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Q: When is it a good time to convert a gravity hot water system to forced circulation?
A: Usually, when the gravity system slows down because of the corrosion which has taken place over the years. Those little nooks and crannies in the pipe slow the flow and stop the heat. The natural response is to raise the temperature to make the water circulate more quickly. But you can only push the temperature so far before you begin to ask for trouble. That's when it's time to convert the system to forced circulation.
Q: What does this involve?
A: You have to add a circulator and (usually) close the system to atmosphere. You'll also have to make some changes to the near-boiler piping.
Q: What changes?
A: The old boiler probably has two outlets and two inlets because the idea in those days was to get the greatest possible gravity-induced flow of water through the boiler. The more holes, the better the circulation. That piping looked like this.
When you add the new circulator, you won't need to use such big pipes coming and going out of the boiler. In fact, you'll want to reduce the size of your near-boiler piping to give the circulator something to "push" against.
Q: Why does the circulator need something to "push" against?
A: So it won't kick itself off on its internal overload protector. A circulator does its maximum work when there's little or no resistance to flow. In a gravity system, the large pipes can't offer much resistance.
Q: Will I still need those double inlets and outlets at the boiler?
A: No, and that's another reason you should rework the near-boiler piping. With two inlets and two outlets the pumped flow might short-circuit around the boiler without moving out into the system.
Q: Suppose I don't want to repipe the boiler?
A: You may have to use two circulators - one on each supply line.
Q: How will I know what size pipe to use on the new boiler?
A: A good rule of thumb is to take the largest pipe, divide it in half and then drop one size from that. That becomes the size of your new near-boiler piping. For instance, let's say the largest pipe is 2-1/2" (if there are two inlets and outlets, you only have to consider one of them). Divide that in half and to get 1-1/4". Now drop down one size to 1" and that's what you'll use all around your new boiler.
If your largest size happens to be two-inch, pipe your new boiler in 3/4". It will look odd, and it might make you feel uncomfortable, but it'll work. Different systems call for different piping techniques. One size doesn't fit all and a gravity conversion is definitely different from a brand-new, forced-circulation job.
Q: How do I size the circulator for a conversion job?
A: It's real easy with these jobs. You're looking for high flow at a relatively low head pressure. A good choice is a circulator similar to Bell & Gossett's Series 100.
Your goal is to move a lot of water around the system as quickly as possible against very little resistance to flow. This type of circulator does just that.
Q: Can't I use a small, water-lubricated circulator instead?
A: These are fine circulators for most modern, forced-circulation systems, but not the best choice here. You don't need to generate much head pressure on these conversion jobs because the pipes are enormous and the resistance to flow is almost nonexistent. Using a small, high-speed, wet-rotor circulator is a poor choice on a gravity conversion because it will do the exact opposite of what you're trying to accomplish.
Q: I'm not sure I understand the difference between flow and head pressure. Can you explain it?
A: Sure! Flow is the "train" on which heat travels. Flow "delivers the goods" to the radiators. Head is resistance to flow and it's important, too, but only in relation to flow.
Q: Well, then what determines the head pressure?
A: In general, the size of the pipes. The smaller the pipes, the greater the required pump head, and vice versa. Since gravity systems have very large pipes, there's no need for a high-head circulator. What you need is high flow.
Q: Where is the best place to install the circulator?
A: It's always best to put it on the supply side of the boiler, pumping away from the compression tank. Piped this way, the circulator will add its pressure to the system's fill pressure and make it easier to get the air out. The system will also run more quietly.
Q: Do I have to use a bypass around the boiler on these jobs?
A: Most boiler manufacturers recommend that you install a bypass around their new boilers when you use them on a gravity system. Here's what that bypass piping looks like.
Q: What's the reason for the bypass?
A: It's there to protect the boiler against condensation and thermal shock.
Q: What's thermal shock?
A: Thermal shock is what happens to hot metal when you hit it with relatively cold return water. If you take a glass plate out of the oven and run cold water over it, it will break, won't it? That's thermal shock.
Q: How does the bypass piping help prevent this?
A: The boiler bypass allows the majority of the return water to bypass around the boiler while just a small portion of that water flows through the boiler, picking up the necessary heat.
Q: You said something about condensation. What's that all about?
A: If the return water temperature is too cool, the combustion gasses can reach their dew point and turn into a liquid inside the boiler. That liquid is very corrosive to metal. It can damage or destroy a boiler in no time at all. By using the bypass, you're mixing hot supply water into the relatively cold return water and raising the boiler water temperature to a point where the gasses can't condense inside the boiler.
Q: Does the bypass serve any other purpose?
A: It allows the boiler to come up to high-limit temperature and shut off. Without the bypass, the large volume of water moving through the boiler often keeps the temperature low and prevent the boiler from reaching high-limit. This does a good job of increasing the fuel bill.
Q: Is there another way to pipe the replacement boiler without using the bypass?
A: You can use primary/secondary pumping techniques.
Q: What's primary/secondary pumping?
A: It's a way of treating the flow through the system and the flow through the boiler as two separate things.
Q: Is there an advantage to this?
A: There is because some boilers require a minimum flow to operate at their maximum potential. This flow may not be the same as the flow you need in the system. If you use a bypass line, someone may adjust it after you've left. This can cause problems with both the boiler and the system.
Q: How do I pipe for primary/secondary flow?
A: Tie the existing supply and return lines together to form a system loop. Then, use two standard tees, set no further than a foot apart, and attach the new boiler to the loop. Like this.
The primary pump serves the system, while the secondary pump takes care of the boiler. You meet the flow needs of both in a very simple way. The not-more-than-twelve-inch spacing between the tees allows the pumps to operate independently. When the secondary pump is off, there will be no flow through the boiler if you keep the spacing within that 12" limit.
Q: Why is that important?
A: By controlling the flow through the boiler, you're taking charge of the stand-by losses of the system. If the burner is off and the boiler pump is stopped, you will have minimal loss to the flue.
Q: How do I control a primary/secondary system such as this?
A: You can have both pumps and the burner come on at the same time. Or better yet, you can run the system pump (the primary) on an outdoor-air reset control, and cycle the boiler pump (the secondary) and the burner to meet the temperature needs of the building on any given day. This is the ideal way to manage an old gravity hot-water system.