New single pipe steam boiler, Boiler feed tank or Hartford Loop?

ofs1

Hi, I'm evaluating 2 bids to replace a boiler in an apartment building. It is a low pressure single pipe steam system. One of the bids specifies a Hartford loop. The other bid recommends using a boiler feed tank rather than a Hartford Loop. The boiler feed tank is a significant extra expense. Roughly 8% of the total system cost.



The contractor suggesting the boiler feed tanks says it is safer for the system to have a feed tank.



Any help on this is greatly appreciated. Thanks.

Danny Scully

How Big?

How big is the system, how big is/are the proposed boiler(s)?

Charlie from wmass

Pumps are used

when you can not get enough water column to get the condensate to push by gravity back into the boiler. The answer maybe neither proposal is right. In the Boston area they love boiler feed pumps. It is just one more thing to go wrong. electric pumps die but gravity seldom stops working. So far I have not had a boiler that had proper height have an issue with return water IF the returns are clean and of proper size.

ofs1

Just under 2.5 million BTUs

thanks

ofs1

It's a single pipe system and the piping is original AFAIK

The building was built around 1920 and we are replacing the original boiler. There is no boiler feed tank existing, but the original boiler has a larger water volume than the modern one will have. Apparently the concern is the condensate return will not be adequate to keep the boiler water level full?

Steamhead

It should be enough

as long as the water can return quickly. And if the return water is slow, it's better to fix the problem than to band-aid it with a boiler-feed pump. 

ofs1

Is there any way to know in advance if the condensate return is fast enough?

and if it's not, it seems finding the problem would be difficult in a 45 unit apartment building of this age.

Steamhead

If you can operate the present boiler

that's as good a test as any.

ofs1

The existing boiler operates fine, but it's water volume is much larger>

presumably it would not run low on water as quickly as the new smaller boiler?

nicholas bonham-carter

2.5 million btu replacement

this project may provide the opportunity for a multiple "staged boiler" system replacement.

as the controller calls for heat, depending on the outside air temperature, all boilers will fire, but as the pressure rises, beyond 5 ounces, the swing boiler will be cut out, and the maintenance of the steam can be handled by the lead  boiler.

all steam boilers are designed with enough water content to fill their rated radiation, as steam occupies 1200 times the volume of the liquid it once was. condensate tanks/ feed pumps should be avoided at all costs except in the case of one boiler supplying separate buildings some distance apart, where gravity may not be enough to return the condensate.

if you look at the manufacturers installation manual, it will have clear and concise instructions as to pipe sizes and layout, and these must be followed at the minimum. also specified is the hartford loop. make sure the new boiler is sized to the radiation count-not the heat load of the building or the capacity of the old one. the books mentioned below will explain this in greater detail.

make sure the proposal contains enough main [not radiator] venting. i have 1 million btu's 1-pipe, and i have 18 big gorton # 2 vents, and you will need more. my system uses a vaporstat to keep the pressure below 10 ounces, and your installer should do the same, and install a good low-pressure gauge [0-15 ounces-gaugestore.com]. the air has escaped from my system at a back-pressure of 2 ounces as the steam is rising. higher pressure steam travels more slowly in the supply piping, oddly enough. condensate in the returns will flow more quickly back to the boiler at these low pressures. higher steam pressures, besides enriching the fuel supplier, will cause the radiator vents to whistle more, especially as the outside temperature begins a steady drop into sub-zero.

there are some very good books available from the shop here which in your case would pay for them selves 1600 times over their cost! it seems that your installers should also read them, judging by their advice!

don't forget to give some thought to the controller of the system- they are now much better than the old thermostat [search tekmar 279].

just remember that these systems, when first installed were quite quiet and economical, and did not bang, or heat unevenly. the trick is to undo any subsequent bad modifications.--nbc

ofs1

This boiler will actually heat two buildings >>>

They are about 20 feet apart from the outside walls. Would that indicate the need for a condensate feed tank? We are being recommended a Weil-McClain LGB-19S. Would a staged boiler system be considerably more expensive?



Thanks

nicholas bonham-carter

2 buildings 20 ft. apart

as long as the condensate seems to return now, without a pump, then i would judge the outside return to be properly piped for a gravity return. usually problems with slow returns are the result of rusty, clogged pipes, or excessive pressure in the boiler [above 1 psi].

what height above the boiler does the outside return enter the boiler room? if it is not at floor level, but at a height coinciding with the waterline, then there can be a problem with "disappearing waterline" as the steam pressure forces the waterline in the return piping higher and tries to achieve a certain level dependant on that pressure in a 30 foot horizontal pipe, which needs a lot of water to raise it up 1.75 inches per ounce of steam pressure [hope i explained that clearly, but it drove us mad!].

the staged burner or staged boiler system could pay for itself in a few years or less, depending on the condition of your present system, as the higher burn is used only to get the steam up and the pipes warm, and when done can coast along at 75% throttle.

have either contractor done a "radiation survey" to verify the needed size of the replacement boiler? that is step one of the replacement procedure-not just replacing what is there from the old boiler name tag!--nbc

ofs1

The old boiler is below grade and the steam pipe that runs to the second building

Is about 4 feet above the ground so the pipe would be about 5 feet above the waterline of the boiler.



The contractor did do a count of the radiators and the sizes of the radiators if that is what you're referring to.

ofs1

The old boiler is below grade and the steam pipe that runs to the second building

Is about 4 feet above the ground so the pipe would be about 5 feet above the waterline of the boiler.



The contractor did do a count of the radiators and the sizes of the radiators if that is what you're referring to.

nicholas bonham-carter

return line between buildings

the steam line at 5 feet above the waterline seems well placed; however what about the return from the other building? that should be either up above the waterline, but lower than the steam supply [as a dry return], or well below the waterline.

doing a steam survey is a good sign on the contractor's part.

there are non-mechanical means of adding a bit of water content to the boiler return, with no pumps [see "reservoir tank"]. the cost is basically the cost of a 50 gallon tank to hold the extra water to tide over the returns, and could be added later if needed.--nbc

ofs1

I don't think there is a return line from the other building >>

It's a single pipe system so it wouldn't have a separate return would it?

ofs1

After speaking with the contractor who recommends the boiler feed tank

He is recommending it because the newer boiler is consideraby smaller in water volume than the original boiler. His concern is that as the boiler fires and makes steam to fill the system the water feed will replace the in the boiler water that has left as steam. At the end of the firing cycle when the condensate returns there is a danger that the returning condensate will now overfill the system so it will not create steam properly in the next cycle. The boiler feed tank captures the condensate so it doesn't overfill the system. He says it is their standard practice when replacing old large boilers with newer smaller ones.

nicholas bonham-carter

steam line to 2nd building

that must be a counter-flow arrangement. make sure that there are plenty of main [not rad] vents at the end of the steam line in the 2nd building so the air will quickly escape from that area.

i would try the boiler without the condensate feed tank and see if you have any problems. perhaps some provision for the later installation can be made to the return piping, but i would not anticipate any problem with low water content.

i would also try the boiler without an auto-feed. there is an advantage to knowing if you have a leak problem, although it does require someone to check it, once a week. if you have an auto-feed. make sure you can turn it off for the first few months, to catch any problems.

how is the system controlled vis-a-vis the 2nd building?--nbc

ofs1

The feed tank is not for low water situations

It is to capture return condensate to prevent EXCESS water in the boiler when the condensate returns at the end of a cycle.



The existing boiler is controlled by a Tekmar outside temperature sensor and a thermostat which is located in an apartment in the second building. It's not a high tech set up but we seem to have the buildings balanced fairly well. When the system is runnin in the winter time we do not get many complaints of apartments being too cold or hot. When we test with thermometers they seem to fall in a pretty close range.

nicholas bonham-carter

excess water?

where has the excess water come from, if not from the boiler itself? other than that the auto feed is the culprit-make sure it can be turned off.

the result of higher steam pressure is a higher level in the return, preventing the water from entering the boiler as quickly as it should. for every ounce of pressure in the boiler, the level in the returns rises 1.75 inches. in large pipes, that 24 inches represents a significant volume, and resistance, and if the auto-feed is trying to compensate, and over fills, then turn it off.

as far as the condensate feed pump, find out what abnormal condition would require its use, and correct it. if your building were 200 feet away, then it might be a benefit, but not in your case unless really required.

the only situation which could cause a low water condition would be a long wet return from the distant building which slows the water returning to the boiler. steam [especially at low pressures] travels quickly through out the mains, once the air has escaped. a long main will have more volume to fill with steam, so maybe with a very long run, you may need to spend the extra 8%. if you do install it, make sure you can valve it out of the system, or return it for complete refund, when you see it is not needed.--nbc

crash2009

Gravity is always preferred

its free, requires no maintenance, is automatic, and will always be in abundant supply.  Over the years, it is quite possible, that your building, has settled here and there, and during this settling has moved a pipe or two.  Boiler feed tank will compensate for that, but it might not be absolutely necessary.  Put the new boiler in and get the near boiler piping right.  See how it runs your building before deciding on whether or not to fix a problem you may or may not have.  Here are a couple technical bulletins from Weil-Mclean, that may help you and your negotiations with contractors. 

http://www.weil-mclain.com/en/multimedia-library/pdf/weil-mclain-pdf/service-bulletins/SB0606.pdf

http://www.weil-mclain.com/en/multimedia-library/pdf/weil-mclain-pdf/service-bulletins/SB0605.pdf

nicholas bonham-carter

our boiler

cooler nights coming soon will be time for me to check our boiler, which is 1,000,000 btu, with no auto-fill, and no condensate feed tank.

the boiler sends steam up into 150 ft. of 6 in. mains out to the 55 radiators, where it condenses, and returns as water through 6 dry returns returns, with a total aggregate length of another 150 ft-a good-sized volume.

amazingly, all the water which left the boiler as steam comes back as water by gravity in a few minutes. as we know, water can neither be created nor destroyed.

we were not always so lucky, as there were improperly installed piping in the returns, from 1952, when the 1885 coal boiler was replaced with gas. instead of dropping straight to the floor, the returns were piped in a series of steps, whose horizontal elements were slightly above the waterline of the boiler. when the pressuretrol allowed the pressure to rise to 1.5 psi, the waterline in the returns will rise to a certain point above the boiler waterline. in this height were the 6 2 in. horizontals which need so much water to fill them, that suddenly no water in the boiler. after the burner would cut out, all the water would return from its "hiding place" in these returns. if we had had a condensate feed tank, we would never have caught this abnormal condition. if we had had an auto-fill, we would have constantly been over-filling without knowing. now i add water by pulling a spring-loaded valve, when i know the boiler will run [to reduce oxygen corrosion]. fortunately, i live here, and can easily check the boiler, blow-down the lwco,etc.

now with a vaporstat, set for 8 ounces max, the water in the returns [now re-piped correctly to the floor] rises less than a foot, whereas at 1.5 psi it was over 2 feet.

you may not have the luxury of someone on site, but for at least the first 2 months, these frequent visits are necessary. you do not want some automatic device masking an abnormal condition and contributing to an early boiler failure.

where is this project located?

another thing to consider is the condition of the old boiler as it is being dismantled. signs of excessive corrosion can be seen, and will point towards a solution. i have forgotten how old the present one was, but we ran 35 years with our old american standard. you may also look forward to lower fuel cost with the new boiler [when other problems are fixed too-lower pressure etc.] our fuel use dropped to 70% correlated for degree-days.--nbc