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Ok guys. I have 3 story building I do service for. They have 4 condos per floor. the boiler is located just about 15' from the one side and the recirc. runs about 15' to the one end and ties into the riser on the first floor just above ground level feeding the hot for the kitchens. They have had a few pin holes in the copper line under the slab. I got them to let me replace the hot and the recirc on that run. they still have about 50' going to the other side and i am assuming it connects at the base of the hot riser on the end units kitchen. its piped in 1/2" copper about 30-40 years old and I told them its only a matter of time before they get a leak over there. they have that recirc running on a B&G 1/12 hp series 100 pump. doesn't that seem like a bit much? do you think going with a 1/25 hp pump would be better? try and slow the water down to buy them some time? also do you think the loop that i replaced would have wornout fast because its a shorter loop from the pump? just want to knock this around and decide what to tell them. the cost on taking care of the other flooded unit came close to tapping out the HOA.
 

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Discussion Starter #3
I'd be concerned with the velocity of flow in that line.
The cheap way out is probably just to use a valve to throttle it down until the temp of the recirculation line cools then reopen it just enough that it stays hot.
I was kinda thinking about going that way but was thinking of going with a smaller pump and maybe throttle down the shorter loop because I am thinking it might be shutting the pump down before the large loop gets hot enough. the pump is connected to a honeywell aquastat on the recirc line, so it turns off when the returning water is hot enough. what do you think?>
 

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Jim
If I may..
I would back your assessment on the piping closest to the supply being the most under stress for wear and eventual failure. The velocities’ at this point in the system are the most turbulent and wear is inevitable. As the flow starts to encounter head pressures, velocities’ will slow down and wear issues not as prevalent.
It sound to me that the 1.5 hp might be a bit overkill. Without seeing the system and looking at things like head pressures, gpm through existing pipe diameters’ with the system closed, and the pump curve capabilities of that particular pump, the overkill suggestion is only speculation. You can go to your pump supplier and by giving them the profile of the head of the building along with total runs and restrictions in the line, they can tell you exactly where you need to be.
Because the system is existing you are stuck a bit with what you’ve got. Building strata’s are usually hard to sell on ripping it out and doing it right. That said, there are some things I would do to alleviate some of the problem. The suggestion of slowing the flow, if the flow rate is excessive, through throttling is a good idea in my estimation. If it were a hot water heating system you need to take more care with throttling. The reason for this is that your front end emitters will disburse more heat than they were designed for and the back end ones run to cold. Your return temperatures back to the boiler can be reduced to the point where condensation becomes an issue.
This is not your case. After all, it’s only re-circ. But the excessive velocities’ if there are, will cause the turbulence against the pipe walls to cause ware. You need to get that in check.

You mentioned the pump goes off if the return temp through the aquastat is to hot. I don’t like this on a re-circ system. I like a good wet rotor pump and I like it on all the time. The pump will last longer and the supply temp to the various units more steady. It there’s a call for water just prior to the temp drop bringing the pump back on, there will be a noticeable temp difference at the tap vs. a top side circ. At least that’s how I like mine plumbed.
The lines in the slab might not be as much as a problem as the exposed ones. Besides velocities‘, the continual heat changes in the pipe, along with ambient room temperatures and humidity levels all add to pipe fatigue. Your encased lines aren’t subjected to such swings. I’ve seen a lot of buildings where the hanging pipe is in terrible shape and embedded lines have stood the test of time to a much greater degree.
I would only suggest to bring your pump and flow rates in line and repair what needs repairing. Other than a major re-pipe, it would probably be the lesser of the evils for the occupants.
Good luck
tl
 

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Pump curve here. Seems to me that pump might actually be a bit small. Its not designed to push more than about 8 feet of head pressure. Am I reading it wrong. The series 100 is in the lower left of the curve chart.
 

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Double a
Your right on the chart data. No, you didn't read it wrong. I like to believe that gravity does alot of the work in these situations. The 8-9 feet of head this pump will deliver is more than adequate in my opinion. Jim mentioned 1/2 inch pipe throughout. The pump needs to assist the natural rise and fall of the hot and cold water. I always believed less was better in these cases. Water racing through the system will defeat the purpose.
It's only one mans opinion.
 

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Hi Jim
Everything in the piping system will need to be added when taking into account head. Lengths of runs both horizontal and vertical, elbows, valves and anything else that will have an effect on flows and pressure. The temperature of the operating system should also be taken into account. Example: the density of the water is directly related to temperature. Density is the number of pounds of a substance (water in this case) to fill a volume of 1 cubic foot. Water for example weighs 62.5 lbs at 50 degrees f. That same cubic foot of water will weigh just under 59 déf at 250déf. That 3-4 lbs will have an effect on the final outcome of your systems preformance. If you have glycol in the system (closed loop hot water heating) the density changes considerably and so do head results.
Head is a term many use to define what a pump can lift and maintain a coloum of water at a steady state. It is in fact this but not only. It is also the pressure differential between the intlet and outlet of the pump. The will be guaged mainly through impeller and flute size. This is where you can get your steep or flat pump curves through sizing those items.

You mentioned in you request that the pump only served the in-floor and not the upper units. Is this correct
You said the circ line was connected to the base of the upper risers. Is this correct
If it is something is amiss. The re-circ for the closed loop floor heating (if I read this right) should not be connected at all to the hot pottable. Something is wrong with this picture. Give me a heads up
tl
 

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Hey Jim
If you’re an old guy like me you’ll remember those old gravity feed heating systems in large houses and small 2-3 story buildings. Big old boilers in the basement usually oil fed or converts. Remember the piping? Always big old cast and 6 inches in diameter. It always amazed me when I first looked at them how they worked. How in heck did they circulate with no pump I thought. The system sure relied on physics didn’t it.

If the pipe went up two stories it could push the water around the system with no mechanical help. How I thought?
It worked on a principal of the density of water. The hot water was lighter and the cold heavier. If the column of water was to lift up the 20 feet it needed I would think it would need a tremendous amount of head pressure to achieve this. But no, not so. The 20 foot column of water that was 6 inches in diameter has a tremendous amount of static pressure at it’s base. But remember. That same water that went up had to come down. Think of the system as in a state of equilibrium. As the static pushes down on the column going up, so is it on the one coming down. The heat in the system and the difference in the cool and hot water densities will do all your mechanical work for you. The pipes, 6 inches in diameter or ½ inch in diameter will have the same amount of static pressure at their bases. Sound strange, but true. The larger pipe simply has more mass to spread the pressure over and consequently no matter the size of the pipe, the static head remains constant.
Form this we got into circulators. People always think bigger is better. Not so. You need very little to assist most systems today and have them efficient. You need a flow rate of between 2-4 feet per second and no more. You need this range to entrain your air bubbles and move them through the system to the separator or your vents. Any faster than 4 and you will more than likely encounter noises in the system. And besides, flow rates in the 6-7 range will wear havoc on the piping if its copper.
Keep the flow down, keep it simple and the system will perc like a fine Swiss watch.
I’m off to the lake for r&r.
See you later
tl
 
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