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Community and Q&A

Water Heater with Air Handler

randy_j | Posted in Mechanicals on

Building a home in the greater Columbia SC area. Climate zone 3.

Basic Home specifications:

Single Story Ranch ~3100 sq/ft
Unfinished/semi-conditioned basement.
All HVAC/Mechanicals will be located in the basement.
Zip R6 sheathing with blown in fiberglass 2×4 walls. (~r19)
Blown in attic with R50
Pella Lifestyle Windows
55g jet tub in the mater bath
Two adults and 3 kids (two oldest are entering their teenage years, which means high water usage)

We are in the beginning stages of our home build, the foundation is poured and rough backfill has been done.

The builder and I had a manual J and D done for the home. My plan is to use either an HTP Phoenix Lite -60 or a Phoenix 50 – 100 or 133 for both the domestic hot water and the heating of the home. The total heating required is ~37.5k btu.

My plan is to use an air handler that will include the AC coils and also a hydronic coil for the heating. Plumb the aux ports on the water heater to a plate heat exchanger then to the hydronic coil. In the future I will also add radiant to the master bathroom floor. This isn’t something that is typically done down here in the SE but the builder is welcome to trying something other then the typical gas furnace or heat pump. I’m originally from the north east (Vermont to be exact) so hydronic heating isn’t new to me. Oddly HTP also shows in their manual you can run the air handler without a plate exchanger, this seems pretty unusual to me to run domestic hot water through the coil, so I haven’t considered this option.

The HTP Phoenix lite is rated 25,000-76,000 btu modulating , where the standard Phoenix models modulate as well and are rated at either 33,000-100,000 or 33,000-130,000 depending on the model.

I’m considering the larger units simply from a recovery standpoint, however my biggest concern is that I won’t have sufficient recovery using the smaller Phoenix Light if the heat is on and the tub is being filled. (Thinking worst case scenario) With the larger BTU Phoenix units my concern is that it doesn’t modulate down as far and I will run into short cycling issues. I called HTP customer support and they don’t do sizing and the local HVAC guys pretty much look at me like a deer in headlights when I tell them what I want. The one outfit that did this type of work has closes shop, the guy retired. So I will be up to doing the heating plumbing myself most likely which I’m fine with.

I would think using a low temp hydronic coil would be best but finding this information has proven illusive and I’m concerned leaving it to the guys down here who order the air handler might not be prudent.

If I left out any important details, I will certainly try to fill in the blanks as required. First time post here.



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  1. Expert Member
    Dana Dorsett | | #1

    Was there a question in there somewhere?

    Clearly a suitably large Phoenix would handle both the heating & hot water loads, but even the 80 gallon the -Light Duty version could become burner-constrained with multiple baths running at a time during the heating season. (How many baths, total? How close are they to one another?)

    But is there something driving you toward going with gas?

    What is the 1% cooling load, per the Manual-J?

    A right sized modulating heat pump (probably a 3 tonner, maybe 3.5-4 if the cooling loads call for it) would be more comfortable for the main zones, low voltage resistance mesh and a floor thermostat can keep the barefoot comfort in the bathroom high, and 1-2 heat pump water heaters in the basement can help keep the basement dry.

    At the rate of the relentlessly falling cost of rooftop solar (with battery backup!) it will be cheaper to make your own electricity than buy 100% from the grid, and it'll be cheaper on a lifecycle basis than subjecting yourself to the volatility of the natural gas markets. You can make your own electricity- can't make your own natural gas. Having bailed on the SCANA nuke project SC won't be stuck paying for another high-cost stranded-asset-at-birth albatross in the grid mix the way the folks in GA are. The current average cost for rooftop solar in SC is currently running $3/watt full-retail (before subsidies), $2.20/watt after the federal tax credits. I'm not sure if or what other incentives might apply in your area. A 10-15kw system (with or without a battery- but make it battery-ready if going solar-only) rolled into a mortgage would keep your monthly operating expenses pretty low, and largely protect you from rate hikes on imported power from the grid. Do the math- if you need more, with a 3000' single story you probably have ample roof area for more. If your house is reasonably air-tight, if the R-values of the rest of the house are similarly above code as your wall assembly it shouldn't need to be crazy-big to go Net Zero Energy on the house, but you may want to add enough for EV charging as well. (You can't make your own gas or diesel either.)

    Natural gas pricing in the US is becoming more & more exposed to the international market prices, which are sky high at the moment due to the conflict with Russia, and if Team USA is committing to massively exporting LNG for the coming decades that will raise domestic prices even further than they already have. And it's not at all certain that natural gas will avoid carbon taxes within the lifecycle of a Phoenix.

    The up side of hydronic heating is greater flexibility in micro-zoning the heating, but it doesn't seem like you're going that route, which puts even more weight on the heat pump side of the scale.

    If this all seems too radical a shift in thinking, at the VERY least make the AC a heat pump (multi stage or modulating is even better) rather than AC-only. Running in the lower 1/3 of their output modulating heat pumps will average a COP of 4 at SC shoulder-season outdoor temps, and 2.5-3 running at max-output, even at the 99% outside design temp.

    Per the EIA, SC electricity rates are arleady among the cheapest in the nation at ~10cents/kwh, which at a COP of 4 is $7.30/MMBTU (million BTU). At a COP of 3 (you should be able to do better than that), that becomes ~$9.75/MMBTU. Before the European natural gas shortage/crisis gas in SC was already nosing up toward $12/ MCF (1000 cubic feet) winter pricing. So even before discounting for the less than 100% efficiency of a Phoenix the gas solution is more expensive to operate. I'm not going to bet on gas being cheaper next year, or for the next decade, but electricity pricing even has room to fall as more zer0-marginal-cost renewable assets get added to the grid (or your roof.)

  2. paul_wiedefeld | | #2

    "I’m considering the larger units simply from a recovery standpoint, however my biggest concern is that I won’t have sufficient recovery using the smaller Phoenix Light if the heat is on and the tub is being filled."

    I think the smaller will be best: boilers usually prioritize DHW over central heating. If this one doesn't out of the box (not truly a boiler, unlike the HTP Pioneer), you can make your own. No need to oversize. You'd probably be fine even during the worst case scenario without DHW priority, 55 gallons isn't that big.

    To be frank, I'd skip the hydronic air handler entirely and use the heat pump version of whatever air conditioner they'd install anyway. I'd even reconsider the in-floor hydronic heating, as your heat load is extremely low for that sqft (makes sense based on the location), so the floor will be very low temperature year round. Possibly do in-floor electric if it's just for the master bath. Hydronics really don't make sense in SC especially for a brand new house predominantly heated with forced air, which is probably why few installers are experienced. You'll probably come out thousands ahead skipping the hydronic route upfront and even more on operating costs using electricity instead of gas for the same level of comfort.

  3. user-6623302 | | #3

    If you are going to have gas and electric in the home, I would heat with a heat pump and a gas furnace/heat pump air handler. Furnace is just for Jan and Feb. Have a separate hot water system, maybe a big HPWH.

    1. paul_wiedefeld | | #5

      I’d agree with this, if not for the location. It’ll be cheaper (upfront and over time) and easier to just use a heat pump for these mild temperatures.

  4. Expert Member
    Dana Dorsett | | #4

    OK this is off topic (and I don't intend to hijack the thread), but the differences between energy pricing and rooftop PV pricing in the US & Europe is striking. I doubt SC will see Euro-style energy rates any time soon, but I read that earlier this year (2022) that some people in the Netherlands are paying more than 35 euro cents/kwh (~39-40 cents US) for power, nearly doubling from a year or two ago, but that natural gas pricing has gotten even more insane. And that was BEFORE the war broke out!

    Rooftop solar there costs roughly half what it does in the US, and at this winter's electricity & gas pricing it's the cheapest energy going. At their lower PV pricing the 15-20 year cost of PV output is now well under 10 US cents/kwh, as long as there is some type of net-metering allowance. (IIRC PV there is fully net-metered for the first few years, then steps down over time.)

    Even before price spikes related to the Ukraine fiasco a Dutch youtuber ran a financial analysis of just how bad it has gotten in Europe this season and noted that at this winter's electricity & gas pricing:

    ...A: heating with a heat pump is substantially cheaper than heating with natural gas

    ...B: PV panels now pay for themselves in less than ONE YEAR in grid savings(!)

    ...C: PV+ resistance heaters is cheaper than current gas pricing (by a lot- it's less than half, more like a third the cost!)

    ...D: Heat pumps + PV are cheaper to run than gas burners even at last decade's gas prices.

    If you can handle listening to (with or without understanding) ten minutes of Dutch, the math in the slides is clear enough:

    (The title "Gas is veel te duur geworden, ik ga elektrisch!" translates to "Gas is becoming far too expensive- I'm going electric!" in 'merican.)

    US energy pricing is dramatically lower than Europe, but solar & batteries continue to get cheaper every year. Even at SC's 10 cent/kwh grid-power a home solar system is likely to provide cheaper energy to US homeowners before 2030. Mind you, the output of PV in SC is substantially more than what it delivers in foggy dew northern Europe! But the installed cost in SC is higher still.

    It's only a matter of time before this math will work at US price/cost numbers. It's pretty clear that in Europe renewables, even at single-home scale have already become cheaper than continuing to import fossil fuels, even without factoring in the security/defense risk cost related to importing Russian coal & gas.

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