What About Higher Efficiency?

Raypak’s residential pool heaters are 82% efficient, meaning that for every 100 BTU of flame, 82 BTU actually goes into the water.  The other 18 BTUs are lost in various ways, primarily as heat going up the stack.

It is possible to achieve higher efficiencies, but there is a downside.

As efficiency rises, less and less heat is available in the stack.  The heat in the stack doesn't heat the water but it does serve a useful function -- it provides lift for the flue gases, in precisely the same way that a roaring fire in a fireplace provides draft to push the smoke and fireplace gases up the chimney.  Because the flue gases are heavy with water vapor, carbon dioxide and carbon monoxide (as well as other vapors formed as byproducts of combustion), it's important to discharge them to atmosphere as efficiently as possible.  Vent pipe sizes are given a carrying capacity based on cross-sectional area, and the height/routing of the stack, and the heat of the gases they carry.  As the heat in the stack drops, less and less lift is created and the gases are less efficiently discharged.

At about 85% thermal efficiency, there is a distinct change in the physics of the flue -- the available heat is no longer adequate to keep those vapors in suspension any more, and it starts to condense.  It forms a watery film on the inside of the flue stack, and combines with other chemicals in the flue gases to form a highly acidic liquid called condensate.  Condensate corrodes standard B-vent flue pipe quickly, so any condensing vent system has to be made of corrosion-resistant materials.  Such pipes are significantly more expensive than normal B-vent.

The higher the efficiency of the heater, the lower the flue temp and the less lift is generated.  When the system efficiency is high enough to condense, you rapidly get to a point where you can't depend on buoyancy from heat (the industry term is “draft”) and instead have to use fans to force the flue gases to go where you want.  That adds cost and a new wrinkle -- the vent pipe has to be made differently again.  In addition, you have to collect the condensate and pipe it away from the heater to someplace safe, etc.  Most areas allow the condensate to be neutralized by running the acidic fluid through a container of a chemical base to form mineral salts and water, which is then disposed of down the drain.  Some areas require that condensate be treated like toxic waste, collected and trucked away for expensive disposal.

Normally, as the flue gases rise through B-vent, the pressure inside the pipe is reduced a bit (Bernoulli's law in effect).  As speed increases, pressure decreases; that's what generates lift on an aircraft wing.  In this case, it means that the pressure inside the pipe is LESS than the air pressure in the room.  If there is a pinhole leak, room air is sucked into the pipe -- vent gases don't spill into the room because the pressure differential goes the other direction.

But when the flue gases are downstream of a fan, they are generally at a pressure HIGHER than room air because of the push from the fan.  Now the vent pipe has to be designed with joints that are leak-proof because the pressure differential goes the other direction; if there is a pinhole leak, flue gases spill into the room.

We do a lot of condensing installations on larger commercial jobs, but it is important to be aware of the other costs associated with them.  The fans and/or high-grade flue pipe can easily add thousands of dollars to the cost of the job, and can raise the installation cost to the point where the payback period is so long that you just can't expect to save money in the long run. 

Because of this, Raypak does not manufacture residential pool heaters with efficiencies high enough to condense.  Such systems can be handled using high-efficiency commercial units.