Steam to Hot Water Conversion

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Customers with steam heating systems oft en ask about the possibility of converting to hot water and it’s been my experience this is driven by three basic needs: #1 is reducing the annual operating costs; #2 uneven distribution of heating; and #3 is safety due to small children or elderly being burned (1-PSI steam delivers 215F radiator surface temperature). Over the past 40-years, I have performed dozens of these conversions. I’ll share my secrets and tips in this article.

Steam boilers are sized to the connected load. The connected load calculation consists of measuring each radiator (column, tube, or baseboard) to determine its EDR (Equivalent Direct Radiation), which is expressed is square feet and converted to BTUs using a chart like the ones found in the Burnham Heating Helper booklet (http://pro.usboiler.net/PDF/htghelper.pdf — see Chapter 5). Adding 30% pick-up-factor to the radiators’ total EDR compensates for the EDR of the piping system.

Radiator sections need to be coupled together on both the bottom and top so that air can be eliminated and water can flow between sections with warmer water rising unimpeded to the top of each section. Steam-only radiators may touch, or not, across the tops of sections and cannot be converted. Radiators designed for use with hot water or steam will have threaded plugs located on an end section — midway for a steam vent and near the top for the water air vent.

In order to determine the proper sizing for hot water, the building’s heat loss, not the connected load, is the rock-solid foundation required and where energy conservation gold is to be discovered that will add value to your designs. Done on a room-by-room basis, you can now go back to compare each room’s radiator to the heat loss and, from that, determine what actual water temperature is required inside that radiator to offset the design-day heat-loss. The tool of choice: ACCA’s Manual-J.

Converting each radiator requires you drill out the upper vent-plug and chase the threads, remove the steam trap and install a union-ell in its place or remove the guts of the trap to permit water to freely pass through, and replace the radiator valve. Thermostatic radiator valves in parallel or home-run piped systems are another great up-sell feature for seldom used rooms.

ZONING: I used to be a circulator guy. A circulator for every zone — hated zone valves due to decades old history of leaks and weak motor failures. Over the past two decades, zone valves became very reliable and leak-free. Today, we have ECM (electrically commutated motor) circulators that use 80% to 90% less power than induction motor circulators and zone valves available using a miserly 1- to 3-watts each. Ignoring the power-consumption side is no longer wise and up-selling jobs based on power conservation has become a key component of my sales pitches.

THE BOILER CHOICE: the heart of your new system. There’s really only one best way to go for your conversion and that’s incorporating a modcon (modulating condensing) warmed water boiler. You have already determined the hottest water temperature required for a design-day. Let’s deal with the real-world efficiency potential VS the stated efficiency ratings. Let’s agree the manufacturers’ ratings are 82% for the steam boiler and 95% for the modcon. At face value, they would appear to be just 13% apart, yet historical data collected from our customers reveals actual reductions in energy consumption averaging 30% to 50% and well above 50% in some cases. How can this be true? Here’s why:

• Every time the boiler runs, the modcon burns at a much lower rate than would have its steam counterpart.
• The golden reward (your design work) for your customers lies here: The modcon adjusts its fi ring-rate as outdoor weather temperatures moderate and couples that with a sliding scale of upper water temperature limits. If we need 110F at 10F outdoor air temps, we certainly have no need to make the water that hot if it’s 50F outdoors. For every 3F we lower the boiler’s upper limit, we will enhance the systems energy consumption- efficiency by 1%. We carved out a significant chunk just by dropping from 215F steam to 110F water (35%) and more will be gained as your finely-tuned outdoor reset curve steps in to lower the upper limit over time.

I am more conservative with customers regarding projections of lowered energy consumption, and, for a steam to hot water conversion, I’d be hesitant to suggest (note I do not promise) more than 30%: under-project and over-deliver!

Posted In: ACCA Now, Hydronics

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