Can you size a hot water boiler BTU based on given Steam Boiler Btu ?

banep

Customer is using steam boiler to heat water for their heating system. The steam is piped to a shell and tube HX along with the water for the building heat. The HX is leaking and needs to be replaced. Customer would like to remove the HX and steam boiler and replace with hot water boiler. Current steam boiler nameplate reads 300,000 Btu/hr ( input ), D.O.E. Heating Capacity 240,000 But/hr, Net I=B=R Output Water 209 MBH, Steam 180 MBH. If we want to replace this steam boiler with hot water boiler, would we match Btus for Btus? In other words, if we chose a hot water boiler with gross output of 240,000 Btu/hr would it be equivalent?

EBEBRATT-Ed

Mabe maybe not. It would be large enough but may be too large. It would be better to do a heat loss or a survey of the load attached to the boiler to come up with an accurate size.

GGross

You always size equipment based on the load the equipment needs to meet, not what is existing. This is true for all residential heating equipment. Sizing replacement equipment based on what is currently installed is not what a customer is paying a professional contractor for.
Step 1 what is the load of the space being heated?

banep

I understand both of your points, however assuming the steam boiler was sized properly, my question is BTUs for BTUs, would it be the same? steam or water makes no difference?

GGross

It depends. Many would size a steam boiler based on the attached radiation EDR, an acceptable practice though it is slightly debated. And then you would depend on the radiation being sized correctly. In this case with a steam to water heat exchanger if you were to size exactly based on output the new water boiler without a separate heat exchanger would likely output more BTU/hr to the space as it isn't going through an additional heat exchanger. the original boiler was most likely not correctly sized though, so an assumption that it was is not practical outside of pure discussion purposes

what type of emitters heat the space?

Steamhead

banep said:

I understand both of your points, however assuming the steam boiler was sized properly, my question is BTUs for BTUs, would it be the same? steam or water makes no difference?

Do not assume the steamer is properly sized. Run the heat-loss calculation which will give you the size you need.

banep

thanks all. I will check on the emitters

Mustangman

By not doing a heat loss / gain, you are guessing that the guy who installed the boiler did it right. A lot of guys are doing the drive by method.. drive by and say... thats about the size of Aunt Jane's house and we put a 125,000 boiler and it works great. Great way to create call backs. I have argued with other Techs about this. Many do measure the radiators for EDR. of the radiators. The problem is, what if someone went to a junk yard and put in radiators that fit the wall space. You could end up with a boiler that will heat the radiators, but is it enough to heat the house? I know its a pain, but do the loss and check it against what you have. I would never consider doing a steam to HW boiler and match the output. I understand that not everyone has Wrightsoft or the time to learn how to use it. Its a great program but very deep. They have a few other programs Right comm-load is a stripped down version of Wrightsoft. Many years ago, I made the mistake of not double checking. I was bidding against 4 or 5 bidders and I knew a few of the guys matched boiler output. I got the job. Beautiful install but was short on BTUs on very cold days. I clocked the gas meter and the boiler was under producing what was listed on the name plate. It was toping out at 175,000 input and I needed around 190,000 - 200,000. I called the boiler company. They sent a new gas valve to install and wanted 4" WC on the burner side of the gas valve. Long story short, the new GV set to 4" WC. It clocked out close to 200K. The next cold day, I was on site and it was perfect. Zones got satisfied. The point is this could have went the other way and I'd be changing boilers. What I didn't know was the landlord had tennants turn their heat off from 8PM to 7AM. They were flooding that boiler with cold water.. all at 7AM. Before the valve change It would bring up the temperature but very slowly. The extra 20K was the difference. Good luck

pecmsg

banep said:

I understand both of your points, however assuming the steam boiler was sized properly, my question is BTUs for BTUs, would it be the same? steam or water makes no difference?

Professionals do not A S S U M E !

Jamie Hall

pecmsg said:

banep said:

I understand both of your points, however assuming the steam boiler was sized properly, my question is BTUs for BTUs, would it be the same? steam or water makes no difference?

Professionals do not A S S U M E !

Well, a BTU is a BTU. You would compare based on the Net I=B=R output. However, boilers for hot water systems are NOT sized based on the connected radiation; in almost every instance there is more -- sometimes a lot more -- connected radiation than is needed. Rather, the boiler is sized based on the heat loss of the building, and then the temperature of the circulating water is adjusted so that the installed radiation provides the needed heat. Steam boilers, on the other hand, are sized on the basis of the connected radiation, with no attention paid to the heat loss of the building.

So... unless the steam boiler is very unusual, it is almost certainly oversized -- possibly by a lot -- for the heat loss of the building as compared with the boiler required for hot water. A like for like replacement is likely to be a disappointment.

Sylvain

If i understand, the steam is only used in the heat exchanger. Why such a set-up?

Was an other use abandonned? Then it is surely oversized.
Can't you just supress the HX and use the existing boiler to heat water?

Does the customer has the fuel/gas bills of the previous years to make an estimation of the heat losses with the degree-days?

edit
What are the specs of the heat exchanger?