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Why you should insulate steam pipes


Dan Holohan
July 16, 2009
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When you take an old house under your wing you sometimes also get to take possession of an old steam heating system and an awful lot of asbestos insulation that goes along with it. There was a day when asbestos was touted as a miracle product. Nowadays, we know better.

Some homeowners choose to contain the asbestos by carefully wrapping it in plastic while others decide to get rid of it once and for all. If you're getting rid of asbestos there are a few things I want you to know.

First, steam is a hot gas that desperately wants to give up its latent heat energy (an amazing 970 BTUs per pound!) and turn back into water. The steam will give up that tremendous amount of energy to anything that’s colder than it is. In most homes, the steam that leaves the boiler is about 215 degrees F. That’s hotter than just about everything else in the house, right? So when the steam hits a cold pipe, it will quickly make that pipe hot by condensing on it and releasing its latent heat energy. And as soon as the steam condenses, it stops moving because, at that point, it’s no longer steam. The colder the pipes are, the faster the steam will condense on them, and that’s why the builders of old houses covered the steam pipes with an asbestos "blanket." Asbestos insulation keeps the steam hot. It prevents it from condensing too soon. It’s like a thermos bottle. The Dead Men wanted that steam to stay hot so that it would travel further – like to the upstairs bedrooms. You don’t need the basement to be 90 degrees, do you?

But let’s say you decide to have that asbestos removed from your steam pipes. Are you going to replace it with a more suitable type of insulation? You should because if you don’t, you’re going to wind up with an undersized boiler. And with an undersized boiler, the rooms in your home will heat unevenly. Some will never get warm enough. Others will be too hot. If your thermostat is in a room that has a cold radiator, your fuel bills will probably also increase because your burner will run longer than it should. You’re also liable to get water hammer noise, especially if the asbestos abatement folks didn’t do a good job of reattaching your pipe hangers. Sags in the pipes cause water to form puddles, and puddles cause problems in steam heating systems.

Insulation makes a huge difference when it comes to steam mains. Here’s an example. Suppose you have a 2-1/2" steel main that runs around your basement. Let’s say it’s 50 feet long. Okay, the original installer covered that main with a one-inch-thick layer of asbestos insulation years before you were born. That was pretty standard practice. When the air in your basement is 70 degrees, the heat loss of that insulated main is going to be about 2,450 BTUs per hour. Naturally, if the air in your basement is colder than that, the heat loss from the main will be greater, but let’s figure that the air is 70 degrees, just for the sake of comparison.

Now, take the insulation off that pipe and watch what happens. The heat loss of the pipe jumps to an incredible 13,250 BTUs per hour! That’s more than five times the heat loss that the insulated main suffered. And if your basement is colder (or if that main runs through a cold crawl space), the heat loss will be even more extreme. And that’s why your basement is 90-degree cozy – even though you’re freezing in your bedroom.

As I said, the load that the uninsulated pipes adds to the system can effectively undersize your boiler. When we size replacement steam boilers, we have to make sure the boiler’s ability to produce steam matches the system’s ability to condense steam. It’s like having an evaporator and a condenser in an air conditioning system. In this case, the boiler becomes the evaporator, and the system becomes the condenser. If the "condenser" (the system) is bigger than the "evaporator" (the boiler) the boiler can run for a long, long time before it shuts off because it will never develop any pressure. This will be most apparent during the spring and the fall because the boiler has to overcome the heat loss of the bare pipe every time it starts. During the winter months, when the boiler runs for a longer time, the pipes won’t have as much of an opportunity to cool off, so the problem won’t be as noticeable. Spring and fall will drive you nuts, though.

The heat loss of the piping is what steam boiler manufacturers call the "piping and pick-up" factor. When they rate their steam boilers they allow for a piping "pick-up" load that’s equal to one-third of the system’s total radiation load. In other words, they measure how much radiation there is, and then they add a third more boiler capacity to that to allow for the heat loss of the pipes that connect your boiler to your radiators. And they base this one-third "pick-up" factor on insulated mains because steam mains are supposed to be insulated. When the insulation’s gone, the "piping and pick-up" factor the boiler manufacturer built into the sizing chart will suddenly be too small. You can wind up with an effectively undersized boiler and cold rooms.

So insulate all the steam pipes you can see. Tuck ‘em in and keep ‘em warm. And remember that the insulation doesn’t have to be fancy to work. You can use something as simple as batt insulation and duct tape if the aesthetics aren’t important to you (who cares what it looks like in that crawl space?). Your goal should be to give steam the best possible chance to get to where you are.