Choosing between High Mass or Low Mass boilers

By Jack Daniels

There has been much discussion on low and high mass boilers as of late. With high mass boilers now being available in highly efficient modulating and condensing configurations only adds more wrenches to that discussion.  The decision of what boiler to use is often made in the boiler room. Today, let’s discuss the real basis on how to choose what boiler is right for you. Wise contractors know the system will tell you what boiler is best.

Taking a look at how the ideal hydronic system should operate is as simple as considering the idea that what is produced is used.  In other words, if we can achieve equilibrium and have the boiler start up in the fall and not shut down until the spring then we will be as efficient as the boiler’s ratings. As we all know, this is not always possible, heck, it is almost never achieved, thus the need for options in the configuration or mass of the boiler.

Suppose we have all of the proper elements of a hydronic system in their proper locales. The heat source (boiler), circulator, expansion tank, make up water, and heat emitters all have their own temperature and flow requirements. The objective is to achieve a reliable, inexpensive, comfortable, and over all efficient system. Failure to look at all aspects of the system will ultimately add to costs (initial and ongoing), as well as under heating, overheating, wasted energy, and most importantly, a disappointed customer.

First, let’s look at the low mass boilers. A few of the obvious benefits include low cost, high efficiency, and space saving. Another benefit to the low mass boiler is recovery time. A low mass boiler will come up to temperature very quickly as it does not have a lot of water to heat. This is ideal in applications that handle heating and domestic hot water needs. So, if I have a 2,500 square foot house that has one zone of in-floor heat the boiler can go from 120 degrees for the heating of the floor to 180 degrees for heating an indirect water tank very quickly.  Sweet! End of discussion. No need for high mass boilers.

What?! Your customer wants the bedrooms a tad cooler than the living spaces and wants to have the master bath warmer than the rest of the house? This creates the need for micro zones.  Now we have created a zone that can only “use” 6,000 BTU’s. Our low mass boiler can only modulate down to 19,000 BTU’s. We are producing more than we can use. Houston we have a problem. The boiler is going to take the water for the small zone to temperature very quickly (which is one of the advantages of a low mass boiler) and be forced to shut down.  The small zone, however, is still calling for heat so the boiler has to fire again and again as it takes the water for that small zone up to temperature very quickly and shuts down.  Repeat. Repeat. Repeat. Considering that each time a boiler fires it takes a short amount of time for the flame to stabilize and be most efficient in addition to stressing the components of the boiler, short-cycling will lead to a very unhappy customer.

So the answer here is to use a high mass boiler.  Awesome! Always use a high mass boiler when micro-zoning. End of discussion.  But now we have to keep all of that mass up to 180 degrees even in the summertime to produce the domestic hot water in addition to needing a mixing valve for the central heating. Though standby losses have been greatly reduced, they still exist. This is a system that should have mass added to the central heating side but not to the domestic water side.  See our last article in Wisconsin perspectives titled, “Ailing hydronic systems: Is there a doctor in the house?”  As you’ll recall, in that article we discussed buffer tanks in detail.

So if we do not have a domestic load in the aforementioned scenario should we use a high mass boiler? This author says yes. It is the best of all worlds. It can take advantage of outdoor reset and handle the micro zones with the energy stored in the higher mass.

Now, let’s take a look at high mass modulating and condensing boilers. We are no longer restricted to using cast iron behemoths that are not very efficient.  Manufacturers have recently introduced very efficient boilers that have the mass or storage needed for the micro zoned applications.  Other advantages of high mass boilers include lower pressure drop, less maintenance, and ease of installation as there is no need for primary secondary piping.

I would also use high mass boilers in any system that you would suspect debris. Retrofit applications replacing cast iron boilers are an ideal place for a high mass boiler. Gone are the worries about plugged exchangers, water treatments, and endless flushing. Let’s not forget high mass boilers were first then the low mass boilers came on the market as higher efficiency options.  Now, with high mass boilers being as efficient as their low mass counterparts, I view them as another option in the arsenal of comfortable, money saving options heating professionals can offer their customer

So the answer about whether to use high mass or low mass boilers stays the same; it depends.

For information from the author, contact Gregory “Jack” Daniels, Hot Water Products, Inc, (877) 377-0011, HotWaterProducts.com

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