    Yes, Radman, this is a system that will use the radiant zones for heat storage. The radiant floor (w/ Raypak 0090 hot water boiler) system already exists, and the solar portion is retrofit.

    I have 160 sq. ft. of collector already mounted. I'm at the point of tying in the plumbing, and my big question is around the importance of bypassing the boiler when solar heat is sufficient. I got, from the responses to my question, that one of my concerns should be using precious BTU's to heat the boiler itself, so I'll reconsider that option. Space and complexity are a concern, so I have to weigh the pros and cons. Are there other cons to NOT bypassing the boiler?

    The tanks shown in the diagram are strictly for Domestic Hot Water.

    By the way, I live just south of Albuquerque, NM where our Clear-Day percentage is quite high.


    Many Thanks, and Happy New Year to all,

    Mart
  • hot_rod
    hot_rod Member Posts: 15,048
    Another option

    this shows the boiler and solar as secondary loop. It will cost an extra pump or two but has much better hydraulics as every pump can be sized correctly.

    Depending on the lift, you may not need a high head pump? I assume you have return temperature protection for the copper boiler. I didn't show that in the drawing.

    Consider a drainback specific solar control, it could run a high head pump at variable speed, or start two pumps and drop one off after several minutes. I believe the thermostat function in the control could fire the boiler if the solar is not enough. Resol and tekmar are a few brands of drainback controls.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Radman
    Radman Member Posts: 70
    Seriously high temperatures...

    I used your current info and ran T-Sol based upon your collector absorber area and existing tank size using Albequerque climate data. Not taking in to consideration the floor as storage, you are going to see really high collector temps even in the winter. If you intend to utilize the existing tanks as DHW only, what are their sizes?

    To answer your question about other reasons not to flow through the boiler, the primary is heat loss to the flue. The boiler will draw heat off the solar loop and lose it through the flue via convection. Regardless of where you divert solar flow after satisfying the DHW tank(s), I don't recommend using the floor as calcuated storage in the system.

    Even with drainback systems, the collectors will stagnate. While there is no fluid in the collector to degrade, your collectors will see temps in excess of 300deg in the summer. Heat is the killer, so plan to use it wisely.

    I noticed that you have no mixing valve on the radiant side of the boiler, so what is the system design temp for the floor? In your case, the average tank temperature is 162deg annually. That is high, so expect solar temps from the collector to be on par where they enter the floor system.

    Most important here to me is that you intend to use the floor system as storage. In seminars I have attended with Robert Bean, we have learned that the specific heat of concrete is closer to air than H2O. Even if you look at various engineering baselines for concrete with stone Vs H2O, you find a great difference between the specific heat of concrete and water. That being said, it is difficult to calculate (not impossible) the storage capacity of the concrete.

    Also, you should consider that in NM, with high insolation you will be passively warming the structure during the day at the same time you are loading the floors with heat energy. This makes a system difficult to control and prone to overheating. Typically, we reset our heating curves at night to reflect lower heating design temps for setback. In this case, using stored energy from the solar at night would be more efficient. At your current configuration, your system efficiency (in T-Sol) is 14%.
    I would consider first loading the tanks in parallel, and drawing heat from the main solar tank via your heat exchanger (that you already apparantly have) to the radiant floors.

    There are many ways to skin the proverbial "Hydronic Cat", so expect many different takes on this. I simply believe that when using solar in a space heating configuration, utilising the energy to it's fullest is a priority. If I were running simulations for a project of my own, I would at least want to see that efficiency number double before I settled on the number of collectors & storage.
    That being said, let's see what else is suggested...
    "If it was easy, they would have called it PV."
  • Radman
    Radman Member Posts: 70
    Is that Hydronicad?

    Hi HR,
    Are you using Siggy's Hydronic Design Studio? I have been using Autosketch for drawings and it stinks. Any reccos here?
    "If it was easy, they would have called it PV."
  • hot_rod
    hot_rod Member Posts: 15,048
    Yes, HydroniCAD

    I think Siggy and Mario are upgrading to a more powerful engine for it. Possible it will have electrical schematics and more pre-built components.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • hot_rod
    hot_rod Member Posts: 15,048
    If you prefer 3 way valves

    and could get a dual coil tank, this is a nice simple concept. Again boiler return protection would be needed and potable water connection to the solar tank.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Martin Romero
    Martin Romero Member Posts: 40
    More on my design

    Heating Helpers,

    I’m grateful for the generosity with which you give your time and talents in assisting others (including myself) on this forum. Special thanks to you, Radman, for running the numbers in T-Sol and to you, Hot Rod, for including the drawings

    With your help, I’ve definitely decided to go with the bypass loop. I fully get that the losses associated with routing through the boiler would be too great. I will use a 3-way motorized valve (which I assume is the same as a zone valve with three ports; A, B, and AB)?

    I want to share a little more about my system:

    My four (4 x 10) SunEarth Empire collectors are mounted in parallel at 50 degrees from the horizontal (latitude plus 15).

    The DHW side of the system is second in the priority line; if/when the radiant floor is satisfied, the three-way motorized valve (Caleffi 1”) closes the radiant side and opens the DHW side. I plan to have 50-80 gallons of storage that will feed my 40-gallon gas water heater. The double-pumped system will work with the “quad rod” HX from AAA solar in Albuquerque.

    The Taco circulator that came packaged with the boiler is designed to deliver 7.5 gpm

    The Radiant Floor bypass loop will use the Carlson 3.5 Stainless, Flat-Plate HX.

    The solar loop will use the Taco 009 or 0011 high-head circulating pump.

    Thanks, Hot Rod, for including the drawings. I understand them for the most part. As far as using the dual coil system, I don’t have enough room for ample water storage, so I am going to try out the system using the zones as storage; I know there are some risks involved, and if it turns out to be too much trouble, I’ll try some other strategies. By the way, there is return temperature protection on the boiler in the form of a tube that routs to the circulator from the boiler (from my reading in Dan’s book, I understand this to be the protection you speak of).

    Radman, you’re right; there is no temperature-sensitive mixing valve on the system. There is only a ball valve that connects the return and supply lines. I’ve never messed with it; I think I would need to install another thermostat in line to really be able to monitor the supply and return temperatures. There’s only one thermostat, and it’s on the boiler body. According to the Boiler installation manual, the system is supposed to work on a Delta T of 20 degrees. I’m not fully up to speed on my understanding of the system, but I’m reading “Modern Hydronic Heating” in hopes of getting a better handle on things.

    I understand that H20 is the best storage medium, and Chuck Marken at AAA solar did some rough calculations to compare the storage capacity of my floor vs. H20. Honestly, I don’t remember much of what came out of that. I just trusted that the system design would be OK, but I’d like to understand it a lot better. My radiant floor system was installed without insulation under the floor (it does have perimeter insulation); I consider this an advantage, since more mass will be available for storage.

    I don’t really expect the summer temps to soar so high. I’ve accepted since the beginning that I will have to cover two or three of the collectors by summer to avoid this.

    Would one of you experts consider a paid phone consultation? I could really use the help in finalizing my plans.


    Peace,

    Martín Romero
  • hot_rod
    hot_rod Member Posts: 15,048
    your plan

    is very close to what the boys at Cedar Mountain in Santa Fe install with the building mass as the storage. Maybe you have chatted with them?

    There are plenty of way solar, as Radman noted. The more we discuss solar the more we all learn. Let us know how you progress.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Radman_2
    Radman_2 Member Posts: 13
    How about some pics...

    Hi Martin,
    Sorry for the delayed response here, got a little busy with end of year to-dos. You could certainly post some pics of what you have currently, I suggest reducing the picture size to about 900x1200 pixels. They post much better here than full size prints. You can certainly contact me toll free at 866-98SOLAR. The out of state design route is challenging because I would not be able to site visit, but I am happy to consider your current situation and lend suggestion. I can also help calc the storage capacity of the floor, though I am still biased against this method. An engineer friend of mine is testing a similar scenario right now at his house here in CO, but final results won't be available until Spring.
    I am glad you are at least aware of the need to de-tune the array in the summer, this is always a consideration and a good one. The T-Sol simulation is real-time, so there are indications of high collector temps at your latitude in the summer. Another good contact is Dave Woycio of Metro Solar, aka MetroMan. You can reach him via the www. He is an excellent drainback resource.
    Tell him I sent you.
    Cheers,
    Bo
  • hot_rod
    hot_rod Member Posts: 15,048
    Cover the DHW load first

    with 4 panels and 50- 80 gallons of storage you will recover that load quickly, then go to the radiant. If you prioritize the radiant, you may never get to the DHW load in cold conditions?

    Also, many of the new solar controllers always "keep looking" at the primary load when they switch to the second load.

    For example... If the first load was DHW, and the second a swimming pool. It's possible the pool would never be satisfied and the control would never go back to DHW when it's temperature dropped.

    For that reason the control keeps looking, or sampling, the priority load and will always go back satisfy it first.

    Also, regarding your boiler. Many of the small Raypak boilers did include a small copper tube bypass lop. It was intended to be a simple return protection method. Usually with a small lever handle ball valve to adjust the bypass. They do work to a point, and under some conditions. But without the ability to sense temperature it was a bit simplistic.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Tombig_4
    Tombig_4 Member Posts: 45


    You DON'T want hot water circulating thru the boiler during air conditioning season.
This discussion has been closed.