Why would oil-fired hydronic boiler short cycle

Question

This is a hydronic heating system at a family member's house -- oil-fired hot water baseboards with two zones, one for each floor. The bedrooms get hot and the bathroom / dining room / living room stay pretty cold. All those rooms are on the same floor. The boiler is relatively new, I'd guess 10 years old at most. The burner is made by Carlin.

I checked and as you might expect the rooms that are getting hotter are at the beginning of the pipe loop for that zone and the ones that stay colder are at the end. The problem seems to be that the boiler is short cycling. With the house at 62° and the thermostat set to 70° the boiler will run for about 2 minutes then shut off for 7 or 8 minutes. The boiler temperature gauge goes up to about 195° before it shuts off so I presume it goes off due to the high limit on the temperature. When the gauge drops back down to around 180 it turns back on. As this goes on the pressure reading is somewhere around 20 or 25 psi and doesn’t change much.

I understand – correct me if I’m wrong – that with the boiler short cycling there isn’t really enough time for the heated water to move through the whole system, which would explain the temperature differential between the rooms. But I have no idea what would cause the short cycling. Any ideas? Thanks!

Answer 1

You might be confusing "normal thermostatic operation" with "short cycling" as you have not mentioned the status of the circulator pump.

If the circulator pump is running, then the boiler burner is firing at 180 and shutting off at 195 and nothing is wrong with that, as heat is being delivered all the time the circulator pump is operating. So generally "until the thermostat (on the wall) shuts off the circulator pump" - the thermostat on the wall has no direct control over the burner on/off temperatures, it just issues a "call for heat" and starts pumping water, unless a low temperature cutout prevents the pump from starting until the boiler is hot. If the burner continued to operate while the circulation was not removing heat at the same rate the burner is supplying it, you'd have an exciting mess as the overtemp/overpressure valve blew off.

So long as the pump is running when heat is being called for, the described behavior is normal, expected, and unremarkable. Given that the system has to be designed to operate on the coldest expected "design day" it's oversized every other day that's not as cold, and given sloppy design practices it's often oversized for that day too. Which means the burner won't run all the time. Typical oil burners are "all or nothing" rather than having a throttled output to match the load. As such, you'll see intermittent thermostatic operation.

Heat is "charged" into the "thermal capacitor" of the boiler metal and water, and "discharged" by delivery to the heating loops.

Close the flaps on the baseboards in the warmer rooms and open them on the colder rooms to balance the heat delivery relative to the thermostat setting. That's also standard, normal, and expected behavior. The baseboards early in the loop get hotter water, but they all get heat, and balancing the delivery in long loops is all controlling how much air goes through the baseboard in different parts of the loop.

Answer 2

You could get similar results with air in one or more of the radiators. Be sure you bleed them to get rid of any air. Also you said two zones, Is this happening in both zones? Be sure the flow valves if you have any are open enough.

A failed circulator could also cause this type of problem, are you sure it is OK. Do not confuse pressure with flow. My system is at about 70 PSIG (potable water system) all the time but unless the circulator is operating nothing gets hot. If in fact the boiler is short cycling that indicates you are not removing heat at the needed rate again caused by the circulator not moving water. I have them fail over time where the volume of water slowly decreases as the impeller fails. Ten years is I believe a reasonable life for a circulator. They are not hard to replace nor that expensive, I would try a new one. If that does not fix it there will be a spare available immediately if the new one fails.

Yes convection could cause this. The effectiveness of this is system and temperature difference dependent. You could put a flow meter in the line and note its value maybe once a month, over time it will show when your pump is not performing properly.

Answer 3

The boiler temperature gauge goes up to about 195° before it shuts off so I presume it goes off due to the high limit on the temperature.

correct, that's what's suppose to happen

When the gauge drops back down to around 180 it turns back on.

correct, that's what's supposed to happen; both are controlled by the aquastat or hydrostat.

oil fired boilers typically operate based on a minimum of ~160°F water temp, absolute minimum around 140° to prevent damaging condensate from forming in the flue and heat exchanger. The burner on/off cycling based on 180° and 195° is in range and fine (upper limit < 210° to prevent boiling is the only real concern)

As this goes on the pressure reading is somewhere around 20 or 25 psi and doesn’t change much.

it should not change at all as the expansion tank (i.e. amtrol) accounts for thermal expansion. Depending on how high the hydronic piping goes above the boiler (i.e. 2 story house) will be requirement of hydronic system pressure. Between 20-25 psi is fine. Expansion tank bladders fail over time, you might need a new expansion tank, nothing major, but the expansion tank would not cause boiler short cycling.

I understand – correct me if I’m wrong – that with the boiler short cycling there isn’t really enough time for the heated water to move through the whole system, which would explain the temperature differential between the rooms. But I have no idea what would cause the short cycling.

There are a handful of reasons why a boiler would short cycle. The burner from its perspective (aquastat or hydrostat controller) is operating as it should. It could be bad circulator pump(s) not providing enough flow or too much flow, the boiler could be oversized for the given demand for heat, thermosats, zone valves, natural heating difference between heating zones not accounted for in existing piping/pumping setup. you would want to educate yourself on hydronic heating and check out taco university and caleffi websites online u can easily educate yourself on the subject to understand what's causing your perceived problem. It's not uncommon for whoever built the house and the person(s) who installed the original boiler and hydronic setup to have done it far from optimal if not wrong, as well as subsequent boiler replacements to make things worse (i.e. let's replace with a bigger boiler than what was originally there, oftentimes the original was oversized to begin with especially oil fired boilers)