With the consistent climb in equipment costs, structure owners and engineers are constantly in search of means to decrease initial and energy expenses for their HVAC jobs.

You are watching: Fire tube boiler vs water tube boiler

This has thrust the sector typical to pumps on variable rate drives, and high performance condensing boilers.

These condensing boilers have the right to be split into 2 subcategories based on the volume of water they have the right to host, and also leads installers to consider High Mass Fire-tube Boilers vs. Low Mass Water-tube Boilers.

Fire-tube Boilers vs. Water-tube Boilers

There are a number of components to think about as soon as determining which kind of boiler mechanism is optimal for your application.

As you think of the demands of your system, compare these differences to validate that you are picking the ideal sort of system:

Fire-tube Boilers

Water-tube Boilers

Hot flue gases circulation inside the tube and also the water outside the tube.Water flows inside the tube and also the flue gases external the tube.
These boilers are almost constantly internally fired. The heating system is inserted at the one end of the fire tube.These boilers are mostly externally fired
Requires a larger shell diameter because the fire tube situated inside the boilerRequires a smaller shell diameter bereason it is externally fired
Normally have actually reduced press drops in the selection of 2-15 ft. H2O, depending on dimension and also circulation rate.Generally have higher pressure drops in the selection of 10-30 ft. H2O, relying on size and also circulation price.
Contains a relatively large amount of storage, upwards of a number of hundred gallons for larger systems.Contains a fairly tiny amount of storage, never acquiring past 100 gallons even on the largest units.

A Huge Difference Between Fire-tube Boilers and also Water-tube Boilers

One significant difference between these 2 boilers that has actually influenced the design of devices is the reality that better mass boilers perform not should maintain a constant flow.

Fire-tube boilers deserve to take care of a large selection of capacity with them.

By utilizing simply one set of variable speed pumps to serve our boilers and also terminal units we have produced what is referred to as a Variable Main System.


Variable Primary Systems have actually gave an alternate to the conventional Main & Secondary Design which is still required for water tube condensing boilers and other boilers via a low mass design.


With a Variable Key Flow style, we eliminate the require for a second collection of pumps within the boiler loop, which can reduce the initial cost of our system.

One renowned topic of conversation about these large mass boilers, when they are designed in a Variable Main System, is the boiler’s minimum circulation rate and just how it affects the system.

The Boiler’s Minimum Flow Rate

One formula we will certainly must understand also for this topic is a heat move formula that is offered rather often in HVAC:


Considering that HVAC jobs in the United States generally meacertain water flow in gallons per minute, not pounds per hour, and that the c worth of water at 60ºF is about 1.0, the formula is often simplified to read:


When relating this equation to a boiler we deserve to analyze it as:

The total warm created by the boiler = 500 x Flow with the boiler in gpm x Temperature increase in °F

So just how low deserve to we circulation water with our boiler?

Some necessary information necessary to fix this equation is the minimum capacity of our boiler.

If this is not provided, it have the right to be calculated making use of the turndown of the boiler and its maximum capacity.

The last piece of information necessary is the temperature rise.

Generally this will be designed by a mechanical engineer and is frequently collection somewhere in in between 20ºF and also 60ºF.

To discover the minimum circulation price feasible with the boiler, we will need to consult the boiler’s hands-on and also look up the largest temperature climb allowed per the manufacturer.

This climb is usually 80ºF or 100ºF. All boilers have a maximum temperature climb.

If the inlet and also outlet temperatures are any wider, it will certainly produce enough thermal stress on the heat exchanger to break it.

As an example, let’s say tbelow is a 3,000,000 BTU/hr boiler that is capable of a 10:1 turndown.

That indicates the minimum output is 300,000 BTU/hr.

If the boiler manufacturer states that the max temperature climb of our boiler is 100ºF we obtain the following equation.


Solving for the flow price, the minimum circulation is 6gpm.

If the circulation is any type of reduced the boiler will shut itself off in order to defend it from damages.

All boilers have a minimum flow rate at which the boiler deserve to repetitively fire also if it is implicitly proclaimed.

Taking a look at the operations manuals for a number of major manufacturers of Fire-tube boilers, they all have actually a relatively equivalent sequence of operations.

They all usage a device supply temp sensor and also follow the (simplified) order of operations below:

When there is a speak to for warmth, the boiler will certainly revolve on at low fireIf the collection point is not got to the boiler will increase its firing rateIf the collection point is exceeded then the boiler will certainly decrease its firing rateIf the collection point is exceeded at the minimum firing rate then the boiler will certainly shut off

Using the boiler from the previous instance and also a mechanism style temperature rise of 40ºF the heat carry formula looks like this:


This offers a minimum mechanism flow price of 15gpm.

Any much less than this and the boiler will certainly surpass its set suggest and shut off until the supply temperature drops aget.

It is important to limit the number of times the boiler will cycle on and off in a offered duration of time.

The even more often the boiler cycles, the more typically the boiler will certainly have to be serviced.

Frequent cycling of the boiler will certainly also bring about an increase in energy intake as a result of the loss of the thermal energy stored in the warm exchanger and vent being lost to the outside.

If a structure is going to have regular durations of very low warmth demands compared to its style point, an excellent solution would be to have numerous smaller sized boilers to boost the boiler plants turndvery own.

While any fire-tube condensing boiler can take care of bit to zero flows with the heat exchanger while the unit is not firing, no boiler can sustainably proceed to fire once circulation via the boiler is less than its natural minimum running circulation.

In order to properly size a boiler plant, the designer requirements to look at the complete selection of warmth tons that will be compelled for the structure and also be aware of the restrictions of the devices they are choosing.

All of This Being Said

In a hydronic mechanism, both fire-tube and water-tube boilers can be viable services.

A water-tube can be a great selection on a retrofit job via limited space for a large boiler.

A primary additional erected is incredibly basic to incorporate with an existing device.

A fire-tube deserve to additionally be the much better solution if you are developing a mechanism with just one set of variable speed pumps in mind.

They have actually the ability to take care of differing flows via their warmth exchangers without risk of damage.

It is still crucial to keep in mind that also fire-tube boilers have actually their limits in regards to circulation variance.

See more: Why Is Trans More Stable Than Cis, Cis Isomers Can Be More Stable Than Trans

Every building is distinctive, and likewise the ideal boiler plant solution will certainly depend on the mechanism you are looking at.

Written by Application Engineer, Will Dargan, place in our Annapolis Junction, MD office. We offer and service Lochinvar boilers; call us for more info!