I am tasked with optimizing the heating system of a quite large building. It currently has two gas boilers working on four separate circuits (all joined together of course when they return to the boilers). I am doing this on a volunteer basis and at the moment I am at the basics of understanding how these systems exactly work.

My current main confusion relates to what exactly makes a boiler to turn on, burn gas and heat water. So far what I've understood is that 1) a thermostat can send a signal for it to turn on, 2) you can just turn it on manually and it will stay on if there's no thermostat control, 3) the temperature of the returning water matters too (I might be wrong on any of these of course). But I'm not sure how these factors are related, how they play together, etc.

E.g. when the thermostat is in a room where the set temp has been reached, can/will the boilers turn on if the returning water is cold from passing through other, less warm rooms? Also, what is the exact role of the temp difference of the outgoing and returning water in general?

I'd much appreciate some basic info on this, even just some pointers/links as to what to read to understand this better.

  • "quite a large building", two boilers, four circuits... Is this a home or a commercial installation of some sort (church, school, etc.)?
    – FreeMan
    Commented Nov 15, 2022 at 14:07
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    Usually two different thermostat systems. One to call for heat in the rooms, and one on the boiler to maintain the water temperature. The one on the boiler will turn on the gas when the water temperature drops a certain amount. This can be if the water is just sitting in the boiler and cools off or when the rooms call for heat and the returning water cools down the water in the boiler.
    – crip659
    Commented Nov 15, 2022 at 14:15
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    @crip659 That looks like a good solid answer. Commented Nov 15, 2022 at 14:28
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    A lot going on here, this will proabably have to be a multi-comment! A lot depends upon the type of boiler: Make & Model will help us give good answers. It's a big difference whether you have older boilers or newer condensing boilers. Next: how many thermostats are there? One in each room? One per floor (if multilevel structure)? Without knowing more, generally speaking, a zoned system, which is what you have with multiple t-stats, if one of the t-stats call for heat and if the boiler isn't already running, it will turn on the boiler. ....continued below: Commented Nov 15, 2022 at 17:07
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    There are a variety of factors that determines when a boiler fires up. Some have outdoor thermometers that both determine when to fire and set point. Is there a buffer tank? A lot more info is needed from you to provide good answers. Commented Nov 15, 2022 at 17:10

3 Answers 3


It's a basic logic and latch circuit, traditionally relay based. The schematic diagram may actually be attached inside the relay box, or available on the web.

Simplified description:

First, of course, a thermostat switch cools enough to close. That low-voltage circuit pulls a relay closed.

That relay does several things. First, it enables the pump which drives this heating loop. Second, it tells the boiler that it needs to be hot.

I'm going to gloss over the logic which controls the gas burner itself; the presence of a safety circuit complicates it a bit, and "moderating boilers" have some additional states. Basically, it has its own thermostats, and a relay circuit which starts the burner when heat is requested, turns off the electric starter when it detects that the flame has caught, powers the selected pumps when the temperature is high enough to be useful, and stops the burner when the temperature hits its design maximum.

With the pump or pumps going, we're now sending heat. Eventually the remote thermostat hits the temperature it was set for and opens, which releases the relay which selected the pump and stops telling the burner that heat is needed.(Again glossing over a step; typically there is a delay circuit so the pumps run a bit longer to usefully extract heat remaining in the boiler.)

Note that the boiler runs (or cools a bit and then restarts) as long as any of the thermostats is still calling for heat. And that if multiple thermostats are running their pumps at once, the heat must be shared and everyone will warm up a bit more slowly than if they were alone. And that the boiler must be large enough to be able to heat all the zones simultaneously, in the worst case.

Does that help, or just increase confusion?

  • It did help! Thank you. The unclear parts for me were 1) that thermostats and relays control pumps and ask for heat as well (not just the latter), 2) if multiple thermostats are present, then they are summed with an OR logic (that is, if any of them asks for heat, the boiler runs). Commented Nov 16, 2022 at 11:32
  • I am going to EDIT my answer to include the setup we plan to install, it has an RVS36 controller element in it as well which monitors outside temp too (in addition to other inputs). Commented Nov 16, 2022 at 11:34
  • Eventually I've decided to post a separate question with the intended setup: diy.stackexchange.com/questions/260607/… Commented Nov 16, 2022 at 14:14
  • Monitoring outside temp is the thing I referred to as a "moderating" mechanism. (I may be misremembering; the term may be "modulating", but either way it's a way if being more efficient when less heat is needed.)
    – keshlam
    Commented Nov 16, 2022 at 17:09

So there can be several levels of control:

  1. time: so to prevent boilers being called to heat overnight in certain situations then there is a time control to limit the working hours for the boiler. So, starting at 6AM to make sure sufficient water for showers etc and bring living spaces up to temperature.

  2. hot water tank: calls for heat with thermostat as needed.

  3. heating system: calls for heat with one of more thermostats as needed.

Are the occupants charged based on what they used or is the bill just divided equally? ie are there meters to register individual use? This would make it much more complicated so unlikely.

  • We plan to include heat transfer meters to see which part of the house consumes how much heat, just for monitoring purposes. Commented Nov 16, 2022 at 11:36
  • @BenjaminMárkus that sounds fun, will you be monitoring the heating water to each apt? Flow meter? Temperature difference? Lots of things to consider…
    – Solar Mike
    Commented Nov 16, 2022 at 12:11
  • I've posted a new question with the intended setup: diy.stackexchange.com/questions/260607/… Commented Nov 16, 2022 at 14:14
  • @BenjaminMárkus why? do you expect us to design a complete control circuit for you?
    – Solar Mike
    Commented Nov 16, 2022 at 16:13

Here's what our church has for heating - similar to a small commercial building. I am just starting to get familiar with this.

We have one main boiler that runs during cold weather to keep the hot water at an appropriate temperature, around 175 deg F, I think.

I am not certain what controls this central boiler. It may be controlled by an ORing of the thermostats in the various parts of the building. That is, if any individually controlled part of the building needs heat, the boiler runs. Or it may just be a single thermostat that senses the water temperature in the loop. Whatever it is, the water in the boiler and the main loop needs to be kept above some minimum temperature (say 140 deg F) so that hot water is available immediately when heat is called for in part of the building.

This hot water then circulates through a large hot water pipe loop that runs around the entire building. The pump for this loop is at the boiler. So during the heating season, whenever any part of the building needs heat, there is always hot water circulating in this loop.

We then have smaller loops that come off the main loop with individual thermostat controlled pumps that turn on to provide hot water to heat individual parts of the building.

It's gets even more complicated in that some of the newer additions have separate mini-split systems that heat and cool and those parts of the building, and are not part of the main heating system.

  • This is very similar to our case! Commented Nov 16, 2022 at 11:35

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