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Viability of evacuated tube radiant floor for supplemental shop heat?

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  • Viability of evacuated tube radiant floor for supplemental shop heat?

    I recently found these forums and am having a great time reading through these various posts and gaining a much deeper (and unbiased) understanding of these various technologies. I recently received some terrific feedback from several people on the off-grid section so I wanted to solicit some opinions here as well. One of the ideas I've been considering is some evacuated tube panels for radiant heating a shop floor but I really have no idea how efficient that really is or if it makes since in this application

    The details...


    I'm in the beginning stages of planning/designing a new wood shop for our very rural weekend (future retirement) property. We're on a 40 acre wood lot and I have a band saw mill that produces a lot of wood waste so we have a practically unlimited supply of the original solar heat source - wood. The building will be approximately 24' x 32' w/ 10' ceilings and the property is located in the Hudson Valley of NY with very long, cold winters at 47 lat.


    Winter heating requirements should run about 30,000 BTU hour and I plan to primarily heat the building when in use with a wood burning furnace. The building will most-likely be timber framed with SIPs panels so very well insulated with insulated concrete floors, proper frost wall, etc. The gable roof will be SW facing and I've calculated my kWh/m2/day for Nov at 3.25, Jan at 2.8 and March at 5. The garage doors will also be SW facing however so I will not have much room on that facade for windows/passive heat and I would want to avoid skylights on the SW facing roof to keep summer temps low as we generally don't need much AC there.


    I know from experience what kind of heat a 150,000 BTU stove can put out but my primary concern is those ealy Sat mornings when the building (and concrete slab) have been sitting at 20 degrees F for the past 5 days so I'm weighing some supplemental heating options. I won't be running water to it so freezing is not a concern. Our house is heated with oil but we do get a propane delivery for cooking but it is expensive and this building will need its own fuel storage.


    Anyway, one of the things I've been considering is a couple of evacuated tube panels on the roof that could heat a closed glycol loop but I have no idea how much it will cost or even if it would do what I need. I know radiant flooring can work efficiently with cooler water temps than a traditional hydronic system and I won't really need to maintain a specific temperature when I'm not there but starting from 50 degrees every Sat morning when I fire up the stove would be a lot faster and comfortable than starting at 20 F. If these systems are relatively low maintenance and installation costs are not much more than what a traditional supplemental heating system would run, it may make sense. Or maybe not, I really have no idea which is why I just wrote this essay, lol. If you made it this far - thanks!

    EDIT: Also open to a DIY solution as discussed here as well... https://www.solarpaneltalk.com/forum...hop-floor-heat

    EDIT #2: https://www.builditsolar.com is an awesome find from the thread above. Thanks Mike90250
    Last edited by nybhh; 12-06-2018, 06:57 PM.

  • #2
    From what I've seen, evacuated tubes fail too often. And they are a "high heat" device, where you only need "low heat" You will have to insulate the slab from the ground, with pea gravel or something, to break the thermal path, or you are going to be heating a lot of dirt. Getting the floor temp up to 70F would be needed to take the chill off. You need to lay PEX tube in the slab before pour, insure it's got enough thickness to survive the large "gaps" the pex is going to make in the slab. Make sure there is good thermal expansion provisions in the slab - seams, fiber reinforcement, what ever.
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    • #3
      Thanks Mike. I suppose you are right regarding conventional collectors over evacuated tubes. Most literature recommends them for colder clients but that is also to produce much hotter temps than what is needed here.

      I've got some experience with radiant flooring in a more traditional applications and tend to dismiss them for occasional/periodic use due do the heating lag but this application would be more similar to the solar providing a baseline freeze protection with supplemental being used to bring temperatures up to a comfortable level, although system failure, snow or cloudy weather doesn't result in frozen pipes here so a little less mission critical which makes some of the DIY options interesting.

      I'm probably looking at about 350 square feet of south/southwest facing (200 true north) roof and perhaps another 80-100 square feet of vertical surface at that same orientation if I only use 1 large carriage/garage door. Just not sure how to calculate if that would do the job or be cost efficient. I'm a capable "DIY-er" and can also weld and fabricate with metal so some of the options on the build-it-solar site are intriguing. I'm going to go through those this weekend and see if I can find some reference for calculations.

      Thanks again.

      Comment


      • #4
        I also live in the Hudson Valley. Just think about the the last few month's weather that we have had. Dam awful, even if we had a sunny winter the average amount of sun would only be about 4 hrs per day. Not enough to give you any usefull amount of solar hot water for heating purposes, or even domestic hot water unless the system is grossly oversized.
        I have a solar domestic HW system and a 5.2 KW PV system that has been installed for 10 years. I have kept records and I can tell you that from November through the end of February there isn't much solar energy to capture "just when you need it the most".
        Save your money on the solar thermal space heating and spend it on an efficient gasification woodboiler connected to radiant floor heating.
        The SRCC solar panel rating web site has all the information that is required for calculating output.
        Last edited by LucMan; 12-07-2018, 05:36 PM.

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        • #5
          Thank you LucMan. That is exactly the sort of reality check I was looking for. I'll be curious to run some numbers but not at all interested in covering the whole building in collectors for a 5 degree temperature increase. My wood pile looks more like mid January right now than early December. Hang in there, only 4 more months!

          Comment


          • #6
            Originally posted by LucMan View Post
            I also live in the Hudson Valley. Just think about the the last few month's weather that we have had. Dam awful, even if we had a sunny winter the average amount of sun would only be about 4 hrs per day. Not enough to give you any usefull amount of solar hot water for heating purposes, or even domestic hot water unless the system is grossly oversized.
            I have a solar domestic HW system and a 5.2 KW PV system that has been installed for 10 years. I have kept records and I can tell you that from November through the end of February there isn't much solar energy to capture "just when you need it the most".
            Save your money on the solar thermal space heating and spend it on an efficient gasification woodboiler connected to radiant floor heating.
            The SRCC solar panel rating web site has all the information that is required for calculating output.
            Big +1 on that one. For one thing, Nybhh will need about an order of magnitude more solar thermal surface than you're talking about to begin to see a noticeable bent in your space heating load.

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            • #7
              Space heating with solar just doesnt work well as the time you need to heat the most is when the resource is the worst. It works great in the summer but not many folks want to heat a space in summer. John Siegenthaler (search for him) is recently advocating air to water "mini splits" hooked into radiant heating. The COP is quite high and depending how solar friendly your state is you can put in solar panels and bank watts in the summer to heat your shop in the winter. I do agree if you have free wood its hard to beat a wood boiler. The one caveat is unless the wood is seasoned well, modern gasifiers dont like to burn poorly seasoned wood. Slab wood does dry quick but it still need several months at a minimum to really season.

              Comment


              • #8
                Originally posted by peakbagger View Post
                Space heating with solar just doesnt work well as the time you need to heat the most is when the resource is the worst. It works great in the summer but not many folks want to heat a space in summer. John Siegenthaler (search for him) is recently advocating air to water "mini splits" hooked into radiant heating. The COP is quite high and depending how solar friendly your state is you can put in solar panels and bank watts in the summer to heat your shop in the winter. I do agree if you have free wood its hard to beat a wood boiler. The one caveat is unless the wood is seasoned well, modern gasifiers dont like to burn poorly seasoned wood. Slab wood does dry quick but it still need several months at a minimum to really season.
                I concur with the air to water heatpump systems. There are quite a few mini split manufacturers that market systems in Europe and Canada but few in the US.
                Daikin stopped supplying the Altherma "in the US" several years ago , in my opinion it was an excellent system . Mitsubishi, Panasonic, Fujitsu all manufacture them but are not marketed here. The only one's that I can think of is SpacePac and Sanden.
                A 10 KW PV system would likely get you close to net0 for heat and air for the year (for the average house) if net metering is available.

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                • #9
                  Thanks you all for the comments and input. I'll have to do some research on these mini splits. I think it's interesting though you guys seem to be advocating for a wood-burning boiler over a wood-burning furnace. I'll run PEX in the slab regardless as it is a minimal cost increase to keep future options open but I've not had good luck with on-demand" radiant heating in concrete slabs which is why I was leaning towards forced air for the shop.

                  To be clear, I agree that radiant is much nicer heat and preferable to forced air for a draft-free comfortable environment and we use radiant heat in our house but in a slab that is unconditioned for long periods of time, I feel like it just takes too long to bring it up to temperature where forced air can warm a 600-800 ft space up pretty quick. If it were a space that was being heated all winter, no doubt but I'm just not sure about an occasional use space that will be unheated for a week or more at a time.

                  Comment


                  • #10
                    Once you have the wood burning boiler you can pick up used fan forced unit heaters to crank out quick heat. I agree radiant heat only works well for steady heat. Of course in a workshop you really do not need to heat the air as much as the floor.

                    Comment


                    • #11
                      Originally posted by nybhh View Post
                      Thanks you all for the comments and input. I'll have to do some research on these mini splits. I think it's interesting though you guys seem to be advocating for a wood-burning boiler over a wood-burning furnace. I'll run PEX in the slab regardless as it is a minimal cost increase to keep future options open but I've not had good luck with on-demand" radiant heating in concrete slabs which is why I was leaning towards forced air for the shop.

                      To be clear, I agree that radiant is much nicer heat and preferable to forced air for a draft-free comfortable environment and we use radiant heat in our house but in a slab that is unconditioned for long periods of time, I feel like it just takes too long to bring it up to temperature where forced air can warm a 600-800 ft space up pretty quick. If it were a space that was being heated all winter, no doubt but I'm just not sure about an occasional use space that will be unheated for a week or more at a time.
                      "On demand" heating of a concrete slab is somewhat of an oxymoron. On demand implies very quick times between energy use and consumption such as an on demand water heater that heats the water immediately prior to use and so has as a principal claimed advantage of eliminating or drastically reducing tank or standby losses.

                      A concrete slab has a fair amount of thermal mass. It takes a lot of thermal energy to increase its temp., and depending on the output of the heating device it may take a long time for the heat input to the slab to conduct through the slab and to the surface where the heat is then transferred to the air in the structure and the rest of the structure by a combination of convective and radiation heat transfer. There is nothing instant about the process of heating a dwelling by what's commonly but at least partially incorrectly called radiant heating. Such systems are toasty for the feet, but timing the heat release to the slab in such a way that the slab surface will be warm enough to accomplish the required level of thermal comfort WHEN needed and in a fairly uniform way and/or in whatever spatial distribution pattern may be required is not as easy as often thought. Slab heating also kills most of the advantage of thermostat setback. It takes about as long for a slab to cool as it does to heat up. Harder to get it right when you consider that each (winter ?) day's temp. will be different while the slab's time constant and something called the Biot number (the ratio of the slab's conductive thermal resistance to the convective resistance between the slab and the air above it is mostly constant.

                      Getting radiant heating right in terms of heating source/element arrangements/layouts, slab thicknesses, side/bottom of slab insulation, possible floor coverings and a lot of other things takes some information and experience to get right. Most DIYers are not equipped to get it right.

                      Comment


                      • #12
                        Radiant floor tubing definitely not the best idea for occasional use or setback temps. in concrete, it can take 2 -3 days to stabilize the slab temps.
                        On the other hand low temperature water constantly circulating through the pex is extremely efficient. I installed a system in a horse barn several years ago where the customer leaves the thermostat at 55 degrees, this temp. is comfortable and efficient . The slab water temp is close to 70 degrees when the heat is operating and doesn't put much of an extra load on the oil boiler once the slab is up to temperature. With the proper controls if you are using a wood fired boiler it would not require much additional capacity to heat this area.
                        I have found that a setting of 60-65 degrees with a radiant system is just as comfortable as a 70-75 degree setting for conventional heating.

                        Comment


                        • #13
                          Thanks guys. Yea, the fact this building is still on a weekend property makes some of the systems decisions a lot more challenging. With everything dialed in just right or if freeze prevention temperatures had to be maintained year-around, a boiler keeping the slab at maintenance temperatures would be very cost efficient and maybe even cheaper than bringing the building up to comfort level from freezing once a week but given free wood heat and a thermostat setback of OFF, pretty hard to make the numbers work at this point. The cost to go ahead and run the pex loops is minimal however so I'll probably do that when we pour the foundation so I can convert it over to a full-time system, regardless of fuel source, sometime in the future when we're able to get up there full-time.

                          Thank you all for your input. Much appreciated.

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