To convert or not to convert

Bearses

My two pipe gas steam boiler needs replacement. I have two different options: continue with steam using a Williamson GSA100 Steam Boiler, adding a Rinnai 7.5 hot water heater; or convert to hot water using a Triangle Tube Prestige Excellence 100 with a tankless coil. How much more efficient will a gas hot water system be than a gas steam system (I understand that the Triangle Tube hot water boiler is about 12% more efficient than the Williamson steam boiler)?   

The Steam Whisperer (Formerly Boilerpro)

Things to consider......

Steam radiators are often designed to operatate with 215F steam.   This supplies a lot of heat from a small radiator.  When converting to water, and especially the Prestige and most other condensing boilers, the average maximum raditaor temperature is only about 185F.  This produces alot less heat from a radiator.  Can the radiators still heat the home at these los temperatures?



Do you have pipes or radiators exposed to freezing temperatures?  Steam rarely has freeze up problems, and when there is a problem the repair is usually minor.  Freeze ups in hot water systems can cause enormous damage and repairs.  Also, power outages become a major concern.

Nearly all steam systems run at a few ounces of pressure (1/2 PSI).  Typical home hot water sysetms run about 25 PSI.   Are your pipes and radiators ready for this huge increase in pressure? You'll probably find l leaks you did not know of.

Government Efficiency ratings (AFUE) between differents types of heating units cannot be compared.  The testing done for efficiency only accounts for heating fuel usage and completely ignores electrical usage. Most small gas fired Steam boilers use virtually no electricity, while hot water boilers use at least 10 times more (hot air funaces usea about 100 times more)  "High effieciency" hot water boilers typically use 2 to 5 times more electricity than standard 80% efficiency hot water boilers due to the power needed for draft fans, and additional, and usually much larger, pumps.  Electricity is a much more costly energy source so it needs to be considered.



A quailty two pipe system is often a vapor system and may include additional devices contained in the piping and radiators that will interfere with water flow.  This potentially means alot more work.

Two pipe systems can usually be converted to room by room temperature control and vapor system operation, making the system one of the simplest and most efficient available.

High efficiency hot water boilers are much more complicated than a typical gas steam boiler.  Repairs are likely to be more frequent and much more costly.

Steam, overall, is probably a better heating method than hot water, even if there are currently still some limitations on boilers efficiency for steam, due to simplicity and ability to deal with freezing without damage ( the number of frozen up hot water systems I have seen recently due to foreclosures is frightening).

That said, I am a fan of the Prestige Boilers, and use them for about 2/3 of my hot water boiler replacments. 

Bearses

Thank you boilerpro

I really appreciate your assistance here. You have provided much more room for thought -- leaks, electrical usage, maintenance. Any thoughts on the general efficiency of gas hot water versus steam? I put a conversion burner on my furnace, switching from oil to gas and it ended up costing me about 20 percent per year more (but furnace maintenance has dropped to nearly zero). Also, does hot water heat more evenly given it circulates through the pipes more often?

The Steam Whisperer (Formerly Boilerpro)

Relative Efficiency

All things being equal, with both systems in good shape and properly controlled, the differences in efficiency are minimal.  With cast iron radiator,  both deliver high percentages of more efficient and comfortable radiant heat.  Also, the mass of the radiators allow both systems to heat quite evenly, with  a hot water system on outdoor reset having some advantage. When moving to condensing/modulating hot water boilers, things can change in favor of hot water.  The Prestige is among the best choices because it uses a small circulator for the boiler and may not even need a second one for the system.  Most High efficiency hot water boilers need very large circulators and a second for the system,  so electrical usage is very high.  The advantages of high efficiency hot water boilers with outdoor reset are likely going to begin to shrink as modulating/ condensing/ reset steam boilers begin to make headway into the smaller boiler market.  They haven't quite arrived yet.

Bearses

Radiators

The radiators on my system are convectors with copper tubing, not cast iron. The subdivision I live in (built in 1949-50) has some steam heat but mostly hot water heat, both using identical looking radiators. I found one neighbor who did the steam to water conversion and said he used less energy, but could not remember how much. I've read where you can expect a 16-39% increase in efficiency when going from steam to hot water the average being 27 percent. Is that what you have found? The fuel is natural gas, so my guess is that it would be more efficient with water given that it doesn't need to go from the 160-180 to 212 degrees

Steamhead

Your neighbor's figures

mean absolutely nothing, unless we know what condition the steam system was in when converted. My guess is it needed at least some work- most do. You could probably achieve the same thing by fixing the steam system rather than converting it.



But there are those who continue to beat this drum, trumpeting their savings figures. You have to really get into the fine print to learn the steam system was on the verge of total failure when it was converted. This is typical in schools.



Boilerpro is right- there isn't much difference between the two methods of distributing heat. Stay with the steam. Instead of the Williamson, look into a Slant/Fin Intrepid or Smith 8 series boiler with a power gas burner. These offer 6-7% better efficiency than the typical atmospheric gas steamer.

Jamie Hall

Efficiency

I would be exceedingly wary of someone claiming a 20 plus percent difference in fuel use between steam and hot water of comparable age and quality installation.  As has been noted by boilerpro and steamhead (two of the very best in the business), the gains in efficiency for modern equipment are marginal, at best.  Under certain (rather limited) conditions, a modern mod/con with outdoor reset and sophisticated controls can do better than a modern steam boiler; partly because of slightly better efficiency in the heat exchanger (boiler), but mostly from latent heat recovery from the stack gas.  The difference in temperature mentioned in the 10 AM 27 December post is not relevant; good heat exchanger design will overcome what little difference in heat transfer rate there is between the flame and the water.



Balanced against that are the fact that you have a steam system in decent operating condition, but which could use a more efficient, modern boiler.  The question then becomes one of cost recovery (assuming that you do have adequate radiation to handle the lower temperatures of hot water radiation): will the cost of conversion of the steam system to hot water be recovered in a decent length of time by the slightly better efficiency of the hot water system (don't forget to factor in the additional maintenance which hot water systems require, and the additional power requirements for the mod/con boiler and the associated pumps).  I can't answer that for your specific situation; in the building I supervise, the answer is an emphatic 'no', even with today's very low cost of money.

Tim_75

Expansion, please

I was hoping you could expand on some of your statements.



You say that the two systems in comparison, steam and hot water, both deliver "high percentages of more efficient and comfortable radiant heat". More efficient than what? Efficiency is usually based on a system's performance in regards to the amount of fuel and electricity required to provide a given output to the space.



"Most High efficiency hot water boilers need very large circulators and a second for the system, so electrical usage is very high." Do you have an example? Flow rates for boilers and for systems are dictated by the system design dT, and this does not change for high efficiency boilers. Does it?

The Steam Whisperer (Formerly Boilerpro)

Expansion

Compared to hot air/convection systems.  Radiant heat systems increase the temperature of the surfaces within a space, so the human body loses less heat to its surroundings.  Lower heat loss from the body means lower air temperatures can be used to maintain the same comfort level.  Lower air temperature in turn reduces air ex filtration through reduced pressure at high points and lower air delta tee's between the inside air leaked out and the outside air that comes in.  Also radiant heat systems tend to reduce air temperature stratification within a space, so there is less hot air at ceilings leading to lower temperature differentials through insulation and less induced air leakage caused by hot air rising out of any leaks at the top of heated spaces.



Delta tees remain the same for high efficiency systems, in fact, generally, larger delta tees are better for high efficiency boilers (to certain limits) which reduce pumping power needs.  However, most high efficiency boiler heat exchanger designs have a high resistance to water flow, often to increase turbulence and eliminate layers of insulating water at heat exchangers surfaces.  To pump 10 GPM through most cast iron boilers requires less than 1 foot of head.  To pump 10 GPM through most high efficiency boilers requires 7 feet of head or more.  More head pressure requires more power for the same water flow.

Also, most cast iron boiler designs have little trouble dealing with variable flow found on zoned heating systems, so constant flow through the boiler is not required.  However, high efficiency boilers have minimum flow rates that must be met, so zoned systems typically required two pumps. There appears to be more flexibility in flow in mod cons than many manufacturers let on to, but they cover themselves by requiring minimum sized pumps and primary/ secondary loops.

World Plumber

Electricty

By bringing my steam system up to par I cut my fuel usage to almost a third of previous years. I  started running return piping to convert to hot water. Dan talked me out of I and I am very grateful he did. One other factor to loo at. Depending on the size of your system how much more water will you be heating. In my system I'm heating 8 to 10 gallon of water for steam. Factoring in pipe size and length and the radiator volume from charts I would have 130 or more gallon of water to heat. Pretty much negates if not less than the BTU's to convert to steam.

       I removed the radiator from the kitchen when I wasn't as smart. I have a loop with a modine pushing hot air into the kitchen and laundry room. The blower and pump cost me an additional 40 plus dollars per month when it get cool out. Based on the difference between. nonheating and heating months with everything else electric in the house turned off.

     I have a customer who wants to take some radiators out. Can you guess where they are going to find a new home?

Tim_75

Dissappointing answers

I thought that I would get something other than rote standard plumber response the radiant/steam crew has been pitching for decades. Most of these arguments make a little more sense when applied to IFH radiant heating. IFH, for instance, does tend to limit stratification and can be comfortable at lower air temperatures, but this doesn't apply to point radiant heating systems, like classic steam. Steam radiators are excellent convectors and provide nearly as much heat transfer by that means as radiant transfer. The surfaces they heat directly via radiant transfer also transfer heat to the air in the space effectively. The air temperature difference is minimal, if it exists at all, else those not in direct "sight" of a radiator would be cold. The idea of reduced exfiltration due to a minimal temperature difference is a leap. I haven't heard that one before. You get points for creativity.The difference in water head loss through similar sized high thermal efficiency (condensing) boilers vs lower efficiency non-condensing boiler is also much less than the 7 to 1 you cited (I checked 30 gpm through a PK C300 vs 30 gpm thru a W-M PFG-6, about 2 to 1), and in comparison with the dP in the system piping, control valves and heating elements, is insignificant. Low(er) efficiency boilers have minimum flow rates required through them as well, so there is no difference there. Minimum flow rates do not "require zoned systems to have two pumps". They require the pumps and controls installed maintain a minimum flow, as does any system. Primary/secondary piping arragements are due to the preference (or lack of knowledge/sophistication) of the installer/designer, not the boiler manufacturer. Have a Happy New Year!

Gordan

A cup of kindness

Well, I guess the last sentence sweetens the salty tone of the rest. :-)



You certainly sound like someone who knows his stuff. You should know, then, that boilerpro's statement about pressure loss through most mod-cons should be interpreted in the residential context in which it was made. In that context, it is indeed true. One could still cherry-pick an exception to the rule, but the rule holds.



Likewise, if you consider the rest of boilerpro's answer to your two questions in the proper context, I think you'll find them a lot less disappointing. This is in no way to say that you don't make some excellent points yourself.

Tim_75

Thank you

for pointing out the unecessary "saltiness". My bad.



Point was that given the knowledge and experience BoilerPro has, and the respect herein he has earned, my expectations were very high. If the rules holds, the rule holds. I didn't cherry-pick, I just picked an example that was on my desk.



Some caution should be used, even for seasoned pro's like BoilerPro, that much of the hype pitched around by those of like minds and interest (i.e around here) though never challenged, is still hype. You want to err on the side of "mostly right" or entirely correct?



Don't misunderstand me. I fully believe that these systems are awesome, comfortable and efficient means of heating. Sticking to only factual claims is more than enough. The expansion in to the realm of almost correct disinformation is uncalled for and unnecessary. "Just the facts, ma'am".