Radiators at end of zone circuit not heating

Bridgestone

Three cast iron radiators at the end of a zone circuit are not heating after a new boiler and circulator replacement. We have a 2100 sq.ft. house and last March an installer replaced our old Weil Mclain boiler with a similar boiler, a fixed firing Weil Mclain CGI-5 series 4, 102k btu output with a Taco 007-e circulator. The old boiler was 109K, but I don't know the size of the original circulator. The circultor was replaced many years ago when a contractor added heating load as part of a kitchen remodel. I'm guessing he installed a larger circulator.

We have a gravity conversion system with a large main zone, 2 small zones, and 3 zone valves. The main zone has 4" and 3" pipes connected to 3/4" copper above the boiler. The pipes are reduced to 2" approaching the radiators. The main zone with cast iron radiators is split into a left and right side of the house. When the main zone is operting alone, the 3 radiators at the end of the right-side circuit, a longer run, don't heat.

In the morning, with the thermostate set at 67 from a nght set-back to 63, the boiler will shut down at 180 and start short cycling. With the 3 radiators still cold, the other radiators on the zone are hot. There's no air in the radiators and there's an air separator above the boiler. With the boiler pressure set at 14, it increases to 24 during operation.

The only variable between the old system and the new one is the circulator pump. The btu output and piping are the same as before. Previously, the radiator at the very end of the right-side run would get hot enough to drive people out of the room. The old boiler didn't start to short-cycle at 180.

The installer came back in March and increased the cold boiler pressure to 18. It raised operating pressure to 29, but didn't help. The radiators on the two other zones work fine and heat very quickly when operating together or alone, just as before.

I called Taco Technical Support and asked about the appropriate size circulator for our system. She asked for the pump head on the system and said that without knowing it she couldn't recommend a pump, but hinted that on a gravity conversion system a Taco 0010 might be appropriate. I have no information on the pump head.

Last week's temperature range was 25 to 35. I would appreciate any input on what might be helpful or what I might do next.

Jamie Hall

Keep in mind that water is very lazy, and will take the shortest possible route to get back to the boiler. In gravity system conversions, this can and often does lead to some paths getting very low flow or almost none at all. Are there any valves on the system so that you discourage some of the flow into the left hand side of that misbehaving zone and force it into the right hand side?

Bridgestone

There are ball valves on the left hand supply and return pipes connected to the boiler. The supply valve has been closed down a little. The left hand side feeds 1 large radiator on the 1st floor and 1 large radiator on the 2nd floor. The 2nd floor radiator is turned down a bit. Hate to touch it again for fear it might leak. There is no turn down valve on the 1st floor radiator.

Bridgestone

I should add that the pipes starting to the left side are 3" and the ones to the right side are 4".

EBEBRATT-Ed

Old gravity systems are touchy. As @Jamie Hall said water will take the path of least resistance In a gravity system the pipes are way oversized for the flow needed with a pumped system. This gives the water a lot of different places to go, and any small change can make a big difference.

One thing that does concern me is your statement that the 3" &4" pipe are connected to 3/4" pipe above the boiler. With a boiler output of 102,000 btu those pipes connected to the boiler should be 1 1/4 although you could probably get away with 1".

It's possible a bigger circulator would increase the flow and give you heat but it may not be the fix. I would start by taking temp reading in all the rooms on that troublesome zone. Then figure out how to balance that zone by closing some radiators off in the warmest rooms.

If you can't get it to balance try a larger pump. Or have the contractor download the Bell and Gossett System sizer (it's free) and calculate the pump head.

pedmec

Gravity systems can certainly be a pain to balance once retrofitted. A lot of times someone will throw a high velocity circulator thinking they need to "get that water moving". then they find the opposite happens. All the water rides around the closest radiators like a nascar race. you need a high flow/low head circulator. most of the piping to the radiators are extremely over-sized. this is why in a high velocity circulator the loop just goes thru the closest radiators and back to the boiler. you can try to balance the system by throttling back the closest radiators. I normally will close completely the closest ones and then open one complete turn. then i work from there opening as the weather gets cold and colder because the imbalance will show itself as the weather gets colder.

Bridgestone

Thank you for the repsponses. A little new information. This morning, 23 outside, after 1.75 hours of running the thermostat was satisfied at temp of 67, with boiler temp of 178. After the boiler rested for an hour I set the stat to 68 and the 3rd to last radiator got hot with boiler temp at 156. The last 2 had a faint amount of heat. This happened once before on a 13 degree day when I raised the stat during the day 1 or 2 degrees and then even the last radiator got warm. I haven't been able to repeat these scenerios. As to turning down the radiators, there are 3 before the 1st problem radiator. Of the 3, the largest has no turn down valve. The smaller ones do have valves but I don't know when they were last turned. I'm worred they might leak.

EdTheHeaterMan

There is a chance that the previous circulator pump was a B&G series 100. That seams to be the standard pump in the 1960s and early 1970s. That pump has a unique pump curve that allows it to work well with gravity heating systems that were converted to forced circulation systems.
This shows that in the most common range (3 gallons per minute GPM to 10 GPM) the pump curve is relatively level. And if you compare it to the Taco 0010 curve, the GPM in the same range, the curve is also relatively flat. Not that the technical stuff means anything to you, It is possible that you may have a pump that is not the right fit.
The B&G 100 is the curve that is in the first file below... it has green lines indicating the range your system operates when only one zone is calling, to when all zones are calling.

Can you take a picture of the circulator pump you have, and perhaps a picture of the whole heater from far enough back so er can see the pipes from floor to ceiling. Then post them on this forum here for us to look at. Maybe there will be some clues as to what is happening.

Bridgestone

Attached are 3 images of the boiler and an image of piping on the right loop. I did some measuring. The left working loop has 4 feet of 3"pipe, plus 6 feet of 2 1/2" pipe, serving two large radiators in the 2 largest rooms in the house. The right loop has



11 feet of 4" pipe, plus three feet of 3" pipe, 90* turn, plus 12 feet of 2 1/2" pipe, plus 5 feet of 1 1/2" pipe to end. The loop serves 6 radiators of various sizes. All mentioned radiatos or on the 1st or 2nd floor.

HomerJSmith

Several things can result in low temps at the end of a run. Low flow or low temp supply water. The result is you run out of heat energy at the end of the run. With pipes as large as yours, the flow is very slow with heat energy leaving the supply piping, too. Your pump, I think is too small for the volume of water and the pressure losses thru the boiler and piping. I think I would have piped your sys differently. I would have piped it as a primary/secondary sys with multi speed zoning circulators (Grundfos). I would have used generous supply headers, too. My take.

DJDrew

I’m am not a heating pro, but I had a similar experiences on our system and it took some time to get answers.

1. A system with that much water quantity should be hard to short-cycle with your boiler. Ours did the same at first and it was because the installer left the bypass valve between the supply and return open too much. After discussing the issue here on this site, I ended up installing a Danfoss ESBE thermostatic mixing valve on the bypass line that automatically shuts it down once the system is up to temperature. 

2. Low Flow at the end of the lines. This was happening since our new boiler installed and it was to the point we thought something was wrong with those radiator pipes. As @Jamie Hall mentioned, water is lazy. After looking closely, we noted the lines with the radiators not heating up at the end were the same lines that had an extra set of 90 degree elbows on the supply and return. Water is lazy and those 90s were enough to impact. I attached a photo of what our original installer left us with 2 extra 90s on the supply and 2 extra on the return… the end radiators on those lines never liked to heat up, but radiators at a farther distance on the other lines without those 90s had no problem. I’d suggest a close mapping of your near boiler piping to see if there are any subtle differences that could be throwing the balance of the system off.

EBEBRATT-Ed

The piping around the boiler is too small and restrictive. The pump is on the return pumping toward the expansion tank. That may not be causing your problems.

I agree with @HomerJSmith primary secondary with seperate zone valves or circulators for each zone would fix it.

With such large water content the water is not going where it needs to go.

Bridgestone

No doubt there could be improvements to this legacy system piping, but the system worked well at the beginning of this year, before the new boiler and curculator were installed in March. After converstions on this site, and with the local Taco Rep and Taco technical support, there seems to be a consensus that my gravity conversion system with large pipes, needs a larger circulator. One is the Taco 0010-F3-1IFC, the other is the Taco 0010-MSF2-IFC. The latter has a 3-speed switch. Any thoughts on whether the this type of switch is generally helpful or reliable?

Bridgestone

The installer returned and replaced the Taco 007e with a Taco 0015 msf3-ifc 3 speed pump. Thought I would report on the result. Shortly after the plumber left the pump leaked about a gallaon of water, with boiler psi going to 0. He returned shortly and fixed the seal.

We didn't like the flow noise on speed 3, so I lowered it to speed 2. Then I partially closed the ball valves to the left-side loop of the main zone, and closed the boiler's manual bypass valve to about 1/3 open. Boiler psi at 13 rose to 22, mostly in the 16 to 20 range. Boiler temp ran mostly in the 145-170 range. The radiator at the end of the right loop didn't get hot, but warm enough to adequately heat the room. It's a big radiator in a smallish room. About 15 minutes after the boiler went off, the radiator got warmer, gravity heat I assume. So far I've worked with theromstat settings of 68-69, night turn down to 64, and outside temps of 2*F and 18*F.

When the boiler starts on call from any of the three zones there is a knocking sound that lasts 10 to 20 seconds, more so on cold startup and more so when one of the two small zones starts. It appears that the knocking often coinsides with the sound of rushing water that lasts 3-4 seconds. Thereafter the system runs withou knocking. Don't know if there's air in the pipes or something else. No air in the radiators. There's an air seperator above the boiler. Any ideas about the knocking sound? Maybe still air that has to get out, or water banging against the many turns of piping above the boiler?

Near the boiler, using a digital laser heat gun on both loops of steel pipe, the difference in supply and return temps ranged from 18 to 24. The gun didn't work on the 3/4" copper supplying the small pex zone of 3 steel panel radiators, but by touchiong I think the return was more than 20* cooler than the hot supply.

Regarding the pex zone, I'm thinking about having that zone run only when the main zone runs. To that extent, can I just set the pex zone thermostat to "off" and manually lock the lever at the top of the pex zone valve?

Thanks much for all the responses and suggestions I receved on the radiator issues, and prior information on bypass valves.

hot_rod

Knocking, ticking, tapping sounds are often caused by the expansion of the metal piping as the temperature increases. Expansion noise typically.
Sometimes you can find and fix where a pipe is touching the wood framing, or enlarge a hole around a pipe that is rubbing.

Bridgestone

Bob thank you for the response. My issue is that all three zones knock when they first come on, 5:00, 5:15, 5:45 a.m. (5-7 knocks various loudness) Thereafter, the zones run without knocking. Later in the day when they come on it's the same pattern, but sometimes less knocking. If near the boiler I'll somtimes hear water slushing for 10-15 seconds. There's no air in the radiators and they have heat. Cold boiler psi is 13, can get to 24, usually about 20. A month ago a pump replacemnt (Taco 0015 3speed, IFC) leaked and was reinstalled later the same day. I figured the air seperator would remove any trapped air by now.

Summerizing from prior photos, the zone valves and pump are on the return side, and the fill valve, air seperator and bladder expansion tank are on the supply side. On the forum it's been suggested that there can be trapped air the seperator isn't removing, that with zone valves a check valve in the pump isn't necessary and maybe a cause of knocking, or a problem with "pumping to" the expansion tank. As I read the Weil-McLain manual it does show two circulator positions, both pumping away from the expansion tank. I assume there can be exceptions and that the installer has expereince with these things.

On the phone the installer didn't think it sounded like trapped air, but a tech is coming tomorrow morning to look at things. Re-piping isn't a practical solution. Would purging each zone, and loop, be worthwhile? Or removing the check valve from the pump? If the knocking isn't causing any damage to the system maybe I'll just have to live with it. Any thoughts would be appreciated.