Tankless Retrofit: Connected Load, the Meter, and the Closet
A homeowner wants to replace a 40 gal tank with a 199,000 Btu/h condensing tankless heater in a 6 x 8 x 8 ft mechanical closet, keeping the existing 100,000 Btu/h furnace and a 65,000 Btu/h range. The gas service is 3/4 in. black iron off a 250 CFH residential meter-regulator at 7 in. w.c.; developed length from the meter to the furnace tee is 60 ft, and the range sits 25 ft further on a 1/2 in. branch. The closet has a solid door and no low or high openings, and shares a wall with the kitchen. The installer proposes to tee 3/4 in. off the furnace line, draw combustion air from the closet, and rely on the tankless modulating down because "it never pulls full load anyway."
1. What is the connected load, and will the existing 3/4 in. trunk carry it?
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Connected load: 199,000 + 100,000 + 65,000 = 364,000 Btu/h. At roughly 1,000 Btu/ft3 that is about 364 CFH - but call the utility for the local heating value (Oklahoma gas commonly runs near 1,000-1,030 Btu/ft3) and use it, because it moves the number.
Method, not memory: gas sizing uses connected input, longest developed length from the meter to the farthest outlet, and no diversity credit. A modulating appliance will fire at full rate - typically when the house is coldest and the furnace is also running, which is exactly the design case.
Result: in the 0.5 in. w.c. pressure-drop table, 3/4 in. pipe at 60 ft is on the order of 130-140 CFH, roughly a third of the connected load. So the existing trunk fails, and the proposed 3/4 in. tee fails worse because it also starves the furnace downstream. At 364 CFH and 60-70 ft developed, the answer is generally 1-1/4 in. - pull the actual value from the adopted table and size every segment for everything downstream of it. Do not size to the tankless alone.
2. The pipe is upsized. Is the job now solved? Who has to be involved?
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No - pipe is only half the system. A 250 CFH meter-regulator cannot pass 364 CFH no matter what you hang on the outlet. The utility owns the meter and sets delivery pressure; call them to upsize the meter or to confirm its capacity curve at the delivered pressure. Get that in writing before you install.
Also verify on site, with a manometer and everything firing:
- Inlet (line) pressure at the tankless while the furnace and range are also on. Many manufacturers require a stated minimum inlet pressure under full flow, and a marginal meter shows up here first.
- Manifold pressure set to the manufacturer's spec, not to a generic value.
- Regulator lockup and vent orientation; a regulator that won't lock up explains intermittent lockouts that look like an appliance fault.
If the utility will not upsize, the options are a lower-input heater, a dedicated high-pressure service with its own regulator, or eliminating the tankless - not a bigger pipe.
3. Combustion air, venting, and connections: what does the closet require and what details get flagged at inspection?
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Combustion air: the closet is 6 x 8 x 8 = 384 ft3. The indoor-air volume method allows about 50 ft3 per 1,000 Btu, so 199,000 Btu/h would want roughly 10,000 ft3 - the closet cannot supply it. Either:
- Provide two permanent openings sized per code to a space with adequate volume (one near the floor, one near the ceiling), or
- Better, direct-vent two-pipe PVC to the outdoors per the manufacturer, which also solves the depressurization and back-drafting risk with the furnace in the same space.
Connections and details that get written up:
- Sediment trap/drip leg ahead of the appliance inlet where the appliance doesn't include one.
- Full-port shutoff valve accessible and in the same room, plus a union or appliance connector not exceeding the code length limit and not concealed in a wall or ceiling.
- Support at the code interval table and at changes of direction.
- If CSST is used anywhere in the run, bond it per code - a listed bonding jumper from the CSST to the grounding electrode system.
- Vent slope back to the heater for condensate, approved material only, and clearances to combustibles per the listing.
- T&P relief discharge piped full size, gravity, to a safe point.
Inspect and pressure-test the whole new run before the closet is closed up.