Poor Heating from Series-Loop with 2 Circuits

MRC

Good Morning Forum Members,
Recently I ran a call at a 2500 sq. ft ranch style home with 3/4" baseboard heating elements on the main floor. The home has a full finished basement with drop ceiling. The basement is not heated by this system other than the heat obtained from the uninsulated piping. The boiler is a Weil McClain model CGM-5 series 7. Input 140,000 btuh, output 110,000 btuh, net I=B=R 95,700 btuh. The system was installed as a series-loop with 2 circuits. The larger circuit has approx 65' of 3/4" baseboard. In the boiler room there is a Bell and Gossett series 100 circulator on the return. The supply and return tapping is 1-1/4" but the supply was reduced down to 1" right off the boiler. From there the 1" splits into (2) 3/4" circuits controlled by (2) zone valves. The system pressure (according to the pressure gauge) is approx 12 psig. The Aquastat is set for 200 degrees...OWEEE! . I am guessing in an attempt to improve farthest loop performance.
The issue the homeowner is having is that the last circuit on the loop is not even luke warm bath water, leaving the bedroom it is serving (with 14' of baseboard element) cold. The previous homeowner/original installer? even installed a saddle valve on this element to bleed "air"! The areas before this room are progressively cooler as well.

I am looking for recommendations to solve this issue once and for all. I am sorry but I do not have the feet of baseboard installed on the other (smaller) circuit. I have been studying my I=B=R Residential Hydronis Heating Guide. While onsite earlier in the week my initial thoughts were to repipe the system as a two pipe, reverse return system. This I believe, would be the extreme (possibly only?) remedy. Could this system work in its present configuration with possibly a larger circulator or how about a one-pipe system with diverter valves?

I spoke with one of our salesmen the other day about the project. He contacted our local Weil McClain Distributor and scheduled an onsite meeting this coming Monday with their system expert. I will be attending this meeting as well.
I recommended that we start with a room by room load calculation and proceed accordingly.

I know I have not included all the information that is needed but if this was your project how would the forum members approach this job?

Mike Curran

kcopp

How long is the loop? I would look to see if there are any restrictions along the length of the loop...i.e. mono flow tees.

icesailor

#1: My old dead boss told us to NEVER ever trust a gauge on a boiler. Especially Tridicators. Because the chances are twice as good that one will be off.

I can't tell you how many boilers I have seen piped the way you described and they work and they work like Jack, the dancing seal.

Are you sure that you have 12# pressure in the boiler? If the High is set for 200 degrees, did you set it back to 160 degrees or less and see if the temperature on the Tridicator drops to the same temperature?

I'm not sure what this "rig" the previous owner installed to get the air out, but it sounds to me like the boiler system isn't installed with adequate purging capabilities in place. Is there a tight shut off on the return side of the boiler and a drain place to put a hose? Is there a "fast fill" device on the pressure reducing valve so you can speed up the water filling the system when you purge?

It sounds like the system needs to be purged of air. And with 200 degree High Limit, the water in the baseboards being higher, and a bad High Limit, is causing the water to vaporize inside the pipes. Raise the pressure, drop the High Limit. Drop it to 150. That's as low as you can make a CGM go anyway.


Don't be planning on any re-piping. It doesn't need any re-piping.

There's a vague possibility of a mechanical problem like a broken circulator. Post pictures. Don't start re-piping. Look for the least common denominator first.

Gordy

To add to Icesailor's recomendations the trusty infra red thermometer is a handy tool when used right. Black tape, or blue painters tape on shooting surface to get a realistic reading. Check pipes base boards all over the loops see where the reasonable temps start, and end.

MRC

Ok Icesailor, Jack the dancing seal? That's a good thing...right? !
No sir, I am not sure of the 12 psig in the system. I also did not lower the temp.
There is no power purge valve on the system either.
The saddle valve on the baseboard appears to have been put there to bleed air. But as Dan Holohan has written " if you bled the radiator and don't get air, air is not your problem".
Thanks for the input.

MRC

Kcopp, I do not know what the length of the loop is. I did not see any mono flow tees either. Would I see any on a series loop?

Gordy

65' of element is with in reasonable limits on 1 loop. The upper limit though. As Ice said verify pressure with alternate gauge.

SWEI

Once you verify that there is not an air issue, and if you discover low water temps at the end of the line, you might consider splitting the loop. A full reverse return would be best, but I have seen wonders from splitting series loops in two or three, at minimal cost.

icesailor

"" Ok Icesailor, Jack the dancing seal? ""

Jack will dance for a fish.

Harvey Ramer

Also, check the entering air temp to the baseboard. Most common baseboard is calculated with 65deg entering air. At that temperature, 70' of element is as far as you want to go. After that the BTU's will all be off the train.
Now if the wall has cracks or is leaky at the base, and you are getting, say 55° entering air, that would unload the BTU's even faster making 65' to long of a stretch.

You also mentioned the pipes in the basement are uninsulated. How many linear feet of pipe on this loop is in the basement? It would help to get it insulated.

icesailor

MRC said:

Ok Icesailor, Jack the dancing seal? That's a good thing...right? !
No sir, I am not sure of the 12 psig in the system. I also did not lower the temp.
There is no power purge valve on the system either.
The saddle valve on the baseboard appears to have been put there to bleed air. But as Dan Holohan has written " if you bled the radiator and don't get air, air is not your problem".
Thanks for the input.

Lower the temperature as low as it will go.

If you are considering ANY modification to the system, start with some fitting above the circulator on the return with a ball valve between the purge valve and the circulator. You should be able to purge into a 5 gallon bucket, get cold water, then air, then hot water. Once the water gets hot, open the ball valve. You should be able to feel the return getting hot. Even if you have to go out and buy a 30# gauge and make adapters to put it on a washing machine hose and connect it to the system, you need to know how high the system pressure. Post a photo of the fill valve. It probably barely works. If it is so old that it doesn't have a fast fill feature, it definitely needs to be replaced. There are ways to "power purge" the system but they are complicated and need experience.

Ironman

In addition to what Ice said: if your pressure is too low, and the water temp too high, vaporization can occur. This can be compounded by the fact that the circ is on the return pumping towards the point of no pressure change (PONPC). When the circ is in this position, the pressure differential that it creates is deducted from the static fill pressure. This means if your gauge is wrong and there's little or no static fill pressure, the pump will actually cause part of the system to go into a vacuum. That, in turn, will cause the boiling point of the water in the system to be lowered and vaporization occurs. Turning the temp up makes it worse.

Another ramification of the this is that should this continue, it will destroy the pump impeller from cavitation causing circulation to cease.

icesailor

Ironman said:

In addition to what Ice said: if your pressure is too low, and the water temp too high, vaporization can occur. This can be compounded by the fact that the circ is on the return pumping towards the point of no pressure change (PONPC). When the circ is in this position, the pressure differential that it creates is deducted from the static fill pressure. This means if your gauge is wrong and there's little or no static fill pressure, the pump will actually cause part of the system to go into a vacuum. That, in turn, will cause the boiling point of the water in the system to be lowered and vaporization occurs. Turning the temp up makes it worse.

Another ramification of the this is that should this continue, it will destroy the pump impeller from cavitation causing circulation to cease.

I don't completely agree with that.

With the circulator on the supply, receiving the hottest water, the chance of cavitation increases. The steam/air bubbles from the cavitation increase. With the circulator on the return, the pump is always pulling cooler water into itself for a trip through the boiler. If the flow is restricted by the system being air bound, it will move the water slowly and then cavitate within itself.

I'll bet that this system has had a long history of "air" problems. I'll bet that there are auto coil vents to eliminate "air" problems on upper floors. When in fact, the problem is that on the upper floors, when the system water contracts, it pulls air into the system through the auto vents that vent both ways. Blow air out under pressure, and suck air in under negative pressures.

When the PONC meets reality, its pressure that gets the traction. Every time.

If you walk up to a second floor radiator that isn't heating properly, and you stick a radiator key in it and get a blast of air that weakens to nothing, and stays there for what seems forever, you need more pressure in the system. Every time. If it doesn't hiss air and blast water into a cup, you need more pressure.

I've never seem the water termites eat an impellor on a return, but have when on the supply and not enough pressure is on the system and it is running at high temps on the High Limit.

Water well pumps pump cold water. If the well screen is plaqued up, the water still flows but slowly. There will come a time when the friction of the impellor and the water will cause cavitation. The water termites come out in force. I've never actually seen one. Just what they do. Like finding where they were once in a house.

Ask a good Navy Sonarman about Cavitation.

Ironman

Ice,
Either one can contribute to cavitation: lowering the suction pressure or increasing the fluid temp.



Pump Cavitation Causes

Pump Cavitation Definition



"Pump cavitation is the formation and subsequent collapse or implosion of vapor bubbles in a pump. It occurs when gas bubbles are formed in the pump due to drop in absolute pressure of the liquid below vapor pressure. These gas bubbles occupy space inside the pump and affect the pump’s operating pressure and flow. With vapor bubbles in the low-pressure zones of the pump, the motor’s energy is wasted expanding the bubbles instead of bringing more liquid into the pump. As the bubbles pass into the pump’s high-pressure zones, the motor’s energy is wasted compressing the bubbles instead of expelling the liquid from the pump. The bubbles can collapse as they pass from low- to high-pressure zones in the pump. When vapor bubbles collapse inside the pump the liquid strikes the metal parts at the speed of sound. The noise generated from these collisions of gas bubbles into the metal parts of pump sounds like pumping marbles and stones.
Causes of Pump Cavitation

Drop in pressure at the suction nozzle due to low NPSHa
If the fluid at pump suction is not available sufficiently above the vapor pressure of liquid at operating conditions, then vaporization of liquid and formation of gas bubbles is very likely, leading to cavitation.
Increase of the temperature of the pumped liquid
Increase in liquid temperature at the pump suction point increases the vapor pressure of the liquid. Thus it becomes more likely for operating pressure to fall below this vapor pressure limit, hence leading to bubbles and cavitation.
Increase in the fluid velocity at pump suction


Increase in fluid velocity at pump suction can typically be caused by higher liquid flowrates than the design case. As per Bernoulli’s principle, higher liquid velocity means higher velocity and lower pressure head. Frictional pressure drop in the pump suction also rises with rise in the flowrate, making low pressure and cavitation at pump suction more likely to occur.
Reduction of the flow at pump suction
Certain minimum flow is required by the centrifugal pumps to keep them from running dry, as indicated by the pump performance curves. If liquid flow falls below this limit, possibility of developing vapor in pumps and cavitation increases.
Undesirable flow conditions caused by obstructions or sharp elbows in the suction piping
Sharp elbows, valves, other fittings and obstructions cause more frictional pressure loss in the pump suction, thus increasing possibility of low pump suction pressure leading to cavitation.
The pump is not selected correctly.
Every centrifugal pump has a certain requirement of positive suction head (NPSHr). If the pump is not selected properly NPSHa might fall below this NPSHr limit, causing cavitation."

icesailor

That's true. Absolutely.

But in practice, return pumps are lower in elevation, that supply pumps, and the fluid is colder in the return. As is the return pressure. The higher the heated liquid goes in a closed system, the greater the opportunity for outgassing.

Submarines cavitate. When they do, they go deeper for the greater pressure. They can be running along silently and merrily, and hit a Thermocline of warmer water. If they are hiding under the thermoclines, and cavitate, they slow the prop speed down.

In almost 50 years of doing my own and seeing what everyone else did, I have NEVER seen a system not pump because the circulator was on the return. I haven't seem them not pump on the supply. But if I went on a "no-heat" call where the first floor was heating, and the second floor was not, more often than not, the second floor was on the supply and the boiler was hot. And the system pressure was low. Sometimes, 10 seconds on the fast fill lever of a 1156F was all it took to make the second floor work.


So many here have drank the Kool-Aid of circulator locations and re-piping and moving them that they fail to connect that after they moved the circulator for big buckaroo's, they increased the system pressure and proclaimed another problem resolved. When the problem was a lack of system pressure.

Ironman

I agree pressure is the main issue. And I wasn't suggesting that the circ needed to be moved just for the sake of making it "pump away".

What I was stating was that when it's not pumping away is that it compounds the problem by decreasing the system pressure at the highest or farthest point.

And I'm headed out right now to replace a pump that cavitated from a combination of both issues: hot fluid and restricted suction.

icesailor

How did you get "Restricted Suction"?

Ironman

Fouled plate heat exchanger.

icesailor

So, how do you decide that the problem was "Restricted Suction" and NOT restricted pressure? Pumps develop ,ore positive pressure than negative pressure. In a closes system, it is difficult to measure restriction in a loop, the restriction can be worse on the pressure side than the suction side. Did the pump develop a negative pressure on the inlet side?