Converting From Steam To Hot Water

July 01, 2009
July/August 2009
A version of this article appears in the July/August 2009 issue of Home Energy Magazine.
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Q . Is it feasible to retrofit cast-iron single-pipe steam radiators to work on hot water from a new combined heating/domestic hot water (DHW) system?  And if so, would it ever make sense to integrate that into a solar hot water system (SHW)?  I’ve heard that hot water radiators are more efficient than steam in the San Francisco Bay Area.

Tom White
Home Energy Magazine

Steve Greenberg replies:

STEVE GREENBERG  is a facilities energy management engineer at Lawrence Berkeley National Laboratory and a technical editor at Home Energy.
Such a retrofit is possible, but you might need more area (bigger radiators) if you’re using hot water rather than steam, mostly because the water isn’t as hot as the steam, and partly because phase change heat transfer (that is, condensing steam) is more effective than convection (that is, hot water in the tubes); note that the effectiveness of the heat exchanger is not the same as system efficiency. But if the conversion is being done at the same time as the house is being tightened (better windows, insulation, and so on), it might work well with the existing radiators.

When it’s really cold, one might need to crank up the temperature of the water to get enough heat, which will compromise the system efficiency somewhat (more if it’s a condensing boiler or water heater). One caution is that you’d definitely need to have a heat exchanger to keep the heating water and DHW separate (the DHW will corrode the cast iron, so you need a closed loop, preferably with some corrosion inhibitor in the water); also, any building inspector who cares won’t allow any system previously used for nonpotable purposes to be connected to potable water.

Regarding solar—sure, they can be connected, but there isn’t a good match between solar availability and need for heat. DHW and solar is better (since you need DHW all year), but if you look at when you need the backup heat for DHW, it’s pretty much the same time you need the most space heat. Cold, sunny days might be a good match, but if you have a decent envelope and good passive-solar capability, you probably don’t need heat then anyway. (My wife and I will probably morph our solar DHW system into a DHW/heating hot water (HHW) system, because while our southern facade is blocked by the neighbor’s house, we have pretty decent solar access on the roof.)

Hot water heating systems are more efficient than steam pretty much anywhere. Steam systems can be simpler, but they’re really a vestige of the 19th century.

The steps are as follows:
  1. Determine the design heating load of the house (function of the envelope, design outdoor condition, ventilation, and warm-up oversizing factor). There are standard procedures for this (for example, ACCA Manual J).
  2. Determine how much “radiation” area you have. You can find useful tables on pp. 71 and 72 of Holohan’s Lost Art of Steam Heating.
  3. Decide on your operating temperature and apply a relevant heat transfer coefficient. See, for example Table 8-1 “Heat Emission of Hot-Water Radiators” on p. 284 of Jennings’s Thermal Environment.
  4. If 3 is greater than 1, you’re there. You can do a reset on your HHW temperature (often done) to make the water hotter when it’s cold outside; that helps you get there.

Regarding heat transfer and efficiency—yes, they are different. What matters for system efficiency is how efficiently the heat is generated, and how efficiently it is delivered. Unless you operate a steam system below atmospheric pressure (possible), you can’t get the boiler to operate in the condensing mode. Thus the efficiency is limited to the mid-80% range. With water, if you can get the return water temperature down below about 130°F, you start to condense the water vapor out of the flue gas and push the efficiency above 90%. With return water temperatures below 100°F (but still capable of space heating), you can get the efficiency above 95%. (The numbers are approximate, depending on other factors.) The fact that condensing steam is better at heat transfer, and (more importantly) the fact that the temperature at which it condenses is higher than you would typically run a hot water system, means the radiator part of the system is more effective—that is, you can deliver the same amount of heat with a smaller radiator. That doesn’t affect the efficiency. Often efficient systems use large heat exchangers to make the fluids on either side of the exchanger closer in temperature, which makes the source of heat or cooling more efficient. For example, the larger the radiator, the colder the water can be for a given heat delivery, and the higher the boiler efficiency can be.

>> For more information:
Holohan, Dan. The Lost Art of Steam Heating. Bethpage, New York: Dan Holohan Associates, 1998.
Jennings, Burgess Hill. The Thermal Environment. Indianapolis, Indiana: Addison-Wesley, 1978.

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