Options for small in floor heat load

[Joshua Miller]

So we bought our house 4 years ago with a poor design in the basement apartment we rent out.
2 loops of 1/2" feed 400sq ft of in floor in the main part and 2 4ft baseboards in 2 seperate rooms. All one zone.
(2 vents from our furnace help out the situation)950 sq ft total.
North wall is exposed to outside. All walls have 1.5" foam board and drywall.

It has an old rebuilt NG navien that was open loop feeding the heating side and domestic hot water. Set at 120

I have separated the heating with a heat exchanger knowing that soon I will need to replace.

I need some options. We need a decent amount of hot water with the renters(2 people) plus our family of 5

1.Small mod con boiler like a HTP UFT-80W plus an inderect tank for domestic.
2.I have considered a 75 gal power vent with taps for a heating loop I would hook up to existing setup (this would be cheapest and easiest to replace)
3. Small electric boiler plus a tankless for domestic. Not thrilled with electric since I have gas available.

Any other suggestions or advice for such a small load?
Thanks

 

[Dana]

So the only radiation is a bit of radiant slab and 8' of fin-tube?

Is it a walk-out basement, or all mostly below-grade, with maybe one outdoor stairwell or...???

A condensing tank type water heater (like an AO Smith Vertex or HTP Phoenix Light Duty, both of which come with side-taps for heating) and a heat exchanger for the heating is probably the best solution here, assuming the room-to-room temperature differences are fine. The heat load of a 950' insulated basement is pretty tiny, even if there isn't any insulation under the radiant slab. A ~75,000BTU/hr condensing burner on a 50 gallon tank recovers in half the time of a typical atmospheric drafted tank. The only time you'd need more water heater than that is if you're filling large tubs or running more than 2 showers at a time for an extended period of time. There are 100KBTU/hr burner versions of the Vertex or Phoenix series if it's a recovery time issue (up to 199K for the Phoenix), and bigger volume versions of the 76K burner Phoenix Light Duty, (starting at 5o gallons) if you have a multiple tub-filling situation that would deplete a 50-55 gallon tank. The Phoenix water heater are all stainless tanks and should go 20 years or more (despite only a 10 year warranty) the less expensive Vertex water heaters are glass lined mild steel, like most standard water heaters, and probably won't make it past 12-15 years.

The Phoenix series (including the Light Duty) has more sophisticated modulating burners with a 3:1 turn down ratio, whereas the Vertex are on/off. When it's just the tiny heating system pulling heat out of the water heater the Vertex may be prone to short-cycling despite the thermal mass of the water in the tank. While HTP is somewhat less supportive of using them for anything other than hydro-air space heating (creative backyard designers can make anything short-cycle ) they work just fine for low-load radiant & baseboard systems like yours if you don't over-pump the water heater side of the heat exchanger. (A potable-compatible stainless Taco-003 or similar might be about right, but the stainless AM55-SFVL would have more options for tweaking the flow and uses less power. In it's "overnight setback" mode it's pumping even less than a Taco-003.)

Westinghouse sells both the 50 gallon and 80 gallon sizes of HTP's Light Duty water heaters under their own nameplate, and can usually be ordered through Home Depot if local distributors don't carry or support them. It's exactly the same hardware under the label- the Westinghouse tech support phone line rings at HTP's home headquarters, and is answered by the same techs.

The basic architecture looks like this:

Don't forget to put a small expansion tank on the heat-exchanger isolated heating system side.

 

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[Joshua Miller]

Thanks for the reply.
It Is a walk out basement with most of that wall exposed. Insulation is unknown under slab.

I currently have all the essentials plumbed in as needed. Just looking for options for replacement as the navien is on it's way out.

I was hoping to stick with ao smith as that's what the local supply house stocks.
But if it will short cycle I'm not so sure.

There are 2 women in the house that like to take 30 min simultaneous showers so that would be the most load at 1 time

 

[Dana]

Thanks for the reply.
It Is a walk out basement with most of that wall exposed. Insulation is unknown under slab.

I currently have all the essentials plumbed in as needed. Just looking for options for replacement as the navien is on it's way out.

I was hoping to stick with ao smith as that's what the local supply house stocks.
But if it will short cycle I'm not so sure.

There are 2 women in the house that like to take 30 min simultaneous showers so that would be the most load at 1 time

That's a problem- a pair of 2 gpm simultaneous showers at a 70F temperature rise (35F in at the coldest part of the winter, 105F out at the shower head) is a heat rate of ~140,000 BTU/hr. The output of the biggest-burner AO Smith Vertex 100 (GDHE-50-NG) is about 95,000 BTU/hr. If the showers are upstream of a vertical section of drain 5' or longer you can pretty much double that "apparent" burner size with decent sized drainwater heat exchanger by pre-heating the incoming water stream with the heat that's going down the drain:

EcoDrain's VT1000 series is the current performance leader. The 54" tall x 4" drain VT1000-4-54 delivers more than half the heat from the drain into the incoming stream at the standardized test flow of 2.5 gpm. At 4-5 gpm (two showers) it's performance isn't quite that good but it should deliver better than half.

Renewability's PowerPipe series is also pretty good, though you'd be looking at the R4-60 (60" tall) or R4-66 (66" tall) to achieve fully 50% recovery @ 5gpm.

Whatever the series, the tallest one that actually fits is the "right"one. Heat exchangers in that class run $750-1000 direct from the manufacturers. With your kind of showering use it would pay for itself in short years, since it's a "burner" that doesn't use any gas. It does nothing for tub-filling efficiency though.

 

[Joshua Miller]

I live in a single story house, so I dont think the drain heat exchanger would work.

I'm not aware of my incoming water ever going as low as 35. Is that city water maybe? I'm on a well.

I was thinking of raising my tank temp to 135-140 and tempering down on the domestic side to help capacity.

 

[Dana]

I live in a single story house, so I dont think the drain heat exchanger would work.

I'm not aware of my incoming water ever going as low as 35. Is that city water maybe? I'm on a well.

I was thinking of raising my tank temp to 135-140 and tempering down on the domestic side to help capacity.

Got a ZIP code? Are you on a well? What is your lowest incoming water temp?

This is a deep-well temperature map. Shallower ground water or shallowly buried pipes will have wintertime low temps lower than that:

Even if the incoming water never drops below 50F, raising it to 105F at the showerhead at 4 gpm is still 110,000 BTU/hr, at 5 gpm it's over 130KBTU/hr. It's going to take more than just storing the water at 140F to deliver simultaneous half-hour showers with a 100K condensing burner backing it up.

I live in a 1.5 story house with a full basement, and the only shower is on the first floor, not the basement. The basement ceilings are a bit low- that and other plumbing constraints limited me to a 48" drainwater heat exchanger, but it really does work.

 

[Joshua Miller]

Yes on a well... 44606
Columbus ohio shows 53 degree ground water.
Incoming pipe is 3 feet underground and runs for 8 feet horizontally to the well.

I'm sure those heat exchangers work, they sound very intriguing but I remodeled our basement apartment 3 years ago and did extensive sound proofing along with drywall in the ceiling so that is not an option.

For discussions sake let's say we can avoid simultaneous showers. What is the benefit of a condensing heater on a small heating load like mine vs an ao smith 75 gal 75kbtu power vent. 1500 price dif.

On my return line to the navien it currently doesnt drop under 100.

I'm not sure how much of a factor this heating setup should be for my decision since the shoulder seasons wont need much and coldest part of winter isnt to bad either. Still want to make the best decision though.

 

[Dana]

The differences are that a condensing 76K burner puts out about 72,000 BTU/hr, which is enough to fully keep up with a 2 gpm shower forever even at sub-40F mid-winter incoming water temps, but not a 2.5 gpm shower.

A 100K condensing burner delivers about 95,000 BTU/hr which is enough to support a 2.5 gpm shower forever, with margin.

A 75K powervent only delivers about 60,000 BTU/hr- which is less than what's being drawn by the shower, so getting 30 minutes of showering out of it could be iffy, back to back 30 minute showers may drop the tank temp below 100F, and the recovery times on 75 gallons with only 60K of input is considerably slower than 72-90K into a 50 gallon tank.

You can pretty much ignore the basement heating system load in this analysis- even when it's running it's taking only a small fraction of the total burner output. If you have enough burner to support back to back 30 minute showers back to back (60 minutes total) with any margin, the heating system won't deplete the heat in the tank quickly enough to matter.

 

[Dana]

For yuks, let's assume you have a storage temp of 140F, that should be able to deliver 50 gallons of 135F water out of a 75 gallon tank, after which the temperature starts dropping fast. With a generous 45F incoming mid-winter water temp and 105F at the shower head the hot water is leaving the tank at 1.67 gpm, mixing with 0.83 gpm of 45F water to deliver 105F at the shower head.

The stored heat in the tank is then enough to provide 50 gallons/1.67 gpm = 30 minutes of showering time before the temp slips off a cliff. That means the burner has to keep with the rest. If the burner kicks on 10 minutes into the first shower with 5 minutes of recovery time the powervent might just barely squeak another 30 minute shower, but it's not guaranteed.

 

[Joshua Miller]

For yuks, let's assume you have a storage temp of 140F, that should be able to deliver 50 gallons of 135F water out of a 75 gallon tank, after which the temperature starts dropping fast. With a generous 45F incoming mid-winter water temp and 105F at the shower head the hot water is leaving the tank at 1.67 gpm, mixing with 0.83 gpm of 45F water to deliver 105F at the shower head.

The stored heat in the tank is then enough to provide 50 gallons/1.67 gpm = 30 minutes of showering time before the temp slips off a cliff. That means the burner has to keep with the rest. If the burner kicks on 10 minutes into the first shower with 5 minutes of recovery time the powervent might just barely squeak another 30 minute shower, but it's not guaranteed.

Thanks for the info!

I will say, it is tempting to go with the powervent and try it since it sounds like it would be decent through most of the year.
I have some thinking to do.

Hopefully the navien can get me through the winter.

Thanks dana