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Gas heated steam radiator system

okapi222 | Posted in Mechanicals on

I have a small dilemma regarding heating systems in our retrofit. We are renovating our brownstone row house in NYC such that it will have a tight air barrier envelope and moderate insulation (R30 in cellar ceiling, R 13.5 exterior walls, R6 windows and doors and R 45 roof. The house came with a cheap gas boiler and steam radiators. It didn’t look in great shape and we were hoping to switch to mini-splits. But now we are running out of money and we are considering just getting the old gas boiler tuned up. Does anyone have any opinions on this? What are the implications of having a fairly blunt un-zoned heating solution for this medium performance house? Thanks, Daniel

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  1. GBA Editor
    Martin Holladay | | #1

    A gas boiler and steam radiators, if the system is well designed, can certainly provide a comfortable interior environment. The sizes of the radiators are probably proportional to each room's heat loss. It's also possible to install non-electric zone valves to regulate the heat output of your radiators.

  2. Expert Member
    Dana Dorsett | | #2

    It kind of depends on just what shape the steam system is in: If this is a 100+ year old system with seriously rusted piping, or the pipes are running outside of your new insulation, you may want to consider other options.

    But there are worse roads to travel than re-commissioning a functional steam heating system, and the higher efficiency the building envelope, the less the efficiency of the heating plant matters.

    But there are several ways to boost system efficiency and comfort with old steam systems. To get reasonable temperature balance & flexibility you can effectively micro-zone 1-pipe steam with thermostatic radiator vents for well under $100/room, eg:

    For more money you can even get them with wall-thermostats:

    You can gain quite a bit of system efficiency by going that route, since you don't end up overheating some spaces in order to be comfortable in others, and you can set them back as-desired for sleeping, etc.

    Tuning a non-modulating gas boiler is typically a once every 3-5 years kind of deal, but you should be able to get at least 75% raw combustion efficiency out of any steam boiler built in the past 20 years.

    Tuning up the steam SYSTEM is also an important part of the re-commissioning- verifying that the system vents are working properly not stuck open or closed, etc. making sure the pressuretrol hasn't been over-cranked in response to gunked up vents, etc.

    There is little rocket science involved, but since much of the world has moved on many HVAC pros are pretty clueless about steam (though in NYC you probably can find plenty of pros who aren't.) If you want to understand and maintain the system yourself (recommended), you can do worse than reading the annotated Torah chapters on it:

    The main system vents are usually out of sight, out of mind for the typical homeowner (who may not even be aware of their existence), and it's common to see systems with all new radiator vents and the system pressure cranked to 5 lbs or something ridiculous to make it all work, when the problem was a corroded up main vent. Normally you would only need to run it with the pressuretrol running between 0.5-1.5lbs (unless your Brownstone is 5 stories tall or something? :-) ), and if it's set higher, it's time back it off and to check all your system vents (it's typically just one), not just the radiator vents.

    It's also pretty common to find the Hartford loop nearly rusted through (or actually seeping), since corrosion will be most-active at the boiler's water level in the plumbing. (The Hartford loop is the U-pipe on the return line prior to the boiler- explained here: ) That's the most susceptible system piping, and it's common to see it all shiny and new, hooked up to the miserable old stuff, but if it isn't, that's where you should look for trouble first, and head it off before it becomes a problem if there is any serious question.

  3. gusfhb | | #3 some serious steamheads there, including the author of the linked book

  4. okapi222 | | #4

    Thanks Martin, Keith, Dana,
    These are exactly the resources I was looking for. I am not sure if the pipes are a hundred years old but there are pretty corroded especially in the cellar. I think the boiler itself is only about 7 years old. We will have to make a decision on it pretty soon. One negative is that if keeping steam is only a short term solution, then removing the pipes at a later date after the renovation, is a messy job. Thanks again for your help. Daniel

  5. JREG88 | | #5

    Dan, I'm now in a very similar situation as yours--retrofitting a house built in the 1930s that has a steam system in need of about 10k worth of improvements to get it running right. We'll be insulating the walls as well to R14, attic to R49, and sealing up holes. My dilemma is what to do about heating/cooling. A forced air system could work but I think a hot water system and/or radiant floor system would be more comfortable. I'd use minisplits to handle cooling if I went the hot water route. I'm curious if you decided to keep the steam and if so, how do you like it?

  6. GBA Editor
    Martin Holladay | | #6

    It's hard to evaluate your situation or to make recommendations with the limited information you have posted. But I'm a believer in simplicity. If you plan to install ductless minisplits to handle your cooling, why not use the same minisplits for heating, too? That way you will save the thousands of dollars that you are planning to spend on a steam system or hot water system.

    -- Martin Holladay

  7. Expert Member
    Dana Dorsett | | #7

    As with any heating system, getting reasonably accurate load numbers is essential for making rational decisions. It's highly likely that mini-splits sized for the cooling load can also heat the house in US climate zones 5 and lower, but the distribution may or may not be adequate to heat all rooms adequately with just a few ductless heads, even if it works fine for cooling.

    Steam boilers have to be sized for the radiation to work properly, and the radiation is probably ridiculously oversized for the load even BEFORE insulation upgrades, and LUDICROUSLY oversized for he "after" picture.

    Part of the oversizing was due to common practices that came into place afer 1918 influenza pandemic. It was deemed necessary to over-ventilate to reduce the transmission of the virus, and heating systems were then sized to fully heat buildings even with the windows open. This led to major discomfort and overheating when the windows were closed or the weather less severe. To increase comfort people resorted to painting the radiators with a low-E silvery or glittery-bronze/gold paint to reduce direct radiation, cutting the heat output by 10-15%, and building radiator covers to limit the rate of convective heat transfer into the rooms.

    In all likelihood there is enough radiator to heat the place with 100-115F water instead of 215F steam, but whether the cost of converting it to a condensing hot water boiler using the same radiation isn't clear. Clarity begins with a room-by-room load calculation of the "after" picture of your building upgrades, along with a room-by-room radiator measurement. In most cases single-pipe steam system conversions to pumped hot water are cost-prohibitive, but many 2-pipe systems can be converted without selling a kidney to cover the cost. So, get out your crayon and start calculatin'... heat load first, then radiation, and the load/radiation ratio in each room.

    Use this estimate radiator sizes in equivalent direct radiation (EDR):

  8. JREG88 | | #8

    Martin, Thanks for your reply. My initial plan was to use the ductless minisplits for both heating and cooling but the proposals I received back for the heating/cooling minisplits were significantly higher than a forced air solution or a combination of converting the oil powered steam boiler to gas and adding a few minisplits that would handle cooling only. My sense is the ductless minisplits work best in houses with lots of insulation. I had a blower door test done on my house and it came back with 5,800 cfm50. The consultants I hired said after I was finished insulating and sealing, we should be able to get to 2,000 cfm50. The other challenge I had with the ductless system was how to heat the bathroom upstairs. I guess we could do electric baseboard. I'm trying to achieve higher efficiency than my current steam system but as Dana put it, "without selling a kidney!"

    Dana, I had a steam expert look at my single pipe system and he told me I had 375 sq ft of EDR and my existing boiler was putting out 570 sq feet. After I insulate and seal the envelope, it will be LUDICROUSLY oversized as you stated. The steam contractor is recommending a boiler that puts out 94MBU per hour. Thanks for the link you sent, which helped me calculate my boiler size (375 EDR x 240 = 90MBUH). Based on this, I think the proposed boiler is properly sized. If I keep the steam, I'm sure the house will will heat up very quickly. What I'm having trouble assessing is if it's worth pursuing other systems that can have multiple zones since I currently only have one zone.

    I just used the heat loss calculation on slantfin's website and came up with about 21,000 BTUs of heat loss for my two floor tudor in northern NJ. I'm thinking of using Radiantec's radiant heating solution for my first floor and baseboards for my second floor. Here's a little more detail about the upgrades I'm planning. I'm having Intus windows installed throughout the house (U factor of 0.19). I'm assuming this will reduce my heat loss significantly along with the R-14 walls, R-49 attic and R-37 for the 2nd floor room over the garage. I will be double checking with any contractor I use to make sure they're really taking into account my insulation and window upgrades when designing the heating system. If anyone has insight on the huge variation in the figures I've mentioned, please let me know (I'm referring to the steam boiler size recommendation of 90,000BTU, an HVAC contractor telling me I need 38,000 BTUs of forced air, and my calculation using slantfin's app giving me 21,000BTUs (this last calculation does not include my basement since it is not currently being heated (aside from the steam pipes).

  9. Expert Member
    Dana Dorsett | | #9

    A 94MBH boiler might be reasonably sized for the radiation, but it's all ~5x oversized for the heat load, based on your calculations. The standby losses of a steam boiler are enormous, and the 94MBH boiler will be spending 80% of it's time in standby mode even at design condition.

    If you undersize a steam boiler for the radiation it can't work properly, so in a low-load home you're really kind of stuck.

    Slantfin's simple I=B=R tool reliably oversizes reality, typically by ~25%, give or take, possibly more for low-load homes.

    A condensing gas water heater with a 76MBH burner can run a micro-zoned hydronic system without short cycling due to the thermal mass of the water in the tank. Its not cheap to set up a gazillion zones, but zoning it by floor isn't too tough. It's fine to use cheap fin-tube baseboard with 120F water (well into the condensing zone), but be sure to get the room by room load numbers right for reasonable room temperature balance. At water temps much below 120F the response gets to be very non-linear. Figure on ~200 BTU/hr per running foot with an entering water temp of 125F.. You may find this bit o' bloggery useful when sketching out hydronic heating options:

    Another micro-zoning option is to use room-by-room low profile fan coils, which can be oversized and still work efficiently. HTP is hawking such a scheme for use with their condensing water heaters and modulation condesing boilers, but it's possible to do it with other vendor's equipment. With low temp baseboard it's likely that not every room will have sufficient free wall length to get enough baseboard to fully cover the load, but that's not a problem with fan coils. Fan coils used in conjunction with an air source chiller can also be used for air conditioning. Your house can probably be heated & cooled with a 2-3 ton Chiltrix or similar, at a comparable or cheaper operating cost than condensing gas.

    Don't let the heating contractor run the heat load numbers- 19 times out of 20 they'll screw it up, especially for higher-R houses with low-U windows. Hire a certified professional engineer or RESNET rater to run the room by room numbers using aggressive assumptions (as required by Manual-J, but often ignored by HVAC contractors and others). With the room by room heating & cooling numbers

  10. JREG88 | | #10

    Dana, the chiltrix looks interesting (and affordable). I will try to find a contractor in the Northern NJ area. If you have any recommendations on how to go about this, please let me know. I'll start searching the internet in the meantime...Thanks for the useful links and information.

  11. Expert Member

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