Steam convector to hot water convector on a 17 story building?

Jim Williams

Dear Heating help,

For a boiler replacement project on a 17 story late 60s building, we are considering replacing the steam boiler with a hot water boiler. Recently the absorption chiller was replaced with a screw chiller so they will not need the steam in the summer. Steam was being supplied to either the absorption chiller in the summer or a shell and tube heat exchanger in the winter to make hot water to be supplied to fan coil units. The domestic hot water is from a coil in the steam boiler as well. We are designing two new gas fired domestic hot water heaters.

Five of the twenty units have small steam convectors in either the bathroom or kitchen. I spoke with the head of maintenance and he stated that the valve for the bathrooms and kitchens only opens at a set outside air temperature – around 20 days per year.

I would like to send hot water to these previously steam convectors. That way we can go with a hot water boiler and not need any heat exchanger. There are ¾” connections into and out of the connector with a shutoff valve on the inlet and a what I think is a disk trap on the discharge of the convector. Please refer to the attached picture.

1. If I replace the disc the trap with a piping, would the system work with hot water? There would probably be too much flow through the convector so the shut-off valve would have to be closed down to reduce flow.
2. Could I just take the “disk” out of the disk trap or will there be too much resistance and not enough water?
3. How about the steam supply and condensate header, would they be able to supported pumped hot water?
4. Is there anything else I am missing? Comments? Suggestions?

Thanks,

Jim

Boilerpro_5

A couple come to mind

First is that you will have about 100 psi of pressure at the bottom of the system, so it better be a high pressure water boiler. Or you could move the boiler to the roof, but you would still have very high pressure on the piping and convectors that may be compromised in strength due to condensate corrosion. If you put in a high pressure water boiler, I understand most codes require 24 hour a day boiler attendants. There are reasons steam is the preferred method of heating in almost all structures taller than 5 stories.

jBoilerpro

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Brad White_88

Some thoughts

(and to add to Boilerpro's comments):

The 3/4" connections seem reasonably sized for the size convectors you seem to have if that photo is a representative sample. That alone should carry 30,000 BTUH and convectors of that size if hot water would be a mere fraction of that.

The return main might be your limiting factor; at that level the return main is often quite a bit smaller than the corresponding supply main. The supply main might closely match what a water supply main of similar capacity, within a size, often the same.

Which brings me to the greater question, that of capacity. If you do switch to hot water you will likley have to run at a high temperature unless the walls and glass, in fact the modern adjusted heat load can be handled by the convectors when operating at hot water temperatures ...


The pressure issue Boilerpro mentioned can also be overcome by using an intermediate plate heat exchanger as a pressure break....

Bottom line is, I can see why staying with steam is a very good idea.

As to the need for a 24 hour engineer for high pressure, that depends on the jurisdiction. Here in MA it applies only to steam boilers over 15 psig and over 9 boiler horsepower (a bit over 300 MBH).


My $0.02,

Brad

Christian Egli_2

No tall order for steam

To see if I understand correctly, except for five units that have some steam radiators, the whole building has a pumped water loop. Then there is the boiler that makes steam downstairs in the boiler room, this steam travels a few feet to a heat exchanger and warms the water loop. And the steam does not travel upstairs (except for the odd units) and there are no steam to hot water heat exchanger placed, oh say, every five floors?

Same question for the domestic hot water, does all of it come from downstairs? There are no steam to domestic hot water heat exchangers higher up?

If this is all correct, then I don't find the steam boiler particularly useful beyond code pressure issues. You could install a low pressure hot water boiler with the heat exchanger to go from low pressure water to high pressure water -but that does not remove the exchanger, which is what you wanted to do.

Perhaps the odd units could be kept as is with their own cute little steam boiler, this may be easier and cheaper to install than to change the piping issues.

Here's what peeked my curiosity

What's wrong with the absorption system? Is it broken beyond repair? or is it just the market situation right now that make cooling with natural gas an unexciting idea?

What if in a year or two, electric rates will have soared so much that your electric chiller has become annoying?

If possible I would really maintain the absorption system on top of adding the new chiller. Divert the saved demolition costs to add valves, that way you don't have to predict what the market will do, you can adapt on the spot. This scenario would change your decision about what to do with the steam boiler at this point.

For the rest, you already got the better suggestions from the two gentlemen above.