Residential Boiler short cycling i think?

Cjmentrup

So i'll start from the beginning. I've lived in this house for 6 years and was previously heating with a Weil-Mclain 100,000 BTU boiler from 1981. Our system is a 3/4 slat fin series loop. The house is a ranch style house on a crawl space with the boiler placed in garage area on the slab. The system has always worked well and heated with zero issues up until earlier this year i moved the system due to turning the existing garage into a master bedroom.

This is when my problems started to my knowledge. Upon adding a hydronic towel rack and a small in wall heater with a fan off the main loop using 3/4 x 1/2 tees for the new master and bleeding the system and firing the boiler in its new home it was short cycling on the temp limit. It was fairly warm out side so i chalked it up to the fact the house didn't really need heat maybe that was the issue.

Fast forward to a few weeks ago, i had forgotten about the issue total and noticed the system was struggling to heat and seemed to be on and off a lot, shortly after this the heat exchanger cracked. Since I've replaced the unit with a CGA-4 unit (75,000 BTU). This is when i fully discovered i still had a short cycle issue. Since I discovered this I re plumbed the system at the boiler to use a garden hose to force bleed the system and i'm 100% positive i have no air in the system.

At this point in time when the thermostat is calling for heat the system is heating to 180 (limit set in boiler) and having short cycles of about 3-5 min off and on. Its about 20* outside and with the house at 68* if i set the thermostat (Nest) to 70 it takes over an hour and a half for it to achieve 70.

Things I've tried to fix this:

New CGA-4 Boiler with new Grundfas pump
Run TSP though the loop to try and clean anything out
Clean all slat fins and make sure they have good airflow bottom to top
Force bleed loop
set boiler limit higher (195) - this did help heat better
adjusted city make up water down to 10psi with no change in cycle time
New CGA-4 Boiler with new Grundfas pump
Added bleeder vent to top of supply off boiler
Added bleeder vent to baseboard furthest from boiler

Things I've noticed from when the system used to work well:

Takes to long to heat the house now
The baseboards no longer pop and crackle when the system cycles on
All baseboards are hot and about the same temp
supply and return going into the boiler is hot


settings:

High temp limit 180
cold pressure 12 psi
running pressure seems to be around 20 psi
auto air bleed vent 1.5 turns from all the way in
Pump setting on High
Econ setting on boiler all the way down (have tried this both ways doesn't seem to help)
Expansion tank set to 12 psi

System summary:

3/4 slat fin series loop with a secondary loop tied in using 3/4 x 1/2 tees running towel rack/ in wall forced air heather
CGA-4 boiler
Grunfaus 3 speed pump

Looking for any advice as i don't feel this system is going to be able to handle the temps closer to zero we often have in January/February. I'm also concerned this short cycle issue is going to reduce my efficiency and life span of the new unit.

Zman

It sounds like you have a piping issue. Can you post a drawing?

Cjmentrup

Here is a very crude drawing i did really quickly. Hopefully you get the idea.

Cjmentrup

Cjmentrup

I timed the cycles here is what i found

Temp 160 on to 180 off

Pump set to low on for: 2:16 min
Pump set to low off for: 3:25 min
Pump set to high on for: 3:02 min
Pump set to high off for: 3:09 min