Boiler only fires up when indirect water heater pump is on

Do you still have hot water?

Prob not an issue

Then no issue. Dont be concerned.

You will have plenty of time. If it making hot water it will make heat.

What is the problem? You're not heating the space right now, so of course it's not going to fire unless the indirect calls.

A cold start boiler only runs when there is a call for heat- they don't "maintain temperature" unless there is an active call for heat from the thermostat or indirect. If you switch to heat mode right now and set the stat to 65*, the boiler still won't fire unless the air temp falls below 65* (for example).

To make it a little more clear, let's use a more typical December day. One or more heating zones may call for heat at different intervals. Zone 1 calls while Zone 2 is off. Zone 1 becomes satisfied, and the boiler temperature starts to drop from 180°F to 148°F when Zone 2 happens to call for heat. At that point, the burner operates to heat Zone 2, and the boiler temperature rises again until Zone 2 is satisfied.

The boiler temperature may then drop to 138°F when you decide to take a shower. The indirect DHW tank calls for heat, and the boiler temperature increases back to 180°F during your shower. When your shower is finished, the DHW tank reaches its set point, the burner shuts off, and the boiler temperature drops to 157°F when Zone 1 calls for heat again... and so on.

In this scenario, the boiler temperature always seems to stay above the so-called "low limit" temperature. However, that is not happening because of a low-limit setting on either the Nest or the L7224. It is simply a coincidence in the timing of the different calls for heat that keeps the boiler temperature above the boiler room's ambient temperature, which might be around 64°F.

Does that make sense?

To make that happen set your L7224 low limit temperature to OFF. That will make you system a "Cold Start" boiler and result in the lowest operating cost.

The L7224 comes in several different configurations, and most of them can be configured as either a cold-start boiler or a minimum boiler temperature boiler for maintaining the tankless coil temperature.

You are asking why you might want to maintain a minimum boiler temperature. That applies to a boiler that has a tankless coil installed inside the boiler. It is not the best approach, but it is somewhat economical because you save the cost of purchasing a separate water heater tank and all of the accessories needed to operate it.

Water heaters with storage tanks include your indirect water heater need a circulator pump aa thermostat and extra piping to connect it to your boiler.There are also gas-fired automatic water heaters where the burner is located under the tank and is controlled by a thermostat. There are electric water heaters with electric heating elements inside the tank, but they typically cost two to three times more to operate because electricity is generally much more expensive than oil or gas for heating water. There are even oil-fired water heaters that have their own dedicated oil burner to heat the water in the tank. With those you have a second boiler sucking oil from that fuel tank.

Every one of those tank-type water heaters adds cost to the mechanical system in your home. If you purchase a boiler with a tankless coil, however, you do not need to buy a separate water heater. The same L7224 control can maintain the boiler water surrounding the tankless coil at a minimum temperature all the time. Then, whenever you want to take a shower, need a cup of hot water for a recipe in the kitchen, or want to wash dishes, the hot water is immediately available.

That is the purpose of the low-limit setting.

Since you do not have a tankless coil in your boiler, you do not need to maintain a minimum boiler temperature. The low-limit feature is included in the L7224 because the same control is also used on boilers that do have tankless coils. It is there for those installations—not for yours.

Not exactly. @GroundUp. The person who taught you that concept was mistaken.

The purpose of the low limit is to maintain a minimum boiler-water temperature for a tankless domestic hot-water coil. A reverse-acting aquastat is usually involved. It opens on a temperature drop and stops the circulator from operating when cold return water from the radiators lowers the boiler temperature enough to create a “not enough hot water” condition.

The burner continues to operate while the circulator is off, allowing the boiler-water temperature to recover sufficiently for domestic hot-water use. Once the boiler temperature rises by the amount established by the low-limit differential, the control allows the circulator to operate again and space heating can continue.

Here is the official explanation from Honeywell/Resideo:

ALSO. This low limit situation does not apply to @haunt since they are not using a tankless water heater inside the boiler.

@EdTheHeaterMan that would be true if we were talking about a hot start boiler with a tankless coil, however, we are talking about a cold start boiler with an indirect so what I said was indeed accurate.

Still disagree @GroundUp. the internal wiring does not support your statement:

Low limit is essentially a range to maintain WHEN heating.

Then you stated:

If the boiler burner produces more BTU than the space is using, the water temp will rise. Instead of trying to maintain a constant temp of, say, 180 degrees by starting and stopping every 10 seconds, they employ a range to allow less frequent cycling.

To be more clear I changed the word Boiler with the word Burner. The L8124 with the white double aquastat mounted on the circuit board only has one dial for the high limit. It has a fixed 10° differential and that will keep it from having a 10 second off short cycle. the low limit with the adjustable differential does not come into play once the high limit opens. there is no holding circuit between the low limit switch and the burner the prevents the burner from operating.

Then you continues with

Low limit might be 160 with a high limit of 180, so that if there is a call for heat while the boiler is still 170, it's not starting the boiler burner until that temp falls below 160.

If you look at the B1 terminal you can clearly see that the hot from L1 goes thru the 1K1 relay contacts (the ones that operate the burner) directly thru the high limit to the B1 terminal. (follow the red line)

The low-limit contacts have no effect on the burner when the boiler-water temperature is above the 160°F low-limit setting. The low limit operates the burner only when the boiler-water temperature falls below 160°F regardless of a call for heat or not.

When the boiler temperature reaches the 180°F high-limit setting, the burner stops, while the circulator is allowed to continue moving heat from the boiler to the radiators.

The low limit does not prevent the burner from restarting when the boiler temperature falls to 170°F, as you suggested. Your trainer gave you incorrect information many years ago.

Now I realize that @haunt ’s system uses an L7224 electronic aquastat. However, I used the L8124 to illustrate the operating logic that the electronic control is emulating. It is designed to operate according to the same basic logic.

The L7224 may also include features for use with an indirect water heater and outdoor reset control combination that the analogue control does not have.  but it DOES NOT use the low limit setting to add to the differential of the high limit as @GroundUp has indicated

The fact that there is a solid-fuel boiler in the system has nothing to do with how the oil boiler operates through the L7224 control. It will still operate the same way. When the wood boiler maintains the boiler-water temperature above 180°F, the oil burner will not operate, while the circulators for the different zones will continue to operate.

Adding another boiler does not somehow change the way the control operates.

@EdTheHeaterMan description is correct. the HL and LL are two different controls although if not set correctly they can overlap and intermingle on each other's operation.

The HL stops the burner when the water temp is approaching the highest temp you want under any circumstances. Normal high limit settings are 180, 190 or 200 degrees. Because of the water pressure in a closed loop system you could run 210 or 220 if you needed to…….and that was promoted back in the 50s and 60s.

Now most set them at 180 I am a 190 degree guy myself. The boiler water temp is driven by the radiation installed. Set the HL too low and the burner will cycle while trying to heat the building. Setting the HL at 190 will give the radiation more output to heat the building but the radiation CONTROLs the water temp the HL does not keep the boiler at that temp it just limits the boiler temp, hence high limit.

The LL low limit is a different animal. It is only needed if the boiler has a tankless coil for DHW or if for some reason someone wants to have a hot start boiler. The LL should be set to keep the boiler water at the lowest temperature needed to provide domestic hot water in conjunction with the tankless coil thermostatic valve.

The LL has another function. If the DHW is being drawn and you have a call for heat at the same time you could be standing in a cold shower if the boiler water temp drops too low. For this reason, if the boiler water drops to low the LL will stop the circulating pump(s) to stop the call for heat and insure DHW production.