Most manufacturers rate their condensate pumps in square feet EDR. You can easily convert whatever boiler rating you have to square feet EDR by using these conversion factors:
Once you have the rating in square feet EDR, you can select a condensate pump for that (or a slightly higher) rating from the manufacturer’s catalog. Most stock units will come with their pumps set to discharge at 20 psig pressure. That’s because the pump has to discharge at 5 psi higher than the pressure at which the boiler is operating. Low-pressure steam boilers operate up to 15 psi, so that’s why the condensate pump is capable of discharging at 20 psi. If you’re running the boiler at 2 psig, which is typical for heating, you’ll need to throttle the pump to 7 psi. If you don’t, the check valve at the pump’s discharge will rattle.