Steam heat-x question

Matt_21

who has an existing 300,000 btu output steam boiler. he currently has 9 radiators on the system. he wants to remove 7 radiators and add a hot water heat-x with two hydro air coils. the total btu of the hw coils is 234,000 btu. i want to make sure i still have enough heat for the 2 steam radiators. would i get the size of the radiators, determine the btu and add the 234,000 from the hot water coils?

Brad White_9

You may run into a mass issue, Matt.

The output emissivity of the two systems are not the same, sort of like using fin-tube and cast iron on the same zone. The steam radiators are the least of my concern. The issue I see is that the hot water side may cool down between boiler cycles unless you add a buffer tank for mass to carry you over the dips. Even so you may have a wide swing in available water temperatures.

Consider selecting your coils for the lowest possible inlet water temperatures, maybe 130F and perhaps a 20 degree delta-T if that meets your load. You may have to use a 4-row or deeper coil to do this. Be thrifty with your water flow management and control the water temperature supplied to the coil using a mixing valve off of the buffer tank. Think of "sipping hot water" into a lower temperature coil loop so you do not deplete it and hence extend your mass.

But my other side says: You have 78 percent of your load going to hot water. There are less expensive ways to make that happen using any variety of hot water boilers. The two steam radiators can be dealt with/supplemented.

I am not a fan of taking out a good steam system "just because", but this looks to me like the tail wagging the dog

My $0.02,

Brad

Matt_21

thanks for

the info. we're trying to talk him into replacing the boiler with a hot water boiler and putting some cab heaters on the second floor. we'll see how it goes.