Maintaining constant pressure

DC_Manager

Our building has 4 HB Smith Mdl 19A cast iron sectional boilers set up for steam. In summer we run one boiler to make DHW with a heating coil in a 3500 gal storage tank. This year we added a solar-thermal system to pre-heat incoming water goiing into the tank. On a sunny day the solar panels will provide between 90% and 100% of the energy needed to maintain DHW temp. Our HVAC contractor says even though the storage tank may not be calling for heat from the boiler (it has a motorized steam valve controlling the coil), we still have to maintain a constant 4-6 pounds of steam pressure in the boiler to prevent the cast iron sections from cracking under expansion/contraction. From 9 am to 5 pm we use very little hot water, and from 11 pm to 5 am we use zero hot water - but we must run the boiler 24 hours according to the contractor. Would like a second opinion on the need to maintain constant pressure with this type boiler, and or any suggestions on how to save fuel. Many thanks.

nicholas bonham-carter

constant pressure?

is this boiler also used for heating? most radiator systems will work better at a maximum pressure of ounces, and not pounds. they are used to intermittent firing, without ill effect, as long as it is not short-cycling. some steam coils may require a little more pressure, but in general, less is the new more both in terms of comfort, efficiency, and longevity of equipment.

why not use the find a contractor button above and see who may be in your area to straighten out your system. while you are pushing buttons, visit the shop and order some steam books which will answer many questions.--NBC

ttekushan_3

Thermal shock

I can see what your HVAC guy (I presume) is getting at.  The demand for hot water, being intermittent, also would cause intermittent boiler firing from a cool or cold state a couple times a day. He's right in that the boiler is protected 100% from thermal cycling and moisture accumulation by maintaining pressure. 



The downside is the heat loss on the header piping, HX coil supply piping, and wear and tear on the steam traps.  But the boiler really likes this situation. Being a Smith 19, I presume it is equipped with a "Low Fire Hold" until the boiler reaches about 200 deg F before going to high fire.  They do this to prevent thermal shock, so the boiler is protected to some degree.



Now, if I was in the boiler's shoes (yes, I presume to speak from the boiler's perspective), I would be just as happy to be held at 180-200 deg F instead.  The boiler is still hot, won't be subject to shock, the oxygen is still driven off of the boiler water, and will come up to pressure quite quickly.  But without producing steam, the heat loss from piping 24/7 and steam trap issues are eliminated.



I've seen this very arrangement used successfully in one particular installation. There's a 200 degree hold switch on the boiler panel.  This solution seems to strike the right balance of boiler protection and energy use as these boilers are over 50 years old and only this year were retubed for the first time (they're steel fire tube boilers).



Regarding the system pressure, I prefer the lowest possible pressures as nbc has noted. But if the system serves a large facility with HX coils (like your water heater) and a substantial number of fan-coil heat exchangers, several pounds of pressure may be necessary.  [but if you have all of these, then you should really be using vacuum return pumps and you can still leave the pressure quite low, but I digress.]



See what your HVAC person thinks of boiler temperature maintenance instead of pressure maintenance.

DC_Manager

Thanks Nicholas and tte

During the heating season the boilers power a HX for a hydronic system with 700 fan coil units. I'm told the burners don't modulate between low fire/high fire, they have only one setting. I'll check for your "Hold" control. That would be a handy solution. Previous buildings I've managed had Scotch-Marine boilers with aquastats we could set at 180F in the summer to make DHW with a tankless coil, but I have no experience with sectional boilers, and don't know if firing them up from cold 3 or four times in 24 hours would be as disasterous as my contractor says. Thanks again.

elfie

cost effective to keep a steam boiler on all the time?

not sure how many units are in your bldg - 3500 gals is a large tank with minimal use most of the time - is tank too large? and how hot do you keep it?



is it really cost effective to maintain pressure in a steam boiler all the time (not sure how many sections you have) and is there a more cost effective alternative to maintaining domestic hot water.



agree that pressure is too high and that there is likely alot of short cycling as demand is absent most of the time (lots of wear and tear on boiler)



HB  Smith has great tech support and they can help with  questions.

DC_Manager

thanks elfie

building has 270 residential units. Storage tank temp avgs 140F. During July and August avg 5,000 gals DHW per 24 hours.

SWEI

What's your average summertime deficit?

How many gallons per day at what rise?  It's looking like a small HW boiler might be able to handle this.

elfie

lots of units

is there a continuous loop of circulating water



is all the domestic hot water piping insulated?



is this a high rise, its interesting to know how domestic hot water is supplied to so many units

DC_Manager

Nine stories

In March we average 8,000 gallons a day. Incoming temp is 50F. Solar panels heat it to 90F. Boilers take it the rest of the way.

DC_Manager

Recirculating loop

nine stories, piping is insulated as is the storage tank.

SWEI

math

8,000 gallons X 50F rise X 8.2 lbs/gal = 3.28 MMBtu, which can be delivered in ten hours at a rate of 328,000 Btu/hr, rather nicely matching the capacity of a 399k boiler at 82% efficiency.