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Tankless Heater for in-floor hydronic heat


Jerry Simon
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You need hot water for the heat, you got hot water.

Don't know where it is, yet. Query from a realtor friend, prior to hopeful contract writing/inspection.

One of his specific questions, though, was if the system would get *enough* hot water. I opined that in such an application, the problems one hears about using tankless for domestic hot water supply probably would be nill in such an application. Seems like it would work quite well.

But what the heck do I know.

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Yeah, I just did the 3rd fl. south.

Icynene foam. Radiant. LG mini-splits for AC. It's a smart, forward thinking conversion of a really cool old building. It's one of the nicest looking buildings on Paulina, and Paulina's got some nice ones.

I'll be interested to see what you think.

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I don't see the energy savings. Last one I inspected the thing ran constantly while I was inspecting at about 185,000 BTU compared to a normal 100,000 BTU forced air that would cycle on and off, maybe run half the time. It's tankless, it's gotta be 'green'.

The rating is the maximum output. Most of the units modulate to demand. The unit you're talking about likely had a minimum output of 11K-12K btus's.

Instead of using twice as much as a furnace, it may have been using only half as much as a stove burner.

Takagi spec sheet... it's rated for boiler duty

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This is one area where tankless is smart. I and a friend stripped and remodeled his house (old, frame, POS) several years ago; whole house, cast iron radiators, radiant bathrooms, etc.

The guy's got (approximately) $80 a month heating bills. It works fine.

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I don't see the energy savings. Last one I inspected the thing ran constantly while I was inspecting at about 185,000 BTU compared to a normal 100,000 BTU forced air that would cycle on and off, maybe run half the time. It's tankless, it's gotta be 'green'.

A heating system that is sized properly should run almost continuously on the coldest day of the year. That's especially true with a radiant system that is sending lower temperature water through the system. They have longer cycles with lower temperatures.

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Tankless for floor heating runs against traditional and conventional thinking. They seem to be doing just fine though, and they're gaining more of a foot hold in the in-floor heating market-at least in our region.

Can't say much about savings though. I don't think the savings are as ideal as people had hoped for. I don't have any data to back that up.

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Savings on square footage and installation is decent. If it's all sized and installed correctly/satisfactorily, and there is appropriate insulation installed all the places it should be, it will save an incremental amount of money on a heating bill.

Based on friends that have done it. It works OK, and I'm predisposed to disregard tankless water heating blather.

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The down side is that most manufacturers limit the warranty to 1 year on units used for radiant heat, and some void it all together. It' good to see that there are now units rated for boiler duty. The limiting factor in the performance of tankless units is their flow capacity. There are low end units that can't make more than 3 or 4 gallons per minute which can easily be exceded with two points of use, and high end units that'll churn out over 9 gpm. It really comes down to reading the specs for the installation in question.

Tom

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Savings on square footage and installation is decent. If it's all sized and installed correctly/satisfactorily, and there is appropriate insulation installed all the places it should be, it will save an incremental amount of money on a heating bill.

Based on friends that have done it. It works OK, and I'm predisposed to disregard tankless water heating blather.

Not having seen one used for this task, I assume they are set up to recirculate, with a water feed valve for makeup water?

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I haven't seen a tankless coil set up to do this, either. However, if warrantees are the only downside, I say: Go ahead! Tankless coil warrantees can't possibly be worse than boiler warrantees when adjusted for equipment price. Witness (many inspectors' favorite) Weil McLain's nearly worthless prorated boiler warrantee that excludes labor and failed gaskets.

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  • 6 months later...

I have discovered that using a tankless water heater for both domestic water and radiant heating is an efficient and inexpensive alternative to a boiler. Keep in mind there are no building codes in my rural part of the country, as there may be in your ‘neck of the woods’. And the cheaper tankless heaters are usually not recommended or warranted by most manufacturers to be used in a radiant heating system and it will probably void your warranty.

We plumbed the radiant heating loop of our vacation cabin to the domestic hot water, making an open loop system. We don't usually drink water from the hot tap - we heat it in the microwave or on the stove, but I wouldn't see anything wrong with that either, as the water is not contaminated or otherwise unsuitable as in a closed loop system. It just goes through (a lot more) tubing and a stainless pump. Fresh water gets introduced into the loop every time we use the shower or DHW.

Originally I tried a 10 gallon tank type water heater, but it took a long time to heat the floor, even after I upgraded to a 220v/4.5kw element. So I decided to try a 7kw tankless 'point of use' heater, and was pleasantly surprised that it heated the cold concrete slab floor to 70 degrees in a reasonable time. The tank type water heater evidently was overwhelmed and thus could only provide tepid water, but the tankless heater put out a continuous supply of hot water which got the job done much sooner. The only drawback was if the floor heat was on, the domestic (tap) hot water would run just warm. This was OK, even for showers, but a dishwasher requires hot water (unless it has provisions for heating the water). And I prefer the hot tap to produce hot water.

So the following year I bought a larger but relatively inexpensive 'Titan' brand N120 tankless (11kw), and (after enlarging the outlet orifice to 1/4" - which will also void your warranty) was happy to find it heated the floor in half the time while providing hot tap water! At 95%+ efficiency, I don't see why this wouldn't be feasible in Northern climates as well, providing your floor or slab is properly insulated , and electric rates are reasonable (electricity here is only 7-8 cents a KWH so is in line with propane costs). Once the initial heating cycle is completed, intermittent use keeps it warm.

We were so happy with this arrangement that we have since converted our home as well. With the money saved from having to purchase higher priced ‘boilers’ we have purchased another backup heater for peace of mind. Time will tell how long they will last, but I suspect we’ll get a good return on our investment. Dave

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I'm surprised that a single electric tankless is heating your house and supplying DHW to your satisfaction because 7.5 KW is just under 24,000 btu/hr. Small cabin? Well insulated & sealed?

What do you mean by 95% efficiency? All electric water heaters are 100% efficient.

Marc

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