help selecting gas boiler size for steam heating system
I am planning on upgrading from an oil heating system to natural gas, using the National Grid incentive program (Burnham boiler). I have a steam system; 4 floor 20’x60′ (4800 sqft) brick rownhouse (plus unheated basement) with one to two radiators at front and back of each floor. Windows are not very efficient (upgrading those is on the to-do list). My current system is a Peerless JOT-5-S “Supersection Oil Burner”. I can’t really tell how old it is, but certainly over 20 years. It is rated at 166,000 BTU steam. I have had about five different plumbing/heating contractors visit and give me quotes over the past year. My problem is that they are recommending a wide range of boiler sizes. One suggested the Burnham IN-6 (175k BTU), a couple suggested the IN-8 (245k BTU), and one even suggested an IN-9 (280k BTU). I think the IN-6 might be undersized, b/c although our current system works, it does take a while to get the steam to the upper floors (or is it just taking a while to make the steam … I don’t know). Any advice on how to figure this out? It seems I don’t want to go too big, as that would be inefficient, right? A related question: a couple of them told me that newer boilers don’t have a coil through which water would run en route to my gas hot-water heater (my current Peerless does have this). Why would that be? Thanks!
djc85
in Boiler Repair 13 years and 6 months ago
11
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11 replies
deancollins | 9 years and 3 months ago
What does an EDR calculation cost…..and I suspect it may take more than 15 mins…..as suggested above.
Master Plvmber | 13 years and 6 months ago
ejalbk, Consider this: If you look at an efficiency graph of a running steam system you may have a vertical column on the left rising in efficiency percentage points while the bottom horizontal line increases in time in minutes going left to right. The graph line (trajectory?) runs flat the whole time the smaller boiler runs and no steam is made because it has to burn fuel to heat the boiler, the boiler water, then finally make steam pressure. Suddenly the graph line spikes because the fuel being burned is directly translating to heated radiators. Now consider that the more time spent in that high efficiency range, the better. Two things are to blame more than any other for the boiler spending a lot of time in the flat, low efficiency part of the graph: 1. a boiler that is too small to produce adequate steam volume for the system it serves and; 2. a boiler that is too large that must short cycle (turn on-off-on-off often) because it is producing more steam than the system can effectively process. So, to answer your questions, yes you’re nuts. Just kidding. Don’t get mad. There is no better one to have when it comes to a too-big or too-small boiler for steam. Steam’s marginal efficiency is unforgiving and can go from ok to very bad in a heartbeat. The boiler’s got to be right for the system it serves within no more than a 10% margin of error at most.
ejalbk | 13 years and 6 months ago
Please correct me if I’m wrong, but there have been a lot of answers reinforcing the idea that too big of a boiler is a headache. In my experience, too small of a boiler is just as much a problem in the following sense. With a one pipe steam system, if the boiler is somewhat undersized, it may take 45 minutes for the boiler to make pressure. I’ve noticed that this long a wait tends to result in apartments that are close to the boiler getting heat long before the apartments far away from the boiler do. OTOH, a boiler that is a bit oversized can make pressure in 15 minutes which gets all the radiators hot within several minutes of each other leading to more balanced heat (although more on/off switching for the boiler). Said differently, radiators don’t get hot until they’ve been exposed to steam for a certain period of time and that steam will condense before it climbs up in the building with a boiler that is undersized. Am I nuts?
djc85 | 13 years and 6 months ago
Thank you everybody for your clear and concise advice. It all seems to make good sense. I found the following website (http://www.colonialsupply.com/resources/radiator.htm) which seems to offer a systematic way to estimate EDR. I will do that for my system and see what I get. That site talks only about the radiators themselves … doesn’t one also need to consider heat loss in the pipes leading to the radiators? (I have insulated my basement pipes, but the ones in the walls/floors are untouched by me. Anything else I need to consider for EDR? Again, thank you all for your time and input! Much appreciated.
shahnandersen
in Boiler Repair 13 years and 6 months ago
The reason you have a tankless coil feeding into your gas hot water heater is that the house once had a tankless coil only which did not provide adequate hot water, so a prior owner added a gas water heater. Since they already had the tankless coil, the plumber decided that he might as well put warm water into the water heater (at least during the heating season–your boiler should be turned off in the summer). As long as you are happy with your gas heater, there is no need to pay for add-ons to your new boiler to provide preheated water to the gas water heater.
Master Plvmber | 13 years and 6 months ago
Good thinking, Slopefarm. If the house is *much* different in terms of heat loss than the day it was fitted with a heating system then certainly one could do that, but at considerable expense. A more practical way is to size the boiler for the EDR and then vent accordingly. A gas boiler can also be down-fired about 10% to reduce output and help balance things out.
Master Plvmber | 13 years and 6 months ago
Regarding the range in boiler sizes: There is only ONE WAY to size a steam boiler properly and that is to do what is called an EDR (equivalent direct radiation) calculation on your system. It takes all of 15 minutes and requires the plumber to count and measure all the radiators in the system and the accessible riser pipes if they give off heat. Those measurements will yield a heating value via a multiplier that is then compared to the published output ratings of each specific boiler. Those ratings will be expressed in “square feet of steam” and have nothing at all to do with square feet of area or living space in the building. It is simply a measurement of steam volume, just like cubic feet of water or natural gas. A steam heating system can only process, i.e. to convey and condense, a volume of steam which is fixed by the diameter and length of the pipes and radiators connected to the boiler’s outlet. My boilers are often much smaller than others’ because I do the math. Sometimes I lose the job because 3 other plumbers are specifying a boiler that is much larger and the customer feels like more must be better. When it comes to heating a building, more steam is simply more operating cost with no added benefit. The put-back-what’s-there-now method is often a terrible practice that adheres you to a 20-year commitment to inefficiency and elevated costs. As far as the tankless coil you’re referring to in the new boilers, it’s true Burnham doesn’t do that in its residential line, but Peerless and Weil-McLain still do. I prefer Burnham, but they’re all good at this level. I also agree with steam man. Steam travel is largely regulated by system main venting, not so much by radiator venting, which is more of a fine-tuning measure. Master Plvmber
steam_man | 13 years and 6 months ago
Radiator vents will have a minimal effect on controling the rate of steam travel to all points inthe system simultaneously. That is accomplished through main vents. The Gorton charts only tell half the story.
slopefarm | 13 years and 6 months ago
Good answers above. I would add to eman’s suggestion that you do a little balancing to see if you can get the heat up to the top floor faster before you assume the boiler is undersized. Put bigger vents on the top floor boilers (try Gorton D) and smaller vents downstairs (Gorton 4 or 5). (Don’t ask why, but Gorton vent sizes go 4, 5, 6, C and D.) You can replace these yourself so long as the heat is off and the radiators cool. Use teflon tape on the vent threads. Keep monitoring how the system heats up and go with bigger vents where you want to draw steam faster and smaller vents where you want to slow it down and effectively divert some of the steam elsewhere. Ultimately, your goal is to have the rooms all be mroe or less the same temp at the same time.
eman134 | 13 years and 6 months ago
an oversized boiler will cause nothing but headaches ,from constant banging to uneven heating distribution, I ‘d be glad to take a look and do an EDR calculation, so that you can make an informed decision. The preferred method of generating hot water is to make up a seperate zone with an indirect heating tank, more efficient than a heating coil.You might also consider going over the sizing of all air vents to correct the slow heating . If you want a free consultation, my contact info is in my profile.
needmtg | 13 years and 6 months ago
Look at this way…if your current boiler delivered enough heat, a more efficient new boiler of similar capacity will do so too. Many installers do neither a heat load calculation nor a radiation capacity calculation (latter particularly important for steam heat.) Do not oversize, very wasteful. If you’re installing new windows, consider downsizing some radiators, maybe on the top floor, typically they are oversized, then you can go with a smaller boiler. If you’re new to this, note you cannot reduce heat from a radiator using the valve. Get thermostatic regulators, which help if you put them on (only) a few radiators. My 4 story 3200 sqft semi-detached house has a 150kBtu steam boiler.