Sizing tankless to replace tank
Last Post 06 Oct 2014 05:18 PM by sailawayrb. 6 Replies.
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dulatoagUser is Offline
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03 Oct 2014 09:07 AM
Good morning ... I have a 550sf living space, concrete slab radiant heated. I live in Upstate New York, so winter's are tough. I currently use a 40-gallon LP water heater to feed the system and it has been adequate. I'm going to try replacing the tank heater with a Takagi tankless ... specifically the T-KJR2-IN model ... 6.6 gpm, 140k btu. A rep at Takagi actually recommended a couple of other models, but the specs are the same as this model. I see some folks using this model for larger spaces (albeit in milder climes), so minus truly intelligent math calculations of which I'm simply incapable, I'm thinking/hoping this could work, and the model gets excellent reviews. Hoping someone here has some thoughts, experiences with these heaters. Thanks so much!
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03 Oct 2014 01:11 PM
I would provide a more detailed response, but I am at airport getting ready to jump on an airplane… With only 550 SF, you are at great risk of over-sizing the heat source. You might also need a buffer tank to avoid short-cycling. You should normally first determine your zone heat loss BEFORE sizing/selecting your heat source. You might be able to do an existing building energy usage analysis (as opposed to a manual J analysis) to determine this. We have much info about all these subjects in the DIY calculator instructions on our website.
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03 Oct 2014 02:55 PM
Posted By dulatoag on 03 Oct 2014 09:07 AM
Good morning ... I have a 550sf living space, concrete slab radiant heated. I live in Upstate New York, so winter's are tough. I currently use a 40-gallon LP water heater to feed the system and it has been adequate. I'm going to try replacing the tank heater with a Takagi tankless ... specifically the T-KJR2-IN model ... 6.6 gpm, 140k btu. A rep at Takagi actually recommended a couple of other models, but the specs are the same as this model. I see some folks using this model for larger spaces (albeit in milder climes), so minus truly intelligent math calculations of which I'm simply incapable, I'm thinking/hoping this could work, and the model gets excellent reviews. Hoping someone here has some thoughts, experiences with these heaters. Thanks so much!

A house that size with any insulation at all will have a heat load under 15,000 BTU/hr, and may easily have a heat load under 10,000 BTU/hr. The min-fire input of the T-KJR2-IN is 19,500 BTU/hr, probably 2x your actual 99th percentile heat load and many times your average heat load.  You may be able to tweak it into working without short cycling by adding mass or some sort of minimum timing scheme on zone calls, but's not very well adapted to the size of the load.

A 3/4-ton or 1-ton cold weather mini-split is probably a more appropriately sized solution for space heating, and will have less than 1/3 the operating cost of a pretty-good tankless at recent years' propane prices.  At -13F these things still have something like 70% of their full-rated capacity at +5F.

Before picking any heating solution it's important to calculate the actual heat load at the 99% outside design temp for your location.  "Upstate New York" covers a fairly broad area, but there are very few locations that have an outside design temp below -10F.  It's possible to size a mini-split correctly for sub-zero temps.

A propane tank hot water heater can work at almost any arbitrary burner size, since it's short cycling can be limited by the thermal mass of the water in the tank and the differential built into the controls.   But the ~80% steady state efficiency of non-condensing propane tanks makes them not super-cheap to run.  It's cheaper than a mini-split solution, to be sure.

I'm currently heating my home with a tankless Takagi (KD20) but the system als includes 48 gallons of buffer tank (with an internal heat exchanger for potable water) on the primary loop with ~7F of hysteresis on the aquastat to establish a minimum burn time (after tweaking flows) and my 99% heat load is about 35,000 BTU/hr, probably 3-4x yours.  If you go the TK-Jr  route it's worth buying the kit that makes it a sealed combustion unit, ducting in combustion air from the outdoors rather than drawing from conditioned space.  You'll have to insulate the duct to keep room air from condensing on it during cold weather, but it won't be sucking air into your house from random places.


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05 Oct 2014 12:24 PM
Dana is right.

But a water heater is not a boiler except it be in the hands of a really smart guy like Dana.

"We" use a radiant heat specific Manual "J" heat load program to properly size all of our radiant floor designs. From much experience with hydronic micro-loads such as yours, a condensing tank type propane water heater would be the answer and one we have specified and installed all over North America and beyond. A well insulated, sealed combustion, direct-vent condensing water heater uses outdoor air instead of indoor "conditioned" air for combustion and has the "buffer" built-in with standby heat losses less than 1/2°F per hour.

"We" use a sub-assembly to isolate DHW from space heating and outdoor reset to properly mix the standing water temperature with the much lower design water temperature required for heating the slab.

"We" could waste a lot more of your time trying to impress you with useless details, but "our" pilot is waving "us" on, as the 60XR does not like to idle...
MA<br>www.badgerboilerservice.com
sailawayrbUser is Offline
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06 Oct 2014 10:35 AM
Of course Dana is right and his recommendation is consistent with my brief initial recommendation too.

Yes Dana, I would agree that a mini split would likely have been a better HVAC solution to begin with. However, it sounds like the OP is wanting to keep the existing HR floor heating system and is soul searching for a better heat source. Just continuing to use the existing hot water tank may be the best solution. A tankless combi (DHW + HR) with modulation may be a good solution too. Without knowing the required HR supply temp, it’s hard to say if a condensing solution makes sense or has acceptable ROI.

Congratulations Badger on being licensed to design/install HVAC systems "all over North America and beyond"...
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Dana1User is Offline
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06 Oct 2014 03:19 PM
I'm not THAT smart- when I run out of napkins & crayons for running the hydronic design math I have to fall back on the lipstick-on mirror approach. ( You wouldn't believe just how messy my place looks! :-) )

It's worth a whole fresh napkin for running at the very least a crude I=B=R type heat load calc on the place before jumping on a tankless water heater as radiant-slab boiler solution. I don't quite get how tankless water heaters became so popular in this application. To do it right still requires the same amount of napkins & mirrors. Some of the pre-packaged tankless slab heater solutions from web-stores are abominations from a design point of view.

It doesn't even take a whole napkin to compare the price of NY propane in a condensing burner vs. electricity, leveraged at ~3:1 with a ductless heat-pump. From there you then have to figure out just how much that warm floor is really worth to you. I like radiant floors too, especially when the fuel is cheap, but doubt I'd spend that much extra every year to run a heat slab with propane if a ~$3000 3/4 ton Mitsubishi would keep the place a granny-warm 75F all winter at less than half the operational cost. (And it can put the chill-on in July for cheap as well.) YMMV

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06 Oct 2014 05:18 PM
Have you ever tried an old fashioned caulk board, LOL.

I suspect that tankless water heaters have become popular for HR systems for the same reasons that tank water heaters became popular. They are both much cheaper than real boilers and they sort of get the job done. Not many people realize how poorly they perform or how short they live compared to other options. The tankless water heaters also take up less space and seem more attractive from that standpoint until you figure out that you might also need a buffer tank…
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