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DHW and Radiant from same large tank?

malik Member Posts: 1
   I live in an off-grid solar powered home in Lakes Region, NH. For DHW and radiant underfloor heating (RUH) we have a 1200+ gallon tank (concrete with insulation and liner) with is heater in summer by solar thermal and excess solar electricity. In winter we need more heat and we have a wood boiler which can lift temperature up 30  - 40 degrees in a day. Both we and wood boiler are getting old, and we can’t find people willing to repair it.
   In winter, the tank loses about 10-15 degrees a day on cloudy days (sunny days raise the temperature, but it can be cloudy for days on end). As long as its above around 105 degrees, we are fine; that’s warm enough for a shower. But that’s not warm enough for RUH.
   My wife would like to use a fossil fuel, like propane, to heat the tank.
   We actually rarely use the RUH as our woodstove keeps us warm on the main floor of our house, but our guestroom in basement is too cold without it being on.  I have been thinking of getting either a tankless water heater or a propane boiler (not sure of the difference). My plan is to set the thermostat so that whenever it gets down to 110 degrees, the propane heat brings the water up to 120 degrees. That way we can run radiant heat if we want it and system will just keep coming back to 120 degrees if we draw it down. 
   Is a propane boiler or a propane Hot Water Heater better for this purpose? I imagine it will take several hours to heat the tank up to 120 degrees. Will a THWH cope with extended burn times. What kind of boiler? It seems to me a combi unit does NOT make sense as our radiant tubing pulls from our warm tank, and warming water from 110 to 120 degrees is a lot less than warming it from 55 to 120 degrees if we used combi unit. How do I find a non-combi propane boiler? Will it let me program it to cut off when tank water (i.e incoming water) hits 120 (the loop would run into tank and come back from tank).  I’ve asked my plumber, but this unique heating system has him stumped, and me too as to upgrading it to make it easier on my wife and I. Any thoughts?


    EBEBRATT-Ed Member Posts: 12,654
    Don't use a tankless WH under any circumstances.

    If you use a propane Mod Con (you have the perfect application for this). So the decision would be a Mod Con or a combi. I think you will want a combi. Separate the DHW from the big tank and the combi will run when you need DHW. This way you don't need to heat the big tank year round.. If your HW load is large maybe an indirect tank.

    You could use an outdoor air sensor to bring on the boiler for the RUH. You can pretty much program them to do whatever you want.

    As far a what boiler Viessmann is thought to be the Cadillac but cost $$$$

    The trick is to find an installer and not just any hvac guy or plumber but someone who knows and installs and services Mod Cons

    For them to last they need yearly service
  • malik
    malik Member Posts: 1
    What is a propane Mod Con?
  • malik
    malik Member Posts: 1
    Reading your response again, it seems as if you are suggesting doing away with the large tank and going entirely to a propane combi unit. To my way of thinking, that would be throwing away thousands of BTU's that I get for free from solar thermal and solar electric heat. The tank occasionally gets below 100°, but heating that amount (from say 100° to 120° is surely way less expensive than heating water via a combi from 55° to whatever I would use for DHW OR RUH. As I understand it, a combi heats two streams of water - DHW at say 125° and RUH at say 180°. Again, it seems to waste all the free (i.e. already existing" sources of effortless heat - the solar thermal and the solar electric.
    One option I've even considered is putting more electricity into the tank (since I waste so much on sunny days). I put in 3,000 watts now - I'm just not sure the tank could take 5,000 or 6,000 watts - Afraid the liner would melt, or the hundreds of feet of pex tubing running thru the tank (carrying the heat from the wood boiler and the solar thermal thru and warming the tank).
    Second, sorry, I don't understand when you say I have the perfect application for a mod con, but you still recommend a combi instead.
    It seems just a small, but often steady for hours, heat input from a propane boiler would be the most efficient way to add the amount I need - say putting in at 150° at one end of the tank and taking out at whatever the tank temp is at the other end. Does a mod come give a modest heat for a long time efficiently?
    One thing I recently found was a Westinghouse Fire Tube (now apparently an HTC UFT), but the new models have 80,000 BTU's as the smallest size, whereas the Westinghouse had a 55,000 BTU unit. I think they don't make that anymore but I might get lucky and find one - if that's the sort of thing I'm looking for. Still unsure about that.
  • Jamie Hall
    Jamie Hall Member Posts: 19,954
    edited April 2021
    malik said:

    What is a propane Mod Con?

    As @EBEBRATT-Ed said, this is about as ideal a situation for a mod/con as I can possibly imagine. And most mod/cons are combis, or can be set up that way easily, but not necessarily the other way around.

    You would be using the mod/con primarily to store heat in that tank when the sun wasn't helping you out (and you are so right -- there are days at a time come winter when the sun doesn't help much -- something a lot of the starry-eyed folks don't quite realise!). The boiler would be controlled by the outdoor reset -- which is a fancy way of saying that there is an outdoor thermostat which determines how much heat the boiler needs to create. In your situation, where you have solar input to the tank as well, that could be taken into account quite easily by a simple aquastat which would turn the boiler input to the tank off when the sun was doing its thing properly. I'd plan on trying to hold the big tank at about 110, which would be enough for your radiant floors (they could get away with less much of the time -- so they will probably not be flowing all the time -- but you do need at least sort of warm domestic hot water, too!).

    Now... domestic hot water. None of us are really keen on using the same water -- especially stored in a big tank at the ideal temperature for bacterial growth -- for domestic hot water as for heating. You could use a combi -- which is basically a mod/con with a tankless water heater heat exchanger thrown in for extra bucks -- to do the job. However, I would very seriously consider using what's called an indirect hot water tank for your DHW. The indirect has a heat exchanger inside it, which takes water (with a small pump) from the heat source -- in this case, your big storage tank) and transfers the heat without mixing to the domestic water in the tank. Recovery would be painfully slow with the big tank kept at 110 , but it would get there, and a big enough indirect would store enough water for two people to be happy, I think.

    Edit -- I didn't really answer the question, did I? A propane Mod/Con is a propane (LP) fueled boiler designed so that it can change (MODulate) the firing rate of the propane to match the load on the boiler, and which is designed to be able to accept cool enough return water (which you have) to CONdense much of the water vapour in the exhaust -- which gets you extra efficiency. A good mod/con, in the setup described above, could very easily achieve its rated efficiency of around 95%.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 12,654

    No I want you to keep the big tank. But your right, just put a propane Mod Con in and heat the big tank with it. Keep the domestic water on the big tank. But how are you keeping the DHW separate from the radiant heat? Do you have a heat exchanger?

    Forget the combi idea
  • archibald tuttle
    archibald tuttle Member Posts: 958
    edited April 2021
    if you have excess solar, put more of it tank, absolutely although with lower sun times and clouds in the winter may not be the sole solution. 3000 watts is nothing in a tank that size and 6000 watts that 2 times time nothing. 60,000 be a problem. how are the current resistance heaters installed, pipe taps near the bottom or . . . ? If you lack easy capability to put it in the tank you could put an electric on demand on the side with a pump. and then you just

    i'm surprised the 105 isn't enough for the floor, maybe not for toasty but if run all the time should be comfortable depneding on insulation of space. but if you ran it all the time you might not maintain the 105 but that is how radiant is generally intended to work. is the floor well insulated underneath and from any outside walls? sounds like you have a somewhat thought out plan so one would hope. you could circ the floor against a thermostat which is cheaper than a fancy mixing valve and as long as you turn it on a week before the guests arrive . . . . warm floor is not a good choice for something you want to only heat for briefer periods. not sure how often or for what periods you need guestroom, but it is sounds like it is under you, so if it's decently insulated the heat is contributed to the rest of the space somewhat even when you aren't having guests.

    @EBEBRATT-Ed I would agree there are some aspects that are not ideal to take full advantage of tankless HW, i.e. ondemand, technology. But some folks have had success balancing these in heating applications. for DHW i like the takagi's although in the article the author who used a Rinnai because it had a higher available output temp which would allow for a better delta in the circumstance of heating water that comes in at 100 or 105 off the bottom of the tank. That would be the main difference between this and DHW service. Now you are going to see more hours and maybe the designs would succomb more quickly than I have experienced in DHW (got Takagis going on 20 years old never had a hiccup). Of course these are all mods but not all cons, and, with higher input temps I'm not sure the condensing ones would live up to the name plate efficiencies. Yes you would get better efficiency from a modcon boiler and the return temps would be ideal for its condensing design, but saving 10% on your propane bill when a good deal is augmented by wood and solar will take a while to pay the difference. The price of modcon boilers is just insane in my mind. The simple flow regulated on demands have, in my experience been super reliable. The main thing you have to do is balance the delta given higher incoming temp than normally anticipated by a DHW style heater (which would also be the case where the on demand was used as a final supplement to some kind of DHW storage or were more than one are placed in series for high DHW loads). But that could be done with a high circ rate as well as higher output temp. I would absolutely agree that such approaches are, generally speaking, off label and require some trial and error so if you are retired in the home and strictly looking for a plug and play solution and not an experiment @EBEBRATT-Ed speaks the truth. For me, I'll be experimenting forever.

    Or, if you are running a woodstove do you have a chimney with flus to work with? or you can get side-vent kits. i love propane, but I would not altogether dispense with the idea of , god forbid, small cast iron oil boiler. you would generally have to return some of the boiler output to the input in order to raise incoming water above condensing temps but its some tried and true technology.