Tankless Coil Hot Water
Abaondonment or Conversion

Abandon, leave in place, or use a Tankless Coil to Heat a Separate Water Tank

Abandoning a Tankless Coil Hot Water System

If you prefer to stop using a tankless coil for making domestic hot water (for washing and bathing) you can certainly do so by installing a different hot water source such as an independent water heater or an indirect water heater that is warmed as a separate heating zone on your boiler system.

Abandoning a Tankless Coil That is Not Leaking

Inspect the face of the tankless coil and its mounting bolts and any other penetrations (such as for an aquastat) at the coil plate for leaks and corrosion.

If no leaks or corrosion are found then your plumber will simply turn off valves controlling cold water in to the coil and hot water flowing out of the tankless coil.

Then she should cut off the cold water line into the coil and hot water line out of the coil, capping those off so that should the coil itself develop a leak in the future boiler water won't leak out of the system.

Separately the plumber will then connect the cold-in and hot-out piping to your new hot water source.

Abandoning a Leaky Tankless Coil

Watch out: if the tankless coil is leaking around the coil mounting plate or its mounting bolts, simply turning off and disconnecting the tankless coil inlet and outlet pipes we mentioned above won't be sufficient to stop leaks that may damage or even ruin the heating boiler itself.

In this case, because it's usually necessary to completely remove the tankless coil anyway to replace a leaky gasket or to re-seal leaky mounting bolts, we recommend replacing the entire tankless coil assembly with a "bliind" round or rectangular plate of the same dimensions as the original coil mounting plate.

The old leaky coil is discarded and the new blind plate mounted in place to seal the front of the boiler.

Really? Well not always. IF, in addition to the "cold-in" and "hot-out" pipes passing through the coil mounting plate there was also a tapping to support a boiler aquastat control, AND IF that control is still needed to control the boiler (it may not be as you're no longer making domestic hot water by that coil) then your new "blind plate" will need to include the same tapping to re-mount that control.

In our photo below a boiler limit control is mounted right through the square tankless coil mounting plate - this control will still be needed aftre abandoning the tankless coil.

If this plate isn't leaky then the plumber will cut and cap the COLD-IN and HOT-OUT pipes indicated in our photo.

Converting a Tankless Coil to Indirect Water Heating

Photo above: at right, a green, indirect-fired water heater, center: the heating boiler, and at far left a water-to-air heating and cooling system. Notice the boiler control aquastat mounted throughthe round black "blind" coil plate (no tankless coil just a plate with a tapping for the primary control mount).

Bottom line: an effective way to convert your domestic hot water source from a tankless coil is to abandon the tankless coil entirely and to install a separate indirect fired water heater - a hot water water tank that provides a larger reservoir of hot water and that is heated as a separate heating zone by your heating boiler.

In this design a circulator pump sends hot boiler water through a loop of pipe that in turn runs water through a heating coil that is immersed inside the indirect water heater tank.

In essence, this approach turns the concept of a boiler tankless coil inside out.

• In a tankless coil installation
  
  domestic hot water is run though a coil that is immersed in the heating boiler (or is along side it as a side arm coil) and the domestic hot water is heated as it passes through the coil.
• In an indirect water heater installation
  
  boiler hot water is run through a coil that is immersed in the hot water tank, and the volume of water stored inside the indirect water heater tank is heated by that coil.

Because the volume of water that is found inside the heating boiler itself and the loop of heating zone pipes, radiators, baseboards, and tne (new) indirect water heater heating coil (that is inside the indirecdt water heater tank) is fixed and unchanging, the problem of corrosivity that plagues many tankless coil hot water systems is avoided.

If you install an indirect water heater, the tankless coil on your heating boiler will be abandoned completely - it will no longer be used - as we describe in detail

at ABANDONING a TANKLESS COIL

What is the difference between an indirect-fired water heater and the tankless coil on a heating boiler?

A tankless coil water heater

delivers "on demand" hot water by sending building cold water through a coiled copper pipe that is immersed in the hot heating boiler's own water. The boiler water heats the coil that in turn heats water passing through it.

However a tankless coil doesn't provide endless hot water because cold water passing through the coil will take heat out of the boiler faster than the boiler's burner can put heat back in.

Tankless coil water heaters are described in detail

at TANKLESS COILS - home

An indirect-fired water heater

uses a separate reservoir tank of "hot water" (a hot water tank) that is heated by circulating hot water from the heating boiler (photo below-right, short and gray to left of the tall blue water tank) through a heat exchanger "coil" mounted in the bottom of the water tank.

The domestic hot water used by the building occupants is inside the water tank.

The quantity of hot water available is a function of the size of that tank, typically 50 gallons, plus the operating temperature range (higher temperature + mixing valve gives more than 50 gallons of hot water). Usually, any indirect fired water heater tank will provide a greater quantity of domestic hot water than can the tankless coil.

At INDIRECT FIRED WATER HEATERS we describe indirect-fired water heaters in detail.

Why do people convert from using a boiler-mounted tankless coil to indirect-fired water heaters?

The specifications for a typical tankless coil / hot water heating boiler indicate that the maximum flow rate through the coil has to be limited, say to perhaps 2.5 to 5 gallons per minute.

Running hot water faster than that will mean the hot water isn't so hot - it needs time (slow passage through the coil) to absorb heat, and it means that you will run out of hot water sooner (sucking heat out of the boiler faster).

Those specifications also typically promise only 5 minutes of hot water supply (that's 25 gallons). Actually some happy users of tankless coils get more than that by combining a flow restricter, high boiler set temperature, and a tempering valve or mixing valve

(see MIXING / ANTI-SCALD VALVES) at the coil .

At HOT WATER QUANTITY IMPROVEMENT we discuss various ways to get more hot water in a home. One of these

Which gives me more hot water: a tankless coil on a boiler or an indirect-fired water heater?

Let's mention again the performance limits of a conventional boiler-mounted tankless coil.

Keep in mind we're talking about a finned copper coil immersed into the heating water found inside a home heating boiler whose primary design purpose was heating the home, not making domestic hot water for washing and bathing.

The hot water quantity available from a tankless coil depends on

• The hot water flow rate - how much hot water you run at a time or how fast you run hot water out of one or more plumbing taps
• The heating boiler's input BTUh rate - the burner's capacity to put heat back into the boiler
• The incoming cold water temperature - how much energy will be required to warm the incoming water
• The setting of anti-scald devices in the building - that control the rate at which hot water is drawn, separately from how far the hot water tap or valve has been opened.

Ultimately the cold water running through the boiler's coil will have absorbed so much heat that the boiler water temperature drops and the burner turns on. But unless the hot water flow is turned off, the burner won't (in a normal residential heating boiler system) be able to heat water faster than it's being run through the coil.

Another Option: On Demand Tankless Water Heaters

Note that unlike a tankless coil, and unlike an indirect water heater, an on-demand tankless water heater (illustrated below) is designed to heat water up to an acceptable temperature for as long as water is flowing through that heater type.

Tankless water heaters are described in detail

at TANKLESS WATER HEATERS

...

Other Tankless Coil Conversion Idea: hook up a circulating pump and hot water tank to the existing tankless coil system?

Some homeowners and fewer plumbers have tried to increase the volume of domestic hot water by keeping an existing tankless coil in use, but using it to heat a separate tank of domestic hot water by circulating water to and from the water tank through the tankless coil.

And indeed, it may be possible to produce a larger volume of hot water for domestic use than the tankless coil can provide "on demand".

This is not an approach that we recommend, particulary if the present tankless coil is leaky and needs repair or replacement, though we've inspected a few installations of this design.