DHW and Radiant from same large tank?

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%.

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.

brian