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solar assisted radiant floor vs. forced air efficiency savings comparison

solarmann Member Posts: 1
We are pricing a solar assisted radiant floor system for a client and would like to know i fthere is any data on expected energy savings when retrofitting a forced air system to radiant floor - anybody out there know?



  • BobbyG
    BobbyG Member Posts: 79
    energy is energy except...


    The only differences I can think of ...

    There would be more air infiltration due to more convective currrents in hot air vs. radiant (depending on air tightness of home - could be 5-30% difference in heat loss).

    The efficiency of the heating unit itself.  If you would be using a condensing boiler as backup - the lower temp. is conducive to better efficiency. (est. 98%) - but solar is less expensive by only paying for the electricity of the pump (unless DC pump is used)

    If you are using radiant sizing software and not familiar with forced air, ask someone what factor they use for infiltration and compare your software (radiant) vs. forced air infiltration.

    BTW, are you the Bloomington solar beer guy?

    Hope this helps,

  • thoughtfulTom
    thoughtfulTom Member Posts: 18
    understanding solar savings


    I think you are asking two questions. One is how efficient is radiant vs. forced air. That is discussed all over the place (not answered, but discussed).

    Then the other question - efficiency of solar. Depends on how you size the solar and where you live. Folks in WI are happy to get 50% solar fraction. I live in cold-climate AZ and we aim for 75%

  • CMadatMe
    CMadatMe Member Posts: 3,086
    Radiant vs Forced Hot Air

    The anwser to your question is in a heat loss. You need 35% less energy to heat a space radiantly versus any other type of hydronic/forced air system. There are many reasons for this. First is infiltration. Since their is very little to no convection in radiant you have slowed down the rate that btu's leave the space unlike baseboard or forced hot air which live on convection. Secondly, in radiant we keep the heat where it's needed. Where you are at your comfort level. In the other systems we stack the ceiling with higher temps leaving cooler temps at our comfort level. Lastly, basic rule of thumb for savings. For every 3 degrees I can run a heating system below 180 degrees I can save 1 percent of fuel. What's the average radiant system run at compared to baseboard? I realize you cannot use this formula with hot air (unless your doing hydro-air) but the heat loss tells it all. It means a smaller heating plant which a potential customer can see thus meaning burning less fuel. 
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Radiant Advantage
    1. Using solar energy with radiant will exaggerate your solar collection, because of the greater efficiencies of radiant heating. This is more evident with higher  ceilings. With radiant, if you only get 25%to 50% of your heating needs from the sun, that energy will be down at the floor where you need it, not up at the ceiling where you don't even notice it. 80 degree water will also be comfortable in a radiant system, while 80 degree air blowing around from a forced air system will feel very cool, because of the wind chill.

    Thanks, Bob Gagnon
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Good answer Bob

    Something  to consider is a radiant floor will bring the water temps down in the solar storage tank so there is a larger delta tee between collector and  system water. This means the collectors can harvest more heat. In the Winter my system water in the storage tank will be about 105 at the end of the day. When the heat turns off in the Summer I can have as high as 170. 
  • hot_rod
    hot_rod Member Posts: 16,286
    Stick with the comfort angle

    there has yet to be an accepted study showing the savings of radiant over FA. I'd be reluctant to put anything in writing or suggest a % decrease in operating cost.

    Remember FA equipment has become very efficient also and has the ability to cool, de humidify and humidify. These are very important parts of comfort. As are air changes in a tight home. It's more than just the temperature of the floor or room.

    Hands down radiant wins for comfort if properly installed and controlled. Zoning and micro zoning is possible with radiant. Panel rads, towel warmers, etc can blend in with radiant or hydronics.

    Use your solar to pre-heat DHW first, it's a year round load. If budget, and roof area allows start picking away at some of the heat load.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Just to add

    In my case I chose to prioritize the RF heat first in my system for the same reason I spoke of above. My storage tank is 120 gallons so by the time I heat that up the day is done and I got little radiant floor heating done. Plus the delta Tee to heat my shower water is smaller than the delta tee to heat my radiant floor. Since  I can harvest more BTU's with a wider delta tee. I can save more money. My goal is to SAVE MONEY. It appeals to the Scottish side of me. Of course I understand my goals can be different than other peoples goals. It can get really complicated deciding how to multi task with solar. 
  • Your Right Wayne

    You will find that most people will disagree with you and say just use solar thermal for domestic hot water, because you don't collect enough hot water for heating in the winter, it's the industry standard. But their wrong. I have two sets of collectors, one high temperature domestic and one low temperature for radiant heating. I can see side by side, how much more energy the low temperature array can harvest, for all the reasons mentioned in posts above. You will collect more energy hour to hour by keeping your tank temperatures low, and your tank temperature won't max out early in the afternoon, halting all harvesting when your tank reaches, say 140 and the sun starts going down about 1 or 2 PM. Your collectors won't be able to make water hotter than 140 in the early afternoon, but it will make 130 degree water for another hour, 120 for another hour, 110 for another hour all the way down to 75 degrees. You won't be able to stick your chest our and say "My solar tank got up to 150 degrees today and it's February" but you will be able to collect two to three times as much energy at the lower temps. I'm sure you know that a BTU at 80 degrees is the same value as a BTU at 140 degrees, it's just easier to collect. We have to find a way to convince other contractors of this. A lot of people talk about thinking outside of the box, but in reality they just do things the same way everyone else is doing it. Sarah Palin had a great quote "I'm a fisherman and only dead fish go with the flow."

    Thanks, Bob Gagnon
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • TonyS
    TonyS Member Posts: 849
    The problem with space heating with solar

    is the storage. I had my roof laid out for 5- 20 tube vac panels. I originally installed 2 panels for domestic and have been studying the weather. The 2 panels do really well in the spring ,summer and fall but it is not uncommon to go 3 weeks without sun in winter. I heat the home with Geothermal HP w/w and have come to the conclusion I would be better off using the rest of my roof for PV panels and bank that electric in the grid. Its hard to compete with PV when you can use the grid for storage for free. Solar tanks are expensive and you still have a very limited time you can store. It would be great if we could collect our solar in the summer and save it till winter( and they are doing that with ground store systems) but for now those system are unrealistic. 
  • hot_rod
    hot_rod Member Posts: 16,286
    every job is different

    why such a large DHW tank? If you had, let's say a 60 gallon, it would recover quicker and present colder water to the collector than a radiant load.

    If you incoming domestic water is 55F, maybe lower, then the solar tank will present that load to the solar. How low can you use the radiant tank down to?

    If you had a small DHW, then go to the radiant I think your SF may be higher because you have a much colder tank to start with.

    Many of the new solar controls have a priority function. You pick 0, 1, or 2. then the control looks at the priority load.

    In handles the priority load, then goes to load 2 but looks back always at PRIO 1. in winter the tSP is used with a 2 minute setting, for example. This gives the control a 2 minute "window" to raise the temperature in the collector, to come back to the DHW or higher temperature load. The tRUN is set to say 15 minutes. That is how long it will go to the secondary load after it decides it cannot load the priority anymore.

    After 15 minutes the control stops the pump to "re-check" if it can load priority anymore. If not it goes on with the secondary load. All these are adjustable parameters depending on what you are trying to do, the size of the storage and amount of sun, etc.

    The control can do a lot more to optimize the solar use than a manual yes or no mode done manually.

    If you get a chance look at the load comparisons for two homes, in I-dronics 6. A small house with 4 collectors, and a large house with 8 collectors. Two houses in upstate NY, two in Colorado.

    During the colder months the larger system, in NY has double the SF of the smaller. BUT on an annual basis the 4 collector system meets 23% of the total load, while the 8 collector system meets 37%. The larger array did not double the energy savings. One reason is lack of summer load for the larger array. For 3 months the larger array provided more than 100% of the load. So the economics of the smaller array, which only hit 100% one month, per square foot of collector is greater than the larger.

    In Colorado the smaller system, due to more sunshine available, has 4 months of over 100% SF. The larger 8 collector array only 2 months. The larger home in Colorado with a larger heat and DHW load, reduces the summer excess, with just July and August at 100% SF So the SF for the larger array is 26% vs 15 for the 4 collector.

    Now if you have a pools or other use for excess, maybe a geo ground loop, then excess my be put to use. Collectors on the roof in the summer with no job to do.... be careful and crunch the number!

    Bottom line when it comes time to sell these Combisystems is run the numbers for all the custom systems. If the load, in the summer months, is far less than the potential of the large arrays, then the economic advantage, which is what many homeowners base their "solar dollars" on, may not look so good.

    You will see this better with the color bar graphs, in I-dronics 6, hard to explain with just text.

    If you are just messing with your own home and have the money, storage and roof space, buying at cost, no labor charge, that's fine. But when you put pencil to paper and run the actual calcs it may surprise you. Generally customers want more than just a roof covered with collectors.

    Show me the money! If the payback for larger systems because of poor energy utilization is non existent, or 50 years down the road, you and the customer should know that.

    Here is a SF graph, an F-Chart simulation from I-dronics 6.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • PV Vs Thermal

    If we can store thermal energy for a week or so , in our houses and in our large storage tanks, it will take care of our hot water needs for most of the time. My domestic tank stays hot for about a week and I could insulate it better. The problem with heat pumps in my area are the HIGH electric rates. commercial rates are 80% above the national average. How are your electric bills? My large storage tanks cost only about a couple of thousand to construct. The prices I see people paying for geothermal heat pumps are a LOT more than that. PV is OK but I do plumbing not electrical, there is not much I can do to make electrical systems more efficient. PV seems easier and in my area there are a lot more rebates and incentives for PV, but in my area we use about 17% of energy dollar for domestic hot water and about 30% for electric. Since most people only do solar thermal domestic, electric seems like a better deal, so that's where they give the best rebates. But when you add in 52% of our energy dollar for space heating, our hot water needs are almost 70%, and with 30% for electric if we can harvest and use thermal solar hot water for half of our hot water and space heating needs, we will be better off than if we collected all our electricity. The PV-Thermal debate may be over soon, when we get integrated combination hybrid hot water and PV collectors like they use in England. They say keeping the PV panels cooler will produce more electricity, about 10% more, the most efficient way to do that is run water through them and store it in a tank. I'm assuming that cooler radiant temps of about 80 degrees might cool off the collectors better and help them produce even more electricity.

    Thanks, Bob Gagnon
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Solarstar
    Solarstar Member Posts: 82
    Re Cooler Pv and Thermal

    Pv Output Vs temp: Speaking from experiance the output is noticeable as the graph shows ,also A nice cold clear winter sky produces real well(above rated output sometimes 15% more ) .  Paul
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