Monoflow system does not heat well

charliechicago

Hello to all

A customer of mine has a monoflow system that is not keeping up with the cold. I've serviced it for 10 years, only maintenance, never needed a repair. This winter, it will not satisfy the thermostat. It's a one pipe system with 1-1/4 main that runs around the perimeter of the basement. Its a one floor house with mostly thin tube rads and a little baseboard. I purged all the rads/baseboard and only get water. Nothing in the system or house has changed. The boiler heats up to 180deg supply temp and shuts off, pump continues to run. All the rads heat up completely, but they seem to not be giving off enough heat. The only way I can get the thermostat to satisfy is if I increase supply temp to 200deg and even then its running most of the day.

Any suggestions?

delcrossv

Either the impeller is getting worn (how old is the pump?) or the system is undersized. 200F isn't unreasonable.

Did it keep up with previous cold snaps? I.E. just this week or earlier when it was warmer?

charliechicago

Thanks for responding.

The one major point is that I forgot to mention I changed the pump. It had a taco 007 and I put another one in. No improvement. It could not keep up even with more moderate temperatures. They have a sensi thermostat and I can look up the history going back to the end of 2018 and they never had close to this much usage.

EdTheHeaterMan

Something changed from before.

1. The Monoflo™ system was installed over 40 years ago?
2. This is the first year that the system can't keep up?
3. Were there any other changes since the previous years when it worked just fine?
   1. Boiler replaced
   2. Expansion tank replaced
   3. Non-boiler relates work done on the home like carpeting that is blocking the bottom intake of the radiators?
4. As you mentioned when bleeding the radiators, no air came out… so it is not an air problem.
5. Are there any service valves that may have been closed, that were not fully opened when the repairs were completed?

delcrossv

Same type pump as you said. Did the new one have check valves on it and the old one didn't?

Thinking there's a flow restriction somewhere.

charliechicago

I presume the system was installed over 40 years ago, house was built in the 50's.

First year it cant keep up. Nothing was changed with regard to the boiler or the house at all. Only water from vents and all valves are open.

Not sure if the old pump had a check valve, but the new one does not.

Thanks again.

bburd

Can you measure the temperature difference between supply and return water? That would tell you if there's a flow problem. Since the system is pressurized a high limit of 200° F should be fine.

delcrossv

Anything change between last year and this one? Might have a bum pump if that was the only change. Or was the pump changed in response to the lack of heat?

charliechicago

When I had the system at 180deg I did more of my testing. The supply was 180deg and the return was 160deg.

I also measured the run time which was around 4 minutes or so for the unit to cycle back off.

No changes from this year to last.

Ironman

I hate to ask this but…are you sure the pump is pumping in the right direction?

Also: a 007 may be a bit too small for a Monoflo system. I’d use at least a 008 or a Grundfos ups15-58 on high speed.

charliechicago

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charliechicago

Bottom picture is most recent. As you can see the usage is way high.

charliechicago

Ironman

Definitely don't have to be afraid to ask, we all make mistakes!

Pump is in the right direction, but I can certainty change out the pump if you think it will help. Thanks

EBEBRATT-Ed

I agree with @Ironman.

I had an issue with my own house LOL.

Had an old boiler with a Monoflo system 1 zone. The system had a Taco 110. When that failed I put in an 007 and it worked fine.

When I sold the house, I put in a new boiler and an indirect used the same 007 but now had (2) 1" zone valves, indirect and heat. I had put in Honeywell zone valves with a Cv of around 5 I think.

Heating load was only 50 K with a 1 1/4 Monoflo loop. Sounded like a low pressure drop to me. On DHW it was ok with the indirect near the boiler.

On heat the 007 just wouldn't do the job the water was real sluggish getting around the main and this was without the indirect calling.

I took the Honeywell's out and put in 1" Taco valve I think the Cv was 7.5 or 8 and that solved the problem. Maybe I could have swapped to a larger circ for less $$$?

delcrossv

B&G 100? Probably what was there originally.