Steam vs hot water boilers: why the sizing is the opposite
One of the most misunderstood things in boiler work: steam and hot water are sized in completely opposite ways. Get it wrong and the system never works right.
Steam and hot water boilers are sized in opposite ways, and most contractors get it backwards. You size a steam boiler to the radiators in the home, not to the house, because the boiler has to make enough cubic feet of steam to fill all the radiators. That usually means replacing a steam boiler with the exact same size that is already there. A hot water (hydronic) boiler is sized to the heat loss of the house instead, which often lets us switch to high efficiency and shrink the boiler considerably.
Why is boiler sizing so misunderstood?
Most people assume every boiler gets sized the same way, to the house. That is wrong, and it is one of the things I see contractors get backwards all the time. Steam and hot water boilers are sized on completely different principles. Mix them up and you either end up with a steam system that cannot fill its radiators, or a hot water boiler that is needlessly huge. Let me break both down.
How do you size a steam boiler?
Here is the part that trips people up. When you replace a steam boiler, you do not size it to the house. You size it to the radiators that are in the home.
The reason is physics. The steam boiler has to be able to create the cubic feet of steam necessary to fill all the radiators in the system. If it cannot make enough steam to fill them, the far radiators never heat and the home is uncomfortable. So when we replace a steam boiler, most of the time we need to replace it with exactly the same size that is currently in there. Going smaller to chase efficiency on a steam system usually backfires.
How do you size a hot water boiler?
Hydronic, or hot water, heat is a completely different story. We are not sizing a hot water boiler to the radiators. We size it to the heat loss of the house, how much heat the home actually loses on a cold day.
This is great news for homeowners, because a lot of the old hot water boilers in Pittsburgh are massively oversized. When we size to the actual heat loss, we can frequently switch the home to a high-efficiency boiler and reduce the size quite a bit. These are some of our favorite jobs, because the homeowner ends up with a more comfortable home and a drastically lower energy bill at the same time.
Steam vs hot water sizing, side by side
- Steam boiler: sized to the radiators (the cubic feet of steam needed to fill them). Usually replaced with the same size that is already there.
- Hot water boiler: sized to the home’s heat loss. Often can be downsized and upgraded to high efficiency.
What this means for your replacement
If you have steam heat, do not let anyone talk you into a smaller boiler to save money, it likely will not fill your radiators, and you will regret it. The right move is matching the steam-making capacity to the radiators you have. If you have hot water heat, the opposite opportunity is sitting right there, a properly sized high-efficiency boiler can mean real comfort gains and serious savings.
Either way, the work starts with knowing which system you have and sizing it correctly. For more, see our guide on how to pick a boiler and the details on steam boilers.
The quick version
- Steam and hot water boilers are sized in opposite ways.
- Size a steam boiler to the radiators, it must make enough steam to fill them. Usually replace with the same size.
- Size a hot water boiler to the home’s heat loss, not the radiators.
- Many old hot water boilers are oversized, so we can often downsize to high efficiency and cut energy bills.
- Shrinking a steam boiler to save money usually backfires and leaves radiators cold.
Everybody thinks you size a steam boiler to the house. It is the opposite. You size it to the radiators, because it has to make enough steam to fill every one of them.
Hot water is the reverse, we size it to the heat loss, which is why we can often go high efficiency and shrink it. Those are some of my favorite jobs, a more comfortable home and a much smaller bill.
David WahlCEO & Master Plumber, Wahl Family
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Call 1-855-GET-WAHLFrequently asked questions
How do you size a steam boiler?
You size a steam boiler to the radiators in the home, not to the house. The boiler has to produce enough cubic feet of steam to fill all the radiators in the system. That is why, most of the time, a steam boiler should be replaced with the exact same size that is already installed.
How is a hot water boiler sized differently?
A hot water (hydronic) boiler is sized to the heat loss of the house, meaning how much heat the home loses on a cold day, rather than to the radiators. Because many older hot water boilers are oversized, this often lets us switch to a high-efficiency unit and reduce the boiler size significantly.
Can I downsize my steam boiler to save energy?
Usually no. If a steam boiler cannot make enough steam to fill all the radiators, the far ones never heat and the home is uncomfortable. Steam boilers are sized to the radiators, so the right replacement is typically the same size as the existing unit, not a smaller one.
Can I downsize my hot water boiler when I replace it?
Often yes. Many older hydronic boilers in Pittsburgh are oversized. When we size a new one to the home’s actual heat loss, we can frequently move you to a high-efficiency boiler that is noticeably smaller, more comfortable, and much cheaper to run.
How do I know if I have steam or hot water heat?
A quick on-site look tells us immediately, but in general steam systems use larger pipes and radiators with steam vents, while hot water systems circulate heated water with pumps and have no steam venting. Knowing which you have is the first step, because the two are sized in completely opposite ways.
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