Collapsing building © Daniel FriedmanTankless Water Heater Installation Procedure & Troubleshooting
Case study details installing a tankless water heater to replace a conventional hot water tank & heater

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How & why to install a tankless or demand water heater.

This article describes the conversion from a conventional LP-gas fired hot water tank & heater to a Bosch Confort 13BP N low-pressure system tankless water heater. With text & photos we illustrate both the old and new water heating systems and the steps required to install the tankless water heater.

The article reports on why the owners decided to convert to a tankless water heating system, and on both their satisfaction and their disappointments in the results.

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Tankless water heater installation details: conversion from a conventional hot water tank to a demand-type Bosch instant water heater

LP gas fired water heater (C) InspectAPedia Daniel FriedmanArticle contents

Why owners decided to install an instant water heater

The LP gas fired 30-gallon water heater shown at above left was installed around 2001 and was still in working condition. But the building occupants were not happy with their hot water supply for several reasons:

How to prepare to install a new tankless water heater

LP gas water heater vent troubles (C) D Friedman

Before starting to work on a water heater or water heater swap-out such as this change from a gas fired conventional water heater and tank to a tankless Bosch instant water heater, don't forget these preparatory steps:

How to remove the old hot water heater in preparation for its replacement

LP gas water heater being removed for a swap-in of a new unit (C) Daniel FriedmanThe steps to remove the old water heater are pretty simple:

Corroded metal flue vent for gas water heater (C) Daniel Friedman

New Water Heater Hookup Prep: check water heater flue vent diameter, routing, length, clearances for combustion gas venting

LP gas water heater vent troubles (C) D Friedman LP gas water heater vent troubles (C) D Friedman

The original LP gas water heater (above left) vented through a 4-inch metal flue (green arrow in our photo). We measured the diameter of the flue vent connector on the new Bosch instant water heater (above right), confirming that it wanted a 5-inch flue vent or chimney.

LP gas water heater vent troubles (C) D Friedman LP gas water heater vent troubles (C) D Friedman

Watch out: some installers opined that the new 5-inch vented tankless heater would work just fine connected to a 4-inch chimney.

We did not agree, nor does the manufacturer who is very clear on this point.

Venting a heating appliance into a flue or chimney smaller than the outlet diameter of the vent connection on the appliance itself is asking for trouble including:

LP gas water heater vent troubles (C) D FriedmanOur photograph at left as well as the above right photo show what was required to install the new 5-inch chimney and vent system for this heater. We had to chop open a concrete wall, remove the 4-inch chimney and install a 5-inch diameter chimney in the same location.

At left you can see the 5-inch flue vent connector and elbows entering the base of the chimney for this appliance.

Water heater Installation tip: because we were working in close quarters we decided to get the entire chimney, elbows and flue vent connector in place before setting the exact mounting position of the heater itself.

That was a lot easier than mounting the heater and then wrestling with custom-cut little vent connection pieces to try to match an arbitrary heater mount location to the exact location of the bottom of the metal chimney.

That screened vent shown in the right side of the photo was an add-on exit vent for an indoor gas-log fireplace; we didn't want any interference between those two gas burning appliances and we didn't want any possibility of drawing carbon monoxide gas into the building through the screened vent when the fireplace was not in use.

Check water heater LP or natural gas supply: piping diameter, flow rates, regulators, controls

At below left you'll notice that we chipped away some concrete where the LP gas pipe penetrates the masonry wall. We confirmed that the LP gas line to this point was 1/2-inch in diameter, meeting the manufacturer's confusing specifications on this point.

We say confusing because while the instant water heater installation instructions recommend feeding a 1/2-inch LP gas line to the tankless heater, the actual gas pipe fittings, valve, and flexible gas line provided by the manufacturer in an installation kit named as intended for this Bosch heater were all 3/8-inch diameter.

We decided to connect a new gas shutoff valve to the existing 1/2-inch gas line at the wall and use the kit's 3/8-inch flexible gas line & fittings for the remainder of the connection.

Hooking up water, fuel, electrical details for a tankless water heater

LP gas water heater vent troubles (C) D Friedman

Before starting to work on a water heater don't forget to locate ahead of time

As we warned in our introduction:

At the photo at below right you can see the new water and gas piping connections at the bottom of this instant water heater.

Yellow arrows point to the LP gas line connections.

Red arrows point to hot water piping connections. Notice that for safety reasons there can be no shutoff valve on the hot water outlet line on the heater.

LP gas water heater vent troubles (C) D Friedman LP gas water heater vent troubles (C) D Friedman

Blue arrow points to the cold water connection; you will notice that there is a cold water shutoff valve on the inlet end of this line.

Our photo at far left of the original LP gas line and shutoff show black staining around the shutoff valve and flare fitting, with some of the black worn away - previously there had been an LP gas leak at this location.

Check water heater installation location clearances, spaces, safety, protection from elements, wind, etc.

Collapsing building © Daniel FriedmanOutdoor installations of tankless water heaters

Although in southern climates it is common to install the instant water heater outdoors, the manufacturer, in this case Bosch, requires that the heater be installed under shelter, protected from rain and wind.

You can see that this heater is in a sheltered location.

By placing it as high as we could on the wall under the enclosed masonry stairs, we were avoiding possible wind problems that had plagued the base-located gas burner in the previous water heater installed in this same space.

Indoor installations of tankless water heaters

Watch out: do not install a tankless water heater (nor any other gas or oil fired heating appliance) indoors without providing an approved, properly installed flue vent connector and chimney to vent combustion gases. Otherwise you risk death of building occupants due to potential flue gas and carbon monoxide poisoning.

Watch out: local building codes or water heater manufacturers may require (and we recommend) installation of flue gas spillage sensors and safety controls on all indoor installations of gas fired heating equipment.

Watch out: other installation & safety recommendations from the Bosch installation instructions for the LP gas piping include

How to Turn on the Tankless Instant Water Heater - 5 easy steps to get the water heater running

1. Turn on water supply to the unit and check for water leaks

2. Turn on the gas supply to the unit and check for gas leaks

LP gas water heater vent troubles (C) D Friedman LP gas water heater vent troubles (C) D Friedman

3. Install batteries in the heater's battery compartment (above left, shown empty of batteries), making sure that the batteries are new, of the proper type and rating, and installed with their + and - ends in the proper position.

4. Turn the heater's ON-OFF switch to the ON position (above right) - our photo shows the switch in the ON position.

LP gas water heater vent troubles (C) D Friedman

Above our photo shows how you can verify that the gas flame is on in this tankless water heater - but it's not easy to see on this model. We listen for flame and we feel the exhaust (be careful about burns!)

5. Turn on hot water at a nearby fixture - you should see the flame indicator lit in the front of the water heater (blue flame showing in horizontal slit in our photo at left) and you should feel hot water delivered at the fixture.

Minimum Hot Water Flow Rate Required for Tankless Water Heaters

The burners of an instant or tankless water heater are triggered by sensors that detect the flow of water. If the flow of water is too low, typically less than ½ gallon per minute, the burners will not turn on and no hot water will be delivered.

Minimum Hot Water Flow Rates to Avoid Instant Water Heater Shutdown

Model [using Bosch Confort low pressure series as example] Minimum water flow rate
Bosch 6L 2.5 LPM - 0.66 GPM
Bosch 10L 4.2 LPM = 1.1 GPM
Bosch 13L 4.2 LPM = 1.1 GPM
Notes: the ignition of the instant water heater depends only on the minimum water flow rates shown in the table above. These flows are for and pertain only to a low-pressure water heater installation. [1a]

Watch out: water-saving shower heads, for example, may not allow enough flow to turn the water heater on, especially when the water heater has a high minimum flow rate. Rheem and Bosch (table above), for example, requires a .66 GPM flow rate to ignite the burner on smaller capacity units and a higher flow rate on higher capacity units. (This is probably to avoid overheating in & damage to the tankless water heater's heat exchanger.)

High Elevation Requires Special Modification for some Tankless Water Heaters

Rheem EcoSense tankless instant water heater (C) DanieL FriedmanSome tankless water heaters may not operate properly at high elevation installations. Some manufacturers address this specifically.

For example, all Rheem tankless water heaters can be programmed to operate at elevations up to 9,840 feet. Our photo (left) illustrates a Rheem Eco-Sense instant water heater for sale in a Home Depot store in Phoenix, Arizona (2012).

Tankless Water Heater Pressure / Temperature Relief Valve Safety Requirements

The temperature/pressure relief valve lets water escape if the temperature or pressure is too high. This valve should be connected to a tube that discharges no more than six inches above floor level so hot water won't scald anyone nearby.

Some codes require that the tube discharge outside the building. The tube should be as large as the tank fitting and the tube end should never be threaded, capped or plugged.

The tube should be able to withstand 250°F temperatures, should have no shut-off valve, and should be as short and as straight as possible.

Combustion Air Requirements for Gas-Fired Tankless Water Heaters

In response to complaints of combustion gas odors, CO alarms sounding, and intermittent loss of hot water where tankless water heaters were installed we inspected several rental properties owned by the same landlord [B.P., SMdA, MX]. We found that the plumber she used to install tankless water heaters at multiple apartments didn't realize that each gas fired unit needed combustion air for safe and continuous operation..

Each tankless water heater had been installed on an exterior wall and to avoid tenants fooling with the heater, the plumber enclosed each unit in a steel box with a lock.

The tenants complained of gas odors and of constant loss of hot water. Because the units were installed outdoors the dangerous carbon monoxide that was being produced had not resulted in a catastrophe.

Combustion Air Requirements for Low Pressure Bosch Instant Water Heaters

Model Minimum unobstructed opening to assure adequate combustion air
Bosch low pressure Series 6 BP > 60 cm2
Bosch low pressure Series 10 BP > 90 cm2
Bosch low pressure Series 13 BP > 120 cm2


The Bosch low pressure instant water heater series used as an illustration here requires combustion air as shown here. Combustion air requirements for these appliances are the same for both LP gas and natural gas: [Other brands and models will of course have different requirements]

How did the new tankless water heater perform compared to the conventional hot water tank & heater that was removed

Shower performance with tankless water heater (C) Daniel FriedmanSurprises with the tankless water heater installation

Well the bottom line is the owners are pretty happy with their new instant water heater, with "endless" hot water on demand and no LP gas fuel consumption when no one needs hot water.

Once the occupants learned how to work with the low pressure instant water heater system the result was the ability to run a single shower with plenty of hot water, on demand, without running out. But there were a few surprises and disappointments to get by first.

In this particular installation, running two showers at the same time at the same building level (the building has baths on three different floor levels) results in delivery of warm but not hot water to both showers. If the occupants want a really hot shower they run just one shower at a time.

The variations are most likely due to peculiarities in individual building water pressure and in the actual routing of existing water piping. Bosch recommends feeding a dedicated cold water line directly to the water heater and carrying separate cold (from the cold source) and hot (from the heater) to the location where hot water will be used. But when installing a tankless water heater at many buildings you may find that the existing piping was not routed by that schema.

Shower head change required for good hot water flow from a swap-in of a new tankless or instant water heater unit (C) Daniel Friedman

Tankless Water Heater Hot Water Supply Sudden On-Off & Intermittent hot water losses

At /plumbing/Tankless_Water_Heater_Repair_Guide.phpters2.htm">TANKLESS WATER HEATER REPAIR GUIDE we explain that some tankless water heaters do not heat continuously and constantly. If a particular model has this problem users will find that in the shower the water runs in cold bursts from time to time.

Watch out: In our own tankless water heater installation experience in the U.S. & Mexico we have confirmed that there can be hot water constancy & flow regulation problems, particularly in low-pressure tankless water heater installations.

Depending on exactly how hot and cold water pipes are routed to the tankless water heater and to various plumbing fixtures in a building, low pressure tankless water heater units are sensitive to reductions in water flow rate when additional plumbing fixtures are in operation in the same building.

The result can be

We suspect that for this reason, after about 2004, Bosch stopped selling low pressure tankless water heaters in the U.S. However as recently as in 2012 these units continue to be sold and installed in other countries including Mexico where some homes depend on low water pressure delivered from a (not very high) rooftop water storage tank.

How to Get the Most from Your Tankless Instant Water Heater & How to Avoid Sudden Cold Water Bursts or Complete Loss of Hot Water from a Tankless or Instant Water Heater System

  1. Careful use of cold water mix-in at the plumbing fixtures in use
  2. Careful choice of bath shower head fixtures to provide sufficient water flow rate to be sure to exceed the low-flow cutoff flow rate at the instant water heater. You may find that you need more actual flow than the claimed low-flow cutoff point stated by the manufacturer, perhaps because of building piping characteristics or from water pressure source variations. Ditto for sink faucets. Most tub faucets run plenty fast and won't give trouble.
  3. Setting down the hot water temperature at the tankless water heater and reducing the mix-in of cold water at the plumbing fixture
  4. Installation of a water pressure boosting pump and pressure tank
  5. Installing a dedicated cold water input line from the water storage tank to the tankless water heater cold-input fitting
  6. Replacing the low-pressure tankless water heater with a high pressure unit combined with installation of a water pressure boosting pump and tank system - the most expensive solution to these complaints.

LP to Natural Gas Conversion for Tankless, Demand, Instantaneous, or Point of Use Water Heaters?

Watch out: most tankless or demand water heaters (such as those made by Bosch & by Rheem) are sold in models designed for a specific fuel such as Natural gas OR LP Gas.

Some instant water heaters such among the Rheem units cannot be converted from LP to Natural Gas or vice versa. So be sure you are specifying the proper model and fuel type when buying a tankless water heater.

Fuel Alternatives for Tankless Water Heaters: Butane, Natural Gas, Propane: proper set-up required

Several readers have asked about the availability of a butane-fueled tankless water heater, and about converting tankless water heaters among fuels using butane (C4H10), propane, or natural gas. Indeed it is possible to convert most gas-fired heating appliances among fuels. But conversion to the proper pressures, combustion air settings, and fuel jets is necessary unless the appliance has been delivered already set up for the fuel you intend to use.

Bosch and some other tankless or point of use water heaters provide a conversion kit to change a natural gas model water heater to a propane or butane fuel source, or vice versa.

Details about the difference between LPG, Butane, and Propane and Natural Gas are discussed

Other tankless water heater accessories & kits include


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