No tankless unit is the direct equal of a 50-gallon tank, because a tankless heater has no stored water to match. It is sized by two numbers. The first is flow rate in gallons per minute (GPM), meaning how much hot water you run at the same moment. The second is temperature rise, meaning how many degrees the unit must add to the incoming cold water.
Add up the hot-water fixtures that might run together, check each fixture's labeled flow, and choose a unit whose rated flow at your temperature rise covers that total. A household that rarely runs more than one fixture needs less than one that runs a shower and a dishwasher at once. The sections below walk through the method.
Four numbers to gather before you shop
Sizing is arithmetic, and each step is something you can look up yourself.
List the fixtures that overlap
Think about your busiest morning. Which showers, faucets and appliances run hot water at the same time?
Check each fixture's label
Showerheads and faucets usually have a flow rating stamped on them or printed in the box. Write each one down.
Add the overlapping flows
The sum is the peak flow your unit must supply. Leave a small margin rather than sizing to the exact total.
Find your temperature rise
Subtract the cold incoming temperature from the temperature you want at the tap. A plumber can measure the incoming temperature, and it changes with the season.
Why temperature rise changes the answer
A tankless heater has a fixed amount of heating power. It can heat a large flow by a small number of degrees, or a smaller flow by a larger number of degrees. When incoming water is colder, the same unit delivers less flow at your target temperature.
That is why manufacturers publish a flow rating at several rises, and why a size that works in summer may struggle in winter. Always read the rating at the rise that matches your coldest months, not the largest number on the box.
What we can and cannot say about Sidney water
The City of Sidney water system draws on both groundwater wellfields and surface water intakes on Tawawa Creek and the Great Miami River. Because those sources mix, your incoming temperature is not one fixed number, and we do not quote one. Ask the City water department what it can share, or have a plumber measure at your tap.
The City's 2022 water quality report listed total hardness of about 113 ppm, in the moderately hard range on the USGS scale. Tankless heat exchangers can scale up over time, so plan for periodic flushing, and ask the installer about service valves that make flushing easy.
Call and we will walk through your fixtures and gas line before you buy.
A sizing guide in relative terms
Exact numbers come from your labels and your measured rise. This table shows how the pieces relate.
| Household pattern | Peak hot flow | Sizing direction |
|---|---|---|
| One person, one bathroom, little overlap | Low | A smaller unit usually fits |
| Two people, a shower and a faucet sometimes together | Moderate | A mid-size unit, with margin for winter |
| Family with two bathrooms and a dishwasher | Higher | A larger unit, or confirm overlap is rare |
| Large household, soaking tub, multiple showers | High | A large unit, or two units |
| Water-saving fixtures throughout | Lower than the household suggests | Smaller may work, check labels |
Always confirm with the fixture labels, the rated flow at your rise, and a plumber's site check.
Checks beyond sizing
Gas line capacity
Tankless burners draw more gas at once than a tank burner. An older, narrow gas line may need to be upsized to feed the new unit.
Vent type and size
Many tankless units use different venting than a tank heater. The old flue may not be reusable, and a new vent route may be needed.
Condensate drain
High-efficiency tankless units produce acidic condensate that needs a drain and sometimes a neutralizer.
Electrical supply
Most gas tankless units need a power outlet nearby for the fan and controls.
Water line size
The existing hot and cold lines should suit the unit's flow, or the delivered flow falls short.
Staying with a tank or going tankless
Tank replacement
- Usually the simpler swap, often reusing the gas line and flue.
- Stored hot water covers brief bursts of high use.
- Stand-by heat loss from the tank is part of the cost of running.
- A tank heater often lasts roughly 8 to 12 years.
Tankless
- Heats water as you use it, so there is no tank to run out.
- May need new gas line, vent and drain work.
- Output drops if too many fixtures run together.
- Needs flushing in areas where minerals can scale the heat exchanger.
What shapes the price of the switch
A tankless conversion costs more than a tank swap in most homes, mainly because of the gas line, vent and drain changes. We do not quote a number over the phone. After a site visit, you get a clear price before work starts.
Insurance usually treats a sudden failure differently from slow, long-term leakage. Ask your insurer what applies to a water heater loss.
Where to go from here
If you are replacing a failing tank, our water heater replacement crew can compare a like-for-like tank with a tankless option at the same visit. Our tankless water heater page explains the unit types, and water heater installation covers the connection work.
Homeowners in Anna and Piqua should know that incoming water and hardness differ by water system. Ask your own utility, or test your tap, before you size a unit.
Common questions
What size tankless water heater do I need to replace a 50-gallon tank?
It depends on your peak flow and temperature rise, not on tank gallons. Add the labeled flows of fixtures that run together and choose a unit rated for that flow at your cold-month rise.
Does Sidney's water temperature matter for tankless sizing?
Yes, because colder incoming water means more rise and less flow from the same unit. The City mixes groundwater and surface water, so ask the City water department what it can share or have a plumber measure at your tap.
Will my existing gas line work with a tankless heater?
Sometimes. Tankless burners draw more gas at once, so an older narrow line may need upsizing. A plumber should check the line size and run length before you buy.
Can I reuse the old flue?
Often not. Many tankless units use a different vent type than a tank heater. Your plumber will say what the unit requires and what the installation needs.
Does hard water hurt a tankless heater?
Scale can build on the heat exchanger over time. The City's 2022 report listed total hardness of about 113 ppm, so plan for periodic flushing and ask for service valves.
Is one tankless unit enough for a large household?
Not always. If peak overlap is high, a larger unit or a second one may be needed. Check labels and run through your busiest morning before you choose.
Do I need a permit for the switch?
Call the Sidney-Shelby County Health Department at 937-498-7249 and ask whether a tank-to-tankless job needs a permit, since the rules we reviewed do not address water heater swaps. Do not assume either way.