Okay, based on some of the comments that I have seen on this forum, I am thinking that there may be some confusion on how to properly design/size, install, and setup an expansion tank for a hydronic heating system. Therefore, I am thinking a dedicated thread on this subject might be worthwhile...and perhaps even entertaining too...

For all practical purposes, hydronic fluid is incompressible, will significantly expand when heated, and can generate high pressure that may destroy an unprotected hydronic radiant floor heating system. Typically, both an expansion tank and a backup pressure relief valve are used to protect a hydronic heating system from this potentially destructive hydronic fluid expansion.

To start out, we prefer to use diaphragm-type expansion tanks to avoid waterlogging the tank (i.e., a standard tank eventually becomes filled up with water because the air is slowly absorbed by the water) and the associated additional maintenance. Furthermore, it is never good practice to allow dissolved air into a hydronic heating system.

So, assuming there is a general consensus on this, what then is the best practice for designing/sizing, installing, and setting up a diaphragm-type expansion tank in a hydronic system?

Relative to designing/sizing a diaphragm-type expansion tank in a hydronic system, I will propose that a properly designed/sized tank allows the system pressure to reach within about 5 PSI lower than the pressure relief valve opening pressure setting when the system reaches its maximum operating temperature. This is consistent with Sieg's design recommendations so there should not be much disagreement here except perhaps from those folks who simply use rules-of-thumb or so called "catalog engineering" to incorporate the hardware that they sell into their so called designs.

Thoughts on this expansion tank design/size approach and subsequent best practices for actually installing and setting up an expansion tank...

My entertaining suggestion is that a properly designed/sized tank is one that does an effective job for the least $, accounting for both purchase cost and time and that a preemptive ad hominem attack like "catalog engineering" doesn't demonstrate integrity or professionalism.

Jonr, we would also agree with you that a properly designed/sized tank is one that does an effective job for the least $, accounting for both purchase cost and time...which is precisely the point of this discussion. A properly designed/sized and set up diaphragm-type expansion tank will not contain any hydronic fluid until the system first begins to get heated and the fluid begins expanding. This allows purchasing the minimum size/cost tank that will accomplish the design requirements.

I assume you mean that "catalog engineering" doesn't demonstrate integrity or professionalism. In which case we would certainly agree and we would add competence to this list too.

An expansion tank is simple stuff as compared to heat pump or condensing boiler for example...which would be good subsequent educational topics.

So do you have any definitive thoughts or best practice guidance to contribute on this subject? I for one am sincerely hoping to learn some new installation tricks.

Well, since you ask, my rule is to size it so that there is no more than 5psi change over the working range of the system and because I do a lot of solar, this means sizing the tank considerably up from typical sizing. As much as some people are parts changers and enjoy billing for service calls, I would rather do new installs and not plan in having to change tanks. Less pressure changes, I believe, means a longer lasting tank. This isn't always possible but that is the goal.

The specific heat of the heat transfer medium. The fill temperature of the medium. The design temperature of the system, or volume/temperature of multi-temp systems. Plug into Siggy's software Hydronics Design Studio "Professional Version" and all done. Since two diaphragm expansion tanks will cover 95% percent of the residential hydronic systems in the N.America, and doubling the bigger of the two, in series of course, will cover 99%, it should be a short subject.

Most will not gain pressure under design conditions, if properly sized. Over-sizing an expansion tank in residential applications is matter of 10-20 dollars, i.e. much less than the cost to install it.

It is not the air absorbed by the old style steel tank, but the fresh "oxygen rich" makeup water that I object to.

Mike, very interesting perspective. I would agree that for solar applications where one would see many heating cycles, sizing up to reduce the magnitude of the pressure cycles seems prudent. Other than the higher initial tank cost, there is no harm in using a larger sized expansion tank and likely a reduced maintenance cost benefit for doing so for solar applications.

Yes Badger, we would certainly agree that using Sieg’s Hydronic Design Studio "Professional Version" software is a good way to determine the minimum expansion tank size and determine the proper pre-pressurization value for the air-side of the diaphragm for a given hydronic system design. We recently added free DIY expansion tank design software to our website which may be used to properly accomplish this design too.

With regard to setup, the pressure on the air-side of the diaphragm must be adjusted to the required design pre-pressurization value BEFORE the system is filled with hydronic fluid. An under-pressurized tank will perform like an under-sized tank and may cause the relief valve to open each time the system is heated.

With regard to installation, diaphragm-type expansion tanks should always be mounted vertically with their inlet connection at the top and should always be installed in the system very close to the inlet side of the circulator pump(s). Failing to do this may result in air being trapped on the fluid-side of the diaphragm (which can result in premature tank corrosion/failure), may reduce the system's ability to expel air (or even result in air being drawn into the system via the air separators), and may encourage destructive circulator pump(s) cavitation.