Hi, sorry to bring this up again as I see this forum has addressed it a few times, but I am wondering about adding a storage tank to my existing system.  I am using an indirect radiant heat system (4 loops, 3 zones) to heat a 900 sqr ft house (R22 walls, R40+ roof, timberframe, very high ceilings).  The heat source is a Takagi-Jr tankless system (propane; NG is not available and electricity in the NYS fingerlakes region is about 0.13/kWhr) run through a short loop to a heat exchanger.  The Takagi modulates the heat; at 40 degrees input water it runs wide open, but when the heat system supply side pump is running, the return water rapidly increases in temp as the house heats up (the tubing is set in a 1.5" micropour).  I am currently paying about $1500 (650 gallons propane) per winter to heat the house (total use in spring, summer and fall is almost trivial).  I know that one recommendation would be to put in a tank system, but the house is unused in the spring, fall and winter for long periods, and the tankless seemed like a better option given that situation.  I am thinking about getting a storage tank with no heat exchanger to allow the system to run wide open and then shut down for longer periods during the heating season.  Is this a reasonable idea - will it help the system to operate more efficiently?

Thanks

A few questions that might help get answers.

Do you keep it a a constant temp or is it cooled down and heated up alot?

Do yoiu have a fan to move the heat dowm from the high ceilings?

What is the temp. that the system is usually set at?Room temp

Is the system in insulated space? or a uninsulated crawl space?

What temp is the water set at, on The Takagaie controller?

A water heater is not a space heater. Most tankless water heaters are low efficiency. ModCon boilers are made for space heating. Have one installed by a professional who has done it before and save money. The proper use of storage tanks is a science of its own and rarely saves money. I design systems for a living and use a handful in a year, nearly all for solar. Shortcuts cost more than long cuts.

proper use of STORAGE tanks can be tricky. Buffer tanks, however, are pretty simple.

That said, your thinslab may already be providing significant buffer capacity, adding more might not help. You may, however, be able to turn down the takagi temperature.

Fans are counterproductive with radiant systems; don't use them.

with a 900 square foot house, I'm not sure I would advocate for replacement with a mod/con. Even if you save 20%.. which you might, but that's a little optimistic... the payback on your bill would be between ten and twenty years for most mod/cons.

Now, if you hadn't bought the takagi in the first place, that shortens up quite a bit, but now, I would be unlikely to advise that.

"Fans are counterproductive with radiant systems; don't use them."

I am interested on why that is the case?

Radiant doesn't stratify much to begin with, so they aren't very helpful in the way that they normally are. On top of that, they induce a draft, which will actually reduce your comfort in a radiant system.

In a FHA or baseboard system you are typically blowing hotter air downward, so that draft effect is of less consequence.

I have a different point of view.
We have a fan operating on low at all times. I think it does well for us during the sunny winter days. Our heating radiant system does not kick on during the day. The fan moves the sun warmed air from the area close to the windows to the rest of the living area. Our home is not built for the best solar gain but the fan does a good job of maxiinizing the gain we do get.

I might try an experiment this winter to see if my thinking is correct on this.

you are redistributing solar gains, that's a different situation than a period of active radiant heating.

the fan may help you, but it's not helping your radiant system's efficiency is all. Luckily, it's not running while you use the fan ;)

"Luckily, it's not running while you use the fan ;)"

But it is! That is why I will have to do the experiment. Maybe I will need to put the fan oin a timer. You gave me something to think about and test!

well, if it doesn't make you turn up the thermostat, then it's not making any other difference; the effect I'm talking about is simply about reduced comfort at a given room temp.

of course, many people will deal with reduced comfort to save a few bucks on heat too, which is fine. but the heat "reclaimed" for a radiant system, with a ceiling fan, is pretty small.

Hi,

Thanks for the input -- additional info is:

Takagi is in a basement w. 3 concrete block walls insulated on outside, and a walkout wall (2x8) full insulation (temp range is 50-60 degrees through 12 months, with no heating source).  Slab is on first floor with no insulation below it.  Temp is house is 64 (comfortable temp) when we are there, 58 when we are not.  We do not leave the ceiling fans on when we are not there; only (in winter) during the day are they used (there is typically some level of solar loading from the windows to move around).

I'm aware of the install cost of a mod/con, so that does not look tempting unless propane increases another $1.50, which it might.   BUT, would a mod/con give me the same performance in the 6 months of the year when heat is not required?  The tankless fuel need during the summer is so low I have to look carefully to see that its using any at all.

Thanks

Sorry - one more question was asked;  the temp on the tankless is set at 122 F.

The heat exchanger shows that the heating side of the loop reaches about 112-115 F as it enters the loop circulation pump.

Thanks

Is the basement floor insulated? Do you use the basement?

My home is 1200 sq feet r30 walls r40 roof over a crawl space( floor was 2/3's insulated last winter) I also heat a 400 square ft art studio( not as well insulated) Radiant heat in the main house( 2.5" ) slab on a plywood deck over the crawlspace. Crawl space walls are not insulated. All cooking, gas dryer etc. on propane. Lived in full time year round, temp at 68 degrees. Heat on last year from mid October to May 1st ( or later). We used 600 gallons of propane. Our radiant is heated with a TK-jr
as your is. We live at 6000 ft in Western WY, winter temps are cold!

I think you may need to insulate below the main slab and keep the heat in the main living area.

The basement floor is not insulated; three walls are underground and the 4th is an insulated walkout wall as noted above.  The temp in the basement is always above 50 F, even when its -5 outside.   The microslab is in the first floor, above the basement:  jmagill, are you suggesting that heat is escaping downward and heating the basement?  Clearly it is not as cold in outstate NY (near Ithaca) as in WY.

Thanks,

"jmagill, are you suggesting that heat is escaping downward and heating the basement?"

Yes! We built ourselves and it took awhile before we insulated the floor below our radiant( just finishing it now) My crawl space was warm! Adding the insulation made a huge difference. You are heating the basement and unless you are using that as living space the heat is also going through the basement floor and is wasted.

Sorry - there actually is foil insulation below the gypcrete (laid on top of the subfloor decking).  I've read elsewhere that this may not work too well as insulation below the thinslab.   Do you think that I'm likely to gain a lot of efficiency if I add a layer to the bottom of the (currently unfilled) joist spaces, or fill the joist spaces w. fiberglass batts?

Thanks

Right now your hydronics is heating the basement and the main floor. The foil insulation is not doing much of anything.

If you do not need to heat the basement, you should place insulation between the joists. You will save at least one third maybe as much as 50% on your propane( that is according to the diifference I saved after finally getting to the insulation.)

Do you use the basement? You may need to insulate water pipes.

even if you do heat the basement, you are making the 1st floor work harder than it needs to, which influences what kind of water temperatures you can run, if nothing else.

downward loss is real. Everyone always talks about "how warm their basements are" without realizing that most of that heat is being sucked out of the first floor.

Thanks for the help!  I'm going to read up on the right type of insulation (any suggestions?) for between the joists; I think 1" foiled backed poly isocyanate (sp?) looks like a likely candidate - if that's too much or too little, please let me know.   I do not use the basement for anything beyond storage, and have a propane space heater hitched to a thermostat that I can dial way down, but which will keep the pipes from freezing.

Thanks to all who responded - this was very helpful.

you want at least an R-19 or so in this case. 1" of polyiso is about an R-7.