Monoflow Continuous Circulation System

From: http://www.heatinghelp.com/article/332/ ... eating-FAQ

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.

That's a very informative article.

Having said that, most of what it says is already on the system. The GE was plumbed slightly differently, circulator on the return, though they did neck down the supply to 1 1/4 inch for a few feet. I've taken my installation down a bit more than that, and my recirculator is a low pressure, high volume TACO (from memory on that one). The pieces I pulled off the old system were rust free, so I don't think I have any issues in that regard.

The only thing that they (and I) did not do is bypass the boiler. However, from talking with Axeman, they seem to think their design incorporates that bypass internally (they dump the return into the middle of the boiler, and the gentleman I talked to seemed to believe that would serve the same purpose as a bypass. In any event, he was not concerned with thermal shock for his boiler under the conditions I described to him).

I suppose the best bet at this point is just to start running the boiler at different temps and different restriction rates to get a feel for what works best.

kstills wrote:In any event, he was not concerned with thermal shock for his boiler under the conditions I described to him.

That may be the case, but you still want the bypass so you can control the delta T through the boiler. Another option is something like this: http://flopro.taco-hvac.com/products/We ... tegory=372

markviii wrote:

kstills wrote:In any event, he was not concerned with thermal shock for his boiler under the conditions I described to him.

That may be the case, but you still want the bypass so you can control the delta T through the boiler. Another option is something like this: http://flopro.taco-hvac.com/products/We ... tegory=372

I saw that yesterday.

That's definitely going on the boiler next summer.

kstills wrote:Next summer, I'm planning on ripping out the old 2 inch mains and going with 1 inch for the supply and return. By then, I'll have read Sting's books so that I can design the system as efficiently as possible. So this is a situation that I'm facing for just this winter.

But it's my new toy, and it's a puzzle, so it's fun to talk about.

This may be a mistake in the planning - think about it -- the system was designed and installed correctly -as per your testimonial of the system worked fine, cept you wanted a coal heat source. So don't screw it up.

a far better approach would be to leave the distribution system alone, but install non electric thermal reacting Danfoos valves in place of each manual radiator valve -

THERE: now you have automatic balanced flow in the load and you just zoned the whole house for temperature control by room/area. OBW you should still fix the near boiler piping.



and on another note: Delta T management this time of year via manual valve twisting is nearly impossible - do some simple management - get used to the coal boiler - then when the degree day load rises over 60, begin your final manual valve system balancing.

Kind Regards
Sting

markviii wrote: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.

This is a bit misleading. The efficiency of a circulator pump varies with the head pressure. The pump designer aims to have the highest pump efficiency at the mid-point of it's operating range. It's a bell shaped curve the falls off at both high head and low head pressures. A typical efficiency curve (from Siegenthaler) is shown below. Adding a circulator to a gravity requires a careful analysis of required water flow (BTU delivery) and then matching the center point of a pumps performance to that flow rate.

Sting wrote:

kstills wrote:Next summer, I'm planning on ripping out the old 2 inch mains and going with 1 inch for the supply and return. By then, I'll have read Sting's books so that I can design the system as efficiently as possible. So this is a situation that I'm facing for just this winter.

But it's my new toy, and it's a puzzle, so it's fun to talk about.

This may be a mistake in the planning - think about it -- the system was designed and installed correctly -as per your testimonial of the system worked fine, cept you wanted a coal heat source. So don't screw it up.

a far better approach would be to leave the distribution system alone, but install non electric thermal reacting Danfoos valves in place of each manual radiator valve -

THERE: now you have automatic balanced flow in the load and you just zoned the whole house for temperature control by room/area. OBW you should still fix the near boiler piping.



and on another note: Delta T management this time of year via manual valve twisting is nearly impossible - do some simple management - get used to the coal boiler - then when the degree day load rises over 60, begin your final manual valve system balancing.

Kind Regards
Sting

I hear you, but here's my concern.

The old system worked even better when the pipes were covered in asbestos. While I've wrapped them with fiberglass, the insulation factor isn't there anymore. So there's a fair amount of loss through the pipes in the basement before the water even gets to the first floor.

My thought was to make each radiator a 'home run' off a central manifold. I'd pipe in 1" to the manifold, then run 3/4" to each radiator (which conveniently lets me valve off each radiator from the basement). That would take out ~15gallons of unnecessary water from the system, plus cut down on my heat loss in the basement making the whole system more efficient (that, and reading your book).

Reducing the heat loss and eliminating the excess water would seem to be the way to go, long term.

Unless you intend to manage and control each "home run" for zone temperature management - consider the surface are of all that extra pipe!

Your heat loss will increase

so do a better job of insulating (and managing) what you have for better payback on investment

Sting wrote:Unless you intend to manage and control each "home run" for zone temperature management - consider the surface are of all that extra pipe!

Your heat loss will increase

so do a better job of insulating (and managing) what you have for better payback on investment

If I assume a very worst case scenario for the new piping, the change in surface area would end up being a wash. In reality, it will probably decrease by a fair amount.

Coupled with the decrease in water required, I'm still thinking this is the way to go.

kstills wrote:Coupled with the decrease in water required, I'm still thinking this is the way to go.

You will be the first to know.

markviii wrote:

kstills wrote:Coupled with the decrease in water required, I'm still thinking this is the way to go.

You will be the first to know.

Actually, I calculated this wrong.

If I essentially double the existing pipework by looping the outside of the basement walls, I'll decrease the surface area by 76%.

That would increase my water a bit, so my savings wouldn't be quite 18 gallons, but it would still be justifiable.