New Gas Boiler "short cycling"

@nelic why open the system up and introduce more air. just shut the supply off and purge thru the boiler drain. Simple

Doh!!

Correct. That is a pressure (and temperature) gauge. A gauge. In one single place. What is happening to the pressure elsewhere?

This problem is quite common in vacation homes. The homeowner is not there during the coldest part of the winter, so they don’t know that this problem may have existed for 30 years, since they are never there on design-temperature days. They also know that on the few cool days when they visit the home, it gets comfortable for the most part.

So let’s look at the whole system.

1. What do the radiators look like? Can we get a picture of some of the radiators? Are there any installation errors, such as placing the fin in the wrong orientation? Are there wall-to-wall carpets blocking the underside of the air intake at the bottom of the enclosure? Is there a kink in a piece of copper pipe that is restricting the flow?
2. Also look at the near-boiler piping for a blockage. I once had a Spirovent get clogged with debris in the system that we could not find for years. When we finally removed it for another reason, we discovered the problem that had haunted us all that time. Once I saw that screen all gummed up, I realized that all the strange moaning noises and circulation problems were the result of that clogged air vent not allowing proper water flow through the system.
3. After ruling out water-flow restrictions and air-flow restrictions through the radiators, then we can look at all the other things that were discussed here.

More radiation would help once the system is working correctly. With those temperatures measured it seems like there is a flow issue, or a measurement issue. I would think this issue should be understood before spending time and money modifying the system.

at a minimum.

the return temperature will vary, throughout the boiler cycle.

Under a heavy heat load, or coming out of setback expect to see a wide delta between S&R

The delta will close as the system distribution warms, the room load gets satisfied. It could drop to 10, even 5 as the thermostat satisfies.

Low mass fin tube will see that delta closing quickly. High mass, large volume radiators will slow the change.

A 20 delta is just a design number. Under a full load, non stop firing it may or may not run a 20 delta. It is a very arbitrary number, That 20 delta.

A 77,000 btu boiler wants to have bout 154’ of fin tube connected. To move that many btu at a 20 delta would require 7.6 gpm, which isn’t likely if it is 3/4 fin tube and piping.

154 X 500/ft=77,000/ft

But the fin tube should be sized to the actual heat load.
You mentioned 80’ of fin, is that accurate? If so, you have the ability to move about 40,000 btu/ hr. That is doable with 3/4 fin and tubing. Ideally a heat load number would bring all the answers.

So you probably have too much boiler, maybe too little fin tube. If the home heats adequately on the coldest day, then you have enough fin tube, but still too much boiler.

This is the cause of the short cycles, the fin tube connected will dictate the boilers operating condition (temperatures) at any given time and load

Regardless of all the numbers ddi g up, the heat load at 40,000, a properly sized boiler, the correct amount of fin tube, on mild low load days the boiler is still going to cycle more. It only knows one speed, full speed.

@Ismellelephant , here is a list of some contractors who advertise here. Not being familiar with MA geography, I don't know who is close to you, but it's a good place to start:

https://heatinghelp.com/find-a-contractor/state/MA/

It sounds to me like a circulation issue, but the travel time from Baltimore to Cape Cod would be a bit much…………

If the old boiler worked as you would expect, not short cycling, and
heating the home as expected. And the new one is short cycling and NOT
heating the home as expected. The question is what significantly
changed. Assuming the new boiler is not significantly oversized.

Do you know the size of the old boiler ?

57’ fin tube X 500 btu/ ft = 28,500 btu: hr that it can transfer into the space

So your 77,000 output boiler is more than twice the size of the attached fin tube probably twice what the heat load would calculate at

Being a very low mass heat emitter and under partial load 80% of the heating season , it is going to cycle, a lot

The boiler should have been sized to the fin tube. A 50,000 input boiler should have been installed

@Ismellelephant I have not read the entire thread, but what are the low, high and differential control settings on the boiler's aquastat? Can you post a photo of it with the cover off?

You certainly have enough boiler capacity and the radiators are the same ones that heated the house adequately with the old boiler. Logically you have either a water circulation problem or a water temperature problem, which the control settings will affect.

adding more fin tube will just have the boiler reach the thermostat setting faster, I doubt this solves the cycling?

Your bottleneck then becomes the 3/4 tubing capacity, in that series loop, about 4 gpm. You’ll be hard pressed to move 77,000 through 3/4 copper tube. Splitting the loop into 2 section would allow more gpm, but your boiler is still twice what you need!

However more fin tube would allow you to run the boiler at a lower temperature, which reduces fuel consumption.

That boiler on fin tube could maybe run a 150- 130 delta. Still warm enough return to prevent the boiler from running too cold.

Bottom line, you need a smaller boiler. Or leave some windows open and increase the load, not recommend 🤓

Not knowing the actual heat load leads to a lot of speculation. But I think you indicated the home previously stayed warm in the coldest day? I suspect, based on the info you provided, the old boiler cycled also. How could it not if it was oversized?

Some more fin tube is throwing good money after bad.

looks like a power vented boiler, that is the inducer fan.

What bothers me with this situation.

1. The old larger boiler worked good, no short cycling.
2. The new smaller boiler short cycles with the burner apparently shutting down on high limit and the thermostat is not satisfied in a timely manor.
3. The temperatures on this diagram. I would expect the supply pipe would be warmer than the return, which also seems odd. Seems like the heat can't get out of the boiler so the burner just shuts down on high limit.

Could the installer have stuffed a rag in a pipe and failed to remove it or something else that is restricting the water flow ?

Is this a leak?

If I'm seeing it all correctly I'd take some hoses and a bucket and verify flow through the boiler and the system. Close the valves at the Red arrows, water feed and the one circulator isolation valve. Feed water from a hose into the drain valve and verify good water flow out the purge valve. Blue arrows represent the water flow, in the drain out the purge valve.