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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.
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If 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.
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.
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.
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.
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
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.
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.
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.
See ANTI SCALD VALVES.
For details on how to diagnose a clogged tankless coil and how to repair it,
see CLOGGED PIPES / TANKLESS COIL DE-SCALE.
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.
As we discussed beginning in the previous section of this article, there are several different hot water problems:
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.
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
at WATER HEATER COMPARISONS, PROPERTIES
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 INDIRECT FIRED WATER HEATERS
Or see TANKLESS COILS - home
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(Dec 6, 2012) oliver said:
very useful information
Thanks; questions are welcomed too - working together makes us smarter.
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