Gas boiler multizone apartment systems need heat turned off, hot water kept on

You need a boiler management control system. Tekmar and Heattimer are just 2 that come to mind off the top of my heat. There are tons of these available.

The information given so far is insufficient to answer the question asked.

1. We know this
   1. there is more than one boiler
   2. some are HB Smith that are old from 1971
   3. some of them are Weil McLain that are newer
   4. these boilers produce space heating and domestic hot water
   5. most of the boilers have 2 aquastats.
2. We don't know this
   1. how many boilers there are
   2. if the boilers are in the same building or if the boilers are in separate buildings: like one boiler per building.
   3. what size boilers they are
   4. are the boilers manifolded together as one large system or if there are separate systems
   5. how the boilers make hot water
      1. indirect tanks
      2. heat exchanger coils with storage tank(s)
      3. tankless coils inside the boilers
      4. or some other potable hot water source that is heated by the boiler(s)
   6. what existing controls are on the boilers
   7. what controls the hot water temperature
   8. what controls the room temperatures in the apartments
   9. if there is even a problem with apartments over heating in the summer as a result of the hot water operation

But without that additional information, we can all answer this with the experience we have all had on similar buildings. I remember shutting down a 220,000 BTU boiler for the winter that was only used to generate heat that goes into a shell and tube heat exchanger that fed a 2000 gallon domestic hot water tank that was covered with 3 inches of asbestos insulation with a plaster coating that was in pristine condition. the reason for shutting the boiler off for the winter was that no one lived in the building in the winter and the entire building was drained of water to prevent freezing water damage.

But that info won't help in this case.

EDIT:

The more info we have, the lower the cost of the solution to the problem: For example @pedmec solution is a complete control system for thousands of dollars. @EBEBRATT-Ed solution is to repair something that is already there. That has to be less expensive.

And since some of those boilers are from 1971, I believe that this situation has come up at least 54 times over the last half century. But that is just a guess

I thought he has 5 boilers in one building. but the more I thought about it, I believe there are many buildings with individual boilers in each building. Why would you have several boilers manifolded together and have 1971 boilers in the same system as boilers that are only a few years old? Makes no sense. that is when I thought of separate buildings. BUT… I am only guessing like you.

@4343 needs to offer more details.

I wonder if that is a new member that replaces @4321 from the other building down the parking lot of the apartment "systems" 🤣

You still need to provide more info. Are you saying the boilers are supplying hot water thru a tankless coil, indirect fired hot water heater. or storage tank? maybe even a sidearm instantaneous? You state one boiler per building if i am reading right? You dont say the type of water heater.

All the old buildings that i take care of use a outdoor temperature control (Honeywell T678 or similar) that turns the circulator on. This is the rudimentary way of suppling heat to the tenants prior to the advent of logic control. Boiler stays on year round so the hot water, if boiler is being used to provide the hot water, is always available. A flocheck valve or valves depending on piping prevent gravity flow during the no heat demands. This is the basic setup for older buildings that haven't

To better understand your system I will explain some of the different items I noticed in your photos.

On the newer Weil-McLain boilers, there are three controls attached to the side of the boiler. Each one has a probe that measures the temperature or level of the water inside the boiler.

The double aquastat marked “A” operates the boiler at a minimum temperature to maintain domestic hot water temperature inside the boiler and in the shell of the shell-and-tube heat exchanger. The LO setting might be set at 160°F, so the boiler and shell contain 140°F to 160°F water, depending on the differential setting on the LO limit side (the red DIFF dial).

When the apartments call for heat, the burners will operate along with the circulator for space heating. If the circulator operates so much that cold return water from the apartment radiators causes the boiler temperature to drop below the minimum temperature (about 140°F) needed to provide sufficient hot water for showers and other domestic hot-water needs, then that control has the ability to stop the space-heating circulator from operating until the boiler temperature increases back to the 160°F LO temperature setting of the control.

Sometimes during extremely cold weather, calls for heat may continue indefinitely from several individual apartments or another, for longer periods of time, causing the burner to operate continuously. When this happens, the boiler temperature will rise above the 160°F LO setting to provide more heat to all the zones. If that temperature ever rises above a safe operating temperature, the HI limit on Control "A" will stop the burner from operating above that temperature (for example 180°F). Once the boiler water drops to about 175°F, the burner will restart and the heating process will continue normally.

In the event that the HI limit on the operating double aquasta "A" fails to keep the boiler at a safe temperature, Control "B" will stop the burner. This is a manual reset control that might be set at 200°F or 210°F. If this control ever shuts off the burner, you will need to manually reset it and determine why the operating limit did not stop the burner from allowing the water temperature to rise above the operating limit (190°F, for example).

Control "C" is a low-water cutoff that will stop the burner if there is not enough water in the boiler. If the water level ever drops below the safe operating level, the burners cannot operate.

Hope this explains some of the functions of your boilers.