Small copper-tube boilers need special consideration for zoning.

OK, so you've read the latest IBR books on system design, you got the book and maybe went to a seminar about old steam systems, and you read the magazine articles on space-cadet advanced systems. You even followed the factory installation instructions. How come the brandy-new hot water system bangs and shudders like it just got a new steam boiler?

What ISN'T in most books and factory instructions is a comparison of what the difference between boilers with a LOT of water in them and the newer low-mass boilers:

Large-mass boilers contain a LOT of water, so it takes some time to heat them up. Lest's say we have a boiler with 6 gallons of water in it and 300 pounds of cast-iron or steel:

Six gallons of water weighs 6 x 8.3 = 50 pounds of water. It takes 50 times 100 to get it from 70F to 170F = 5000 btu.
300 pounds of metal is heated with 1/2 a btu per pound so - 150 times 100 to get it from 70F to 170F = 15,000 btu.

So to get that boiler from 70F to 170F requires 20,000 btu. If the boiler inputs 80,000 btuh into the boiler it takes
(80,000 divided by 20,000 = 4) 60 minutes divided by 4 = 15 minutes to heat it up.

Small-mass boilers with 1 gallon of water in it might weigh 100 pounds.

A frequent problem is using boilers that hold little water in a system that holds little water. Many small boilers are able to provide 100,000 btuh to a system. While a system uses all 100,000 btuh there is little problem; however, many 2000 sq ft floor area houses today require only 25 btuh per square foot due to the good insulation required to save energy since 1992. This means that a house that used to require 40 btuh per sq ft in the 1960's is no longer made, so 'rules-of-thumb' learned by the previous generations no longer apply. A proof of that is the design of radiant-floor heated homes which usually can only supply 30 btuh from each sq ft of exposed floor.

One gallon of water requires 1 x 8.3 = 8.3 gallons of water. It takes 8.3 times 100 to get it from 70F to 170F = 830 btu.
100 pounds of metal is heated with 1/2 a btu per pound so - 50 times 100 to get it from 70F to 170F = 5,000 btu.

So to get that boiler from 70F to 170F requires 5,000 btu. If the boiler inputs 80,000 btuh into the boiler it takes
(80,000 divided by 5,000 = 6) 60 minutes divided by 16 = 4 minutes to heat it up.

SO WHAT? The pump is taking the heat from the boiler and sending it out to the house right?

Single Series Circuit

The answer is MAYBE. Yes, if the boiler is attached to radiation in a series loop and part of that can't be shut off to prevent the heat getting out, then it doesn't matter.

HERE'S THE POINT!

There is this idea of ZONING, which simply means that some of the radiation isn't served when it isn't needed. Then, the boiler burner still is putting in the heat to get the open zones heated; but they don't need the full input. This is not a problem if the controls can modulate the burner down fast enough to prevent overheating the boiler and the high limit is located in a place where the sensor will shut off the burner before the water sizzles and bangs; but that isn't always the situation.

multicirculator zoning

To keep boilers less expensive, many are made with a single-input burner. The burner shuts off when the high limit senses the water at one spot. Hopefully it is the one spot in a bundle of tubes that gets the hottest first (Murphy's law says NO!)

In such a situation, you should have a minimum flow through the boiler to assure that the water won't reach boiling point inside the boiler before it can get out to the piping to go on to the radiation. that is what the boiler-loop system-loop design is about. The water will whiz around the boiler subloop attached to the main because the circulator in the boiler subloop moves the water around at a fixed rate, no matter what happens in the heating circuit.

boiler subloop circuit

BUT, that doesn't solve the problem of the boiler itself having little water and having zones shut off. Whizzing hotter and hotter water around in a circle still can get the boiler to sizzle and bang before the high limit shuts the burner down.

bosubloop

Another way to get enough water available would be to add a BUFFER TANK to the low-mass system.

Then the boiler circulator has the mass of water to heat up in a longer time. The zones can even BOTH be shut off (for the summer for instance) and the tank can be used for making hot water. There are tanks that have large coils inside them to make hot water like an instantaneous heater.

maxsystem

One advantage is that the coil is so large (much more surface than a tankless coil inside a boiler) that it transfers heat from the tank to the domestic water nearly 100%, so the tank can be held at 130F with a condensing boiler used for a slab and the boiler never needs to get hotter than condensing range, assuring maximum efficiency.

.................. http://www.ergomax.com/New-Tanks.htm