Sketch of a tankless coil tempering valve or anti scald valve Carson Dunlop Associates How to Get More Hot Water Pressure, Temperature, & Quantity from a Tankless Coil

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Tankless coil or side arm coil hot water improvement guide: here we explain just how to improve the hot water pressure, quantity, flow, and water temperature obtained from a tankless coil used for making domestic hot water.

We review aquastat control settings as well as using a flow limiter or an anti-scald mixing valve to improve hot water supply. We discuss all of the various ways to improve hot water quantity, temperature, flow rate where a tankless coil is in use, such as shower controls and de-clogging the tankless coil.

Green links show where you are. © Copyright 2017, All Rights Reserved.

How to Increase Hot Water Quantity & Flow When a Tankless Coil is Installed - how to avoid running out of hot water

Tankless coil on a steam boilerIf your hot water pressure is declining because of a clogging tankless coil, be sure to see CLOGGED PIPES / TANKLESS COIL DE-SCALE for diagnosis and repair advice.

Article Contents

A tankless coil used to produce hot water is shown in the sketch at the top of this page and again in the photograph at left. The black rectangle is the mounting plate that bolts the tankless coil to the heating boiler - in this case it happens to be a steam boiler.

On this tankless coil cold water enters at the lower of the two pipes connected to the coil. You'll also see that the cold water line continues to enter the bottom of the anti-scald valve.

The hot water leaves this tankless coil from the upper pipe connected to the coil, where it flows down to enter the mixing valve (tempering valve) at the left side of that valve (see the black temperature setting knob on top of the valve?). We discuss anti-scald valve or tempering valves in detail just below.

Tempered hot water (mixed hot and cold) leaves at the right side of the mixing valve and heads for the plumbing fixtures in the building.

On many heating boilers the tankless coil mounting plate is round, not rectangular, and it may be located on the top of the heating boiler as well as on the front, back, or either side of the heater.

Setting the Aquastat Controls to Higher Temperature for More Hot Water (watch out for scalding)

Adjust the aquastat  controls. As we explain in detail at See AQUASTAT CONTROL FUNCTIONS, you can set the Aquastat HI and LO for the boiler to higher temperatures to increase the heat stored in the boiler and thus the hot water quantity produced by the tankless coil. (But still you must keep the HI and LO at least 20F apart). Now if you also set a higher DIFF (move it to 25) you will run the boiler at higher temperatures and will in general store more heat in the boiler. This will cause the tankless coil to provide a more hot water before the boiler itself runs out of heat.

The Basics of Getting More Hot Water out of a Tankless Coil

Especially if yours is a modern steel boiler of small (efficient) size, the thermal mass of the boiler itself plus the water inside the boiler, is very small compared with an old-fashioned, physically bigger, maybe cast-iron boiler. So the amount of heat stored in the modern boiler is much less than in the older models.

No oil burner on a home heating boiler can put heat into the boiler fast enough to keep heating cold outdoor water coming into the tankless coil and headed up to the hot water side of your shower. [Put another way, a tankless coil on a heating boiler is not capable of giving endless hot water - as you've discovered - while a "tankless water heater" (Tankless Water Heaters) can do that, though depending on size, only at a modest rate.]

So once we use up the heat in the boiler, we are out of hot water. The effect of making the boiler come on as soon as possible while the heat is being sucked out of the boiler and into the tankless coil and thence into the cold water running into the hot water side of shower - the effect of all that, is that we slightly extend the hot water time in your shower. That's all.

So our object in giving advice about setting the LO and DIFF dials on the aquastat is

Setting HI 200, LO 180, and DIFF 25 is about as hot as you can get.

Watch out: check the relief valve for drips, and also use an anti-scald valve to avoid burns

Use a flow limiter to increase the length of time that you can run the hot water and to increase hot water temperature

Tankless coils do not provide infinite hot water. Cool water entering the coil draws heat out of the boiler water and into the house water. The oil or gas burner that reheats the heating boiler cannot pump heat into the boiler as fast as the tankless coil is removing heat. That's because water is entering the coil at 40 to 55 deg F in most cases, and it's trying to leave at the boiler temperature that may be close to 200 degF.

So if you run water too fast through the tankless coil it'll draw heat out of the boiler quickly and you'll have great hot water pressure, but not for very long. Then you'll just have tepid or cold water pressure. Some tankless coils have a flow limiting valve mounted right at the coil to prevent water from flowing through the coil too fast.

Using a flow limiting valve on a tankless coil lets you run the hot water longer before you run out by forcing you to run it more slowly.

Use of a flow limiting valve, because it slows the passage of cold water through the water heater, will also permit the water to arrive at the plumbing fixture at a higher temperature - the hot water will be hotter.

Intelligent use of shower controls to restrict hot water flow can save hot water costs and provide longer shower bathing time

Even when a flow limiter is not installed in hot water piping, a bather can save on hot water heating costs by smart use of shower controls. Some bathers turn the hot water all the way to it's fastest flow position, followed by turning on lots of cold water in order to avoid being scalded. Instead of this fastest-flow best water pressure approach, turn the hot water to a less powerful stream, which will require also turning on less cold water to obtain a comfortable shower temperature.

This method of bathing does not really provide a greater quantity of hot water in a building, but by drawing hot water out of the water heater more slowly, one can either have longer time in the shower, or subsequent bathers can have adequate hot water at less total water heating cost.

Use an anti-scald valve to increase hot water quantity and protect from hot water burns

Photo of an anti-scald tempering valveWhy does an Anti-Scald Valve, mixing valve, or tempering valve on a tankless coil hot water system increases the total amount of hot water available?

The use of a mixing valve or anti-scald valve at a tankless coil permits us to set the Honeywell 6006 limit control switch or other boiler temperature or water heater temperature limit control to a higher number without having to worry about scalding occupants of the building.

Keeping the water at a higher temperature inside the heating boiler (with a tankless coil system) or in the water heater (with a separate domestic water heater or indirect-fired water heater system) means that there is more heat stored inside the heating boiler or water heater.

By adding cold water to the very hot water leaving the tankless coil or water heater, we draw hot water out of the water heater itself more slowly than we would without this addition.

Therefore we can expect to draw a larger quantity of (tempered) hot water from the tankless coil on such a system than otherwise. We will have more total domestic hot water for washing and bathing, and we have safer hot water (non-scalding) than if we omitted this pair of controls.

See MIXING / ANTI-SCALD VALVES for a detailed discussion of how to install, set, and use mixing valves, anti-scald valves, or tempering valves on water heating systems to avoid scalding burns and to improve hot water heater performance.

Check the settings of the anti-scald valve or mixing valve - setting the anti-scald valve to a lower output temperature means that it mixes more cold in with outgoing hot, drawing heat out of the boiler and through the tankless coil more slowly than otherwise. You get "longer hot water on time" because you draw heat out of the boiler more slowly.

Make Sure That the Tankless Coil is Not Mineral-Clogged

For details on how to diagnose a clogged tankless coil and how to repair it,

See TANKLESS COILS for an explanation of how these water heaters work and why they clog up and how to stop clogging up the coil.

See WATER PIPE CLOG REPAIR for a discussion of loss of water pressure due to clogged piping or clogged tankless coils.

At LARGER DIAMETER WATER SUPPLY PIPING we discuss the benefits of using larger diameter water supply piping both to improve water pressure and flow and also to delay the clogging of pipes due to minerals or rust.

See WATER SOFTENERS & CONDITIONERS for a discussion of how to use a water softener to prevent hot water pipe clogging and tankless coil clogging (and reduced hot water pressure and flow). A water softener can prevent mineral-clogging of pipes or the tankless coil but regrettably, installing a water softener after your pipes or tankless coil are already clogged will not fix that problem.

How do we Improve the Overall Hot Water Volume & Quantity?

As we discussed beginning in the previous section of this article, there are several different hot water problems:

  1. Poor hot water flow, or bad hot water pressure, which is usually a piping or clogging problem or an overall building water pressure problem

    In this case the water is hot enough and we may seem to have plenty of hot water, but the flow, or pressure of hot water in the building is too weak and (a good diagnostic) the hot water pressure is noticeably less than the cold water pressure in the building.
  2. Poor hot water quantity: not enough hot water, or running out of hot water very quickly. Insufficient hot water quantity may be due to the way hot water is being made, such as a water heater that is too small (perhaps 30 gallons for a large family), a tankless coil on a new small heating boiler, or an instantaneous water heater that is being asked to supply hot water too rapidly. See the hot water improvement articles listed below.
  3. Hot water temperature that is not hot enough: water temperature is just too cool. Insufficient hot water temperature may be due simply to the setting of a temperature control on a water heater or mixing valve, or it may be due to flowing water too fast through a tankless coil. Hot water that is not hot enough can be caused by quite a list of problems, some of which are easy to correct.
    See TEMPERATURE of HOT WATER is TOO LOW for steps to correct water that just won't get hot.
  4. Slow hot water supply recovery time: we have water that is hot enough, and we have plenty of it (or maybe not), but when we run out it takes too long for the hot water system to "recover", that is, to again deliver hot water after previously running out. The speed with which a water heater re-heats cold water which has entered its tank depends mostly on the type of heater and on its overall size in gallons. Here are some examples:
    1. Oil-fired hot water heaters warm-up faster than gas or electric water heaters
    2. Gas-fired water heaters warm up incoming water faster than most electric water heaters
    3. A 50-gallon hot water heater tank that has been cooled down to a low temperature will take longer to re-heat than a 30-gallong hot water tank of the same type (oil, gas, electric, solar)

Articles Describing Steps to Increase Hot Water Quantity

The articles listed below offer more details about steps one can take to increase hot water quantity, pressure, and flow in a building.

The topics discussed in this article address improving hot water flow and improving hot water total quantity.

A Comparison of Alternative Hot Water Heaters & Sources

The following articles discuss alternative ways to produce domestic hot water for washing and bathing.

The characteristics of various water heaters such as life expectancy, cost, safety, and capacity are discussed

Ways to improve total hot water quantity, pressure, temperature and flow are discussed beginning


Continue reading at HOT WATER QUANTITY IMPROVEMENT or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see TANKLESS COILS - home

Suggested citation for this web page

TANKLESS COIL HOT WATER INCREASE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Questions & answers or comments about how to increase the hot water supply quantity or flow rate produced by a tankless coil or by a sidearm coil.

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman