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Porters, Handymen, and Doorman, or PHD's Blog
  All About Heat & Hot Water  
This article is excerpted from a presentation on Heating and Hot Water to the Supers Technical Association in January and February, 1999.

Making Hot Water
Making domestic hot water with the same appliance that heats the building: In almost every case, hot water should be made by the heating appliance. Costs are as follows:

First Cost: Separate systems need two fuel pipes, two vents, and the associated cost of a larger chimney. In larger installations (more than 12 families) the separate equipment is more expensive to buy. In smaller installations (less than 12 families) the separate equipment may or may not cost less, but the savings from the smaller chimney, etc., will pay for all or some of the cost of the equipment to make hot water with the heating appliance.

Fuel Cost: Numerous studies have shown separate systems burn more fuel.

Maintenance Cost: Separate systems are typically glasslined, direct fired tanks that rot out after a few years, if they don't lime up first. Systems where the heating appliance makes hot water typically last much longer and thus cost much less to maintain.

Advantages of systems that make both heat and hot water: The single combustion appliance runs year round, and thus avoids service problems associated with equipment that is unused for long periods of time, like those experienced with lawn mowers in the spring.

The single flue is properly sized for the appliance, as opposed to a combined flue sized to accommodate two appliances running at the same time. Since the two appliances usually run one at a time the flue is oversized most of the time, thus it runs cool and rots, has back drafts, and other problems.

Very high efficiency boilers are available, while dedicated water heaters are limited to medium efficiency. The savings from buying one appliance only can be applied to buying more efficient equipment.

A typical water heater has a steady state efficiency of about 80 - 85%, but isn't a steady state device, and has a typical AFUE (Annual Fuel Utilization Efficiency) of about 50% due to constant losses up the chimney while not running. A combined system can use a hot water storage tank with no chimney and a direct vent boiler, eliminating practically all off-cycle flue losses.

Advantages of separate systems: Popular myth says that separate systems burn less fuel. This is mostly the result of comparing non-working mixing valves to separate systems. While the myth is not true, is is so persistent that going against it is very difficult.

Thus separate systems have all the advantages Mark Twain understood when he said: "I'm all for progress - it's change I don't like."

How do you make hot water from a heating boiler? There are two good ways:

For buildings with less than about 12 families: Install a Stainless Steel tank with a heat exchanger coil in it.

For buildings with more than 12 families: Install a coil in the boiler and control the water temperature with a working mixing valve.

The reason not to use tanks in buildings with less than 12 families is that the largest tank, 120 gallons, won't handle a larger load. Installing multiple tanks is too expensive; tanks cost $1,200 each and are tricky to pipe properly in multiples. A working mixing valve costs $1,250 to $1,400 and for up to hundreds of families and the coil in the boiler is cheaper than the second or third or fourth tanks that would otherwise be needed.

Here's how the tank makes hot water from the boiler (the blue is cold and the red is hot):

The cold water from the street enters the bottom of the tank and is heated by the coil. The tank both stores hot water and controls its temperature.


The pump moves hot water from the boiler through the coil to heat the water in the tank. The fresh water in the tank and the "rusty" boiler water never mix - the heat is exchanged through the metal of the coil which is exactly that - a coil of tubing inside the tank.

The controls for the tank and boiler work like this: An aquastat (thermostat) in the tank turns the pump on when the tank temperature drops. This should be set at about 115 degrees F to produce water at 120 - 125 degrees F at the faucet.

A separate aquastat in the boiler keeps the boiler hot and turns on the burner flame whenever the boiler drops below a certain temperature, which is about 140 - 170 degrees F, depending on how fast the tank can heat water. Obviously, experimenting with the lowest possible boiler aquastat setting will reduce off-cycle losses.

A fancy way of doing the controls involves keeping the boiler off until it needs to heat up in order to heat water. The simplest way of doing this is to wire the aquastat in the tank to simultaneously turn on both the pump and the burner flame. This is called "cold start" because the boiler is typically cold until needed - the tank stays warm.

A better way is what I call "tentative cold start". A fancy control replaces both aquastats, it has sensors in both the tank and the boiler. It's set to turn the pump on if the tank cools down to 116 degrees F, and turns the boiler on if the tank cools down to 115 degrees. Such close settings are okay when done in the same control; drift won't screw things up.

This arrangement keeps the boiler from turning on if it's already warm enough to heat the tank. On a rise in temperature the boiler turns off at about 120 degrees in the tank, and the pump turns off at 122 degrees in the tank. While the tank warms up those two degrees from 120 to 122 the boiler is off and is being cooled by the tank. This simple arrangement insures that very little heat is left in the boiler, thus minimizing off-cycle losses.

If you're resetting the heating water temperature with weather you'll need either primary / secondary pumping or a mixing valve to allow the boiler to run up to 140 Degrees F or more, while making heating water at 100 degrees or less.

Pumping boiler water: This can be tricky, especially with steam boilers. Hot water heating boilers are easier - just make sure you take the water out up high in the boiler and return it down low in another part of the boiler. This avoids "short circuiting" the water through the boiler.

Steam boilers are more difficult because there is not enough water pressure to please the pump. This problem can be minimized by installing a black steel nipple in the boiler and mounting the pump on the nipple - no elbows, fittings or valves. Pump away from the boiler about 6 - 10 inches below the water line, land return the water into the bottom of the boiler at the other end. Try to avoid the section that contains the water level controls. Use a maximum durability pump in this situation.

With either a hot water heating boiler or a steam heating boiler, wire the pump to turn off with the low water cutoff. This saves buying a new pump next time someone drains the boiler. Also, install a flow-check valve to prevent convective circulation.

How does a mixing valve system work?

A coil in the heating boiler heats fresh water to something up near the temperature of the boiler. This might be 212 degrees in a steam boiler that's steaming, 140 - 170 degrees in a steam boiler that is just running to heat water, or maybe 180 - 220 degrees in a hot water boiler. The point is that the water is too hot to send to the faucet, and its temperature varies depending on the heating load at the moment.

A mixing valve is simply a tee that has 3 connections:

  • Cold water from the street

  • Hot water from the coil in the boiler

  • Mixed to the house

    Here's how to set up a working mixing valve:


Two companies make working mechanical mixing valves; either one, if installed properly, will work for 3 to 4 years before you will need a new thermostat. "Installed properly" means following the parts of the instructions that require the thermal trap and the pump running nonstop on the return. Few people bother, thus the poor reputation of these valves.

Despite the possibility of getting mechanical valves to work, they are obsolete and should not be installed new. A 12 family building would need about a 1 1/2 inch valve, while you can also get an electronic valve which is cheaper to install and lasts for 10 years or more and won't go bad if the return pump stops running. Therefore mechanical mixing valves are obsolete and should not be used. Larger valves for buildings housing hundreds of families cost only a little more money.

A mixing valve.


An electronic mixing valve installation.


The thermometer on the mixer going to the house is important for use in adjusting the valve and to see that everything is working right. The thermometer on the hot coming from the coil is important to see if the coil is doing its job. If there is a complaint someday after you installed the valve, it will be very helpful if you can determine if the valve is getting hot water form the coil, or the valve is not working properly. The thermometers that snap into the 1/2 inch wells seem to last longer than almost anything else for any price.

Tees tapped 1/2 inch for the thermometer will work but are expensive. It's cheaper to buy a regular copper tee with a 1/2 inch opening and sweat in a 1/2 inch Street Copper by female adapter.

Thermometers should go on the outlet of a tank also, to see what the tank is doing. If you put thermometers on the boiler water to and from the coil it will help diagnose any problems you might have. Skipping thermometers is one of the most common and expensive shortcuts.

Several companies make Stainless Steel storage tanks with coils in them. The Tank-in-a-tank might also drain water into a steam boiler and flood it, so that type should be avoided.

The problem with the Stor-Ex is that the fresh water tappings are 1 inch, even on the largest tank. This could lead to pressure complaints. The problem with the Jazz is that the pipe connections are not pipe threads - they use straight threads which are tough to make watertight. The problem with the Veismann is that it costs about twice as much as the others, but it really is the best. Avoid the non-stainless steel types - they rot out and flood the house, never mind the plastic lining (except at the connections).

A Stainless Steel tank installed in a house basement:

Boiler Water from Coil in Tank

The Jazz-Tam tanks cost approximately $650 for 20 gallons, $700 for 40 gallons, $720 for 40 gallons, $840 for 60 gallons, $1,099 for 80 gallons, and $1,590 for 120 gallons - the largest.

Sizing charts can be had from the manufacturer to help you estimate the correct tank size for a given installation. Generally speaking, the 120 gallon tank is good up to about 8 - 12 families, depending on family income, family size, number of washing machines, size of the pump and boiler supplying the coil, etc.

The Veismann charts are the best because they show how the capacity changes with varying flows and temperature through the coil. The Domestic Hot Water consumption And Efficiency" study, # 97-2, available from NYSERDA, call 518-862-1090 (paid for by your taxes) gives a detailed analysis of how much water people use.

In general, the boiler size does not need to be increased to accommodate the load of the tank until you get into 4 or 5 families, but needs to be increased in larger installations where the "peak usage" in more than just a few minutes a day.

Things to remember:

  • Hot water should be delivered at 120 - 125 degrees - no higher.
  • 110 degrees is the legal maximum in some health care buildings.
  • Water comes from the street in New York city at 40-45 degrees, depending on the time of year.
  • One gallon of water weighs 8.3 pounds.
  • One BTU (British Thermal Unit) of heat will heat one pound of water one degree F.
  • The BTUs needed to heat 12 gallons of water per hour from 45 degrees street temperature to 125 degrees faucet temperature:
    125 - 45 = 80
  • 80 degrees X 12 gallons/hour X 8.3 pounds/gallon = 7,960 BTUs/hour. A boiler has to have an input of about 10,000 BTUs to have an output of 7,960 because about 20% of the heat goes up the chimney.
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