best heating unit for small radiant with solar backup?

Discussion in 'HVAC Heating & Cooling' started by essentialagenda, Aug 21, 2008.

  1. essentialagenda

    essentialagenda New Member

    Messages:
    2
    we have just built a new guest house, 600 sf with extra insulation R19 walls, R38 roof, and rigid under concrete, small windows, trellis on south side, overhangs. The pex tubing is in the concrete slab and ready to be hooked up to some type of heating unit. (circuits are ~280 lf, 1/2"). This summer the house has been very comfortable despite the heat outdoors without AC.

    the question is what heating unit would be the best? The plan is for the system to have solar thermal panels to provide backup heat into some type of storage tank (maybe superstor but very expensive, others?) and also to allow all excess heat from the solar panels to dump to the pool to provide some heat there. The heater for the house radiant and DHW should also allow for some occasional heating of the pool heating manifold (i have 1300 lf of pex in the pool shell).

    what heater makes sense?

    -sm mod/cond boiler (though i've been told that a condensing boiler requires expensive annual maintenance that doesn't justify the 10% increase in efficiency over a mod boiler)
    -mod boiler
    -tankless water heater ?
    -high efficiency water heater?

    We're in northern California. the climate is very mild. Not that much heat is needed.

    Also, is an outdoor reset very important? I've heard differing advice.

    thanks
  2. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Look into Baxi mod con boilers. They have a wall hung that will give you heat and domestic hot water. Skip the solar, it will cost more for the equipment and labor than you will ever make back in savings.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,153
    Location:
    New England
    YOu need to do a heat load analysis of the building and decide how much heat you might want to add to the pool. In a mild climate, just lights, cooking, etc. might provide enough heat most of the time with no additional heat. A WH with dual coils could be more efficient than using an external heat exchanger (one set for the boiler and one for the solar).

    Running a boiler to high temps for short durations is not very efficient. RUnning it longer at lower temps is the most efficient. An outdoor reset adjusts the system to maximize the efficiency. What that delta is, depends on the system and how it is set up. A modulating boiler fits in well with the outside reset.

    The smallest boiler will probably have more capacity than you need in a mild climate for the house. You probably need more help than can be provided here to optimize a system.
  4. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    Solar is a great way to save energy in your climate, but i would not think of it as a "backup" heat source.

    Here is what I would do
    1. Tank style electric hot water heater with extra insulation.

    2. Roof mounted box style Solar heater. This could be used to pre-heat water going into the hot water heater and when it get's hot enough the solar can be used to heat the hot water that is in the tank.

    3. Use solar to heat up the slab. Don't consider the slab as the main heat source, but rather as an extra heat source.

    Plumbing wise, I would use a plate heat exchanger to transfer the heat to the hot water heater. It would be placed before the intake to warm the water going into the heater and then use one of the several method's to circulate hot water back from the faucet area back threw the heat exchanger when the solar is hot enough to heat all the water up to the desired temp.

    The solar loop would have a pump, small tank, tempering valve for floor.
    and a boiler of some type as backup.

    Edit:
    The pool could be heated when nothing else wants heat by using a valve and a heat exchanger.
    Last edited: Aug 23, 2008
  5. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    www.caleffi.com down load Idronics 3 and give it a real thourough read through. If you still think you can heat your home or get anymore than 30% of your hot water out of a typical 3 panel solar set you didn't read thouroughly. Adding additiona panels increases the yield but the expense far outweighs any real gains. After all if the system is never going to pay itself back what's the point. Electric water heaters no matter how well insulated are a terrible choice for heating applications from both an energy use stand point and longevity.
  6. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    Solar hot water heaters are actually mandated in some parts of the world and they DO work. It's all about climate and cloud factor.

    I am sure I could get close to 90% if I lived in a desert and by using day/night heat storage.

    There is also the solar experiment in Canada that uses heat during the summer to heat several acres of ground to near boiling and then heats all the houses in the area during the winter. It's not a heat pump, just Hot water.

    It's all about calculating the ROI (return On Investment) and then making the decision. You are correct in that some solar projects never pay off if you factor in interest and the time/value of money, but others pay off quite quickly.

    I think essentialagenda should be congratulated for building such a well built house. The insulation used looks quite high for the climate zone it's in. The insulation costs will more than pay for itself over the life of the house.
  7. BigLou

    BigLou New Member

    Messages:
    138
    Oh great master plumber who knows all. Tell me where exactly it says I can only get 30% or less of my hot water heating needs ? I think you need to re read it and understand their assumptions and re think what you just said.

    I would like to hire you as my stock broker and retirement agent, based on that comment I am going to assume you have the next 20-25 years of fossil fuel pricing figured out. With that kinda outlook I don't know what you are doing working as a plumber I am sure Warren Buffet would hire you in a second. There are also reasons beyond money why some one would invest in something. is everything you buy based on pay off ? or do you sometimes buy things because you think its a good idea and you just want them ?

    I hope this does not turn in to another " I am the plumber so if I say it's so then it has to be true" arguement. kinda like not understanding the difference between a condensing boiler and condensation taking place in the exhaust pipe

    Lou
  8. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Nope. I won't argue with either of you. Yes I have installed a pile of solar systems over the years and the truth is that there's no rocket science here. In fact other than some very slight improvments in panel design over the past 30 years the biggest change has come in modular control systems that eliminate the old mix and match proplems. Now you're gonna ask me why would I install something that I have such little faith in. Two reasons really. Number one Money. Plain and simple. Folks that want solar are going to buy solar and since they are determined to have it they might as well buy it from me. Number two. I would just as soon have them come to us for installation because we have many years of experiance in design and installation.
    Now I note that neither of you is either a plumber, an hvac professional or a hydronic engineer, but I'm pretty sure that you both researched the subject. There are piles of good and bad information online. If you indeed happen to live in the middle of the desert or have an ideal solar location and climate than you may very well do better than 30%. However that 30% is an industry average for the nation. Yes, you can build huge insulated storage tanks and do many many other things to help retain the heat gained by the panels, but the cost of doing so overrides any savings realized. Lastly, any time you read about state or government sponsored solar installation just remember that cost is usually not a issue. Most if not all of the componants are donated by manufacturers. In short, they can afford to spend a half a million to make a couple thousand.
  9. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    [ There are also reasons beyond money why some one would invest in something. is everything you buy based on pay off ? or do you sometimes buy things because you think its a good idea and you just want them ?


    This is in fact the rational that most solar buyers have and there is indeed nothing wrong with that.

    I hope this does not turn in to another " I am the plumber so if I say it's so then it has to be true" arguement. kinda like not understanding the difference between a condensing boiler and condensation taking place in the exhaust pipe


    I guess you have me there cause I have no clue what you are talking about. Why don't you educate me?
    Can I have the Buderus, Viessman or Lochnivar tech rep call you?

    Lou[/QUOTE]
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