The short answer: tankless installs are a house upgrade, not a swap
A tankless water heater heats water on demand with a burner or electric elements, so it needs far more fuel or power at one moment than a tank does. That is why a tankless install is often a gas line, venting or panel project as much as a plumbing one. The plumber looks at those three things before quoting.
If your existing heater is a gas tank in a closet, the new unit may need a larger gas line, a different vent and a place on the wall. If it is an electric tank, the unit may need heavy circuits that your panel cannot supply. Ask for these items in writing, so the quote is complete.
The four checks that decide the job
Each check can add or remove work from the quote.
| Check | What the plumber looks at | What it can change |
|---|---|---|
| Fuel supply | Gas line size and meter capacity, or panel capacity | A new gas line or an electrical upgrade |
| Venting | Path to outdoors, distance, wall or roof termination | Venting runs and wall penetrations |
| Water flow | Supply pressure and pipe size | Larger supply piping in some homes |
| Drainage | Floor drain or condensate route | A condensate line or neutralizer |
Thinking about going tankless?
Call and ask for a visit to check gas, venting and panel capacity.
What happens on install day
A typical visit follows a set order, and the plumber explains each step as it happens.
- Shut off and drain the old tank. Water and fuel are closed, and the old heater is drained and removed.
- Mount the unit and run vent and fuel lines. The plumber follows the manufacturer's instructions for clearances and materials.
- Connect water with isolation valves. Valves let the unit be flushed later without cutting pipes.
- Commission and test. The plumber tests ignition, temperature and flow, and shows you the controls.
Size by flow, not by family size
A tankless unit is rated by how many gallons per minute it can heat at a given temperature rise. In a cold New Hampshire winter, incoming water is colder, so the same unit delivers less hot water than it would in summer. That is why sizing starts with the number of fixtures that run at once and the coldest water temperature you expect.
A household that showers in the morning while the dishwasher runs needs more capacity than the same household using one fixture at a time. Some homes use two smaller units. The plumber adds up your peak demand and picks a model with some margin.
Gas, electric or condensing: how the choice plays out
Gas tankless units come in two broad types. Standard models vent hot exhaust and can often use a single-wall or stainless vent, while condensing models extract more heat and produce acidic condensate that needs a drain and a neutralizer. Both need combustion air and a sealed vent path, so the install location matters as much as the model you choose.
Electric tankless units avoid venting entirely, which makes them easy to place, but they pull heavy current. A whole-house electric unit can ask for several large breakers, and many older panels cannot supply that without an upgrade. Smaller electric units suit a single bathroom or a point-of-use job, which is why panel capacity is the first question the plumber asks.
Whichever fuel you choose, the unit needs access for service. Leave clear space in front for the panel and below for the isolation valves. A tankless heater squeezed into a tight closet is harder to flush, harder to repair and more likely to be neglected.
Winter performance in a New Hampshire house
Incoming water temperature changes with the seasons. In a cold New Hampshire winter the supply can be much colder than in summer, so the heater must raise the temperature further at the same flow. The result is that a unit that handles two showers in July may handle only one in January.
Plan for the worst month, not the best. Ask the plumber to size the unit for winter flow, and consider a model with a higher capacity than the minimum. If the household is large, two units or a unit plus a small tank can be a better answer than one unit stretched to its limit.
Derry notes for a tankless project
The Derry code enforcement office at 603-432-6148 requires 24 hours' notice for inspections, and commercial applications and new houses or additions with sleeping rooms need a code office application and Fire Prevention plan review. Ask the office whether your water heater swap needs a permit before the work is scheduled.
Derry's water department serves about 15,000 residents, institutions and businesses and can be reached at 603-432-6147. If you are on town water, ask what the department reports about your supply before deciding on scale protection. If you are on a private well, the state recommends testing, and the result tells you whether a filter belongs ahead of the unit.
Sources: derrynh.gov code enforcement and water department pages. NH DHHS and NHDES private well testing brochure.
Where to go next
If you want the full service description, see our tankless water heater page. If your current tank has failed, the water heater replacement page covers a faster route back to hot water. We install in Derry and in Londonderry and Windham as well.
Frequently asked questions
Often yes. A tankless unit burns more fuel at once than a tank, so the existing line may be too small. The plumber measures the line and the meter before quoting.
The manufacturer's instructions set the allowed materials, lengths and terminations. The plumber chooses a route through a wall or roof that meets them.
A straightforward swap can take most of a day. Gas line work, new venting or an electrical upgrade adds time. The plumber gives a schedule after seeing the space.
Yes. Descaling keeps the heat exchanger clear. Isolation valves installed on day one make the flush simple.
It can depend on the work, especially for gas and venting. Call Derry code enforcement at 603-432-6148, and the plumber will tell you if one applies.
Get the checks done before you buy
Call to schedule a tankless water heater visit in Derry.