Season transitions and oil usage

HeatingHelpMePlease

I have a Budurus oil boiler for hydronic air. I have the Logamatic controller and an outdoor sensor.

I’ve heard that as far as thermostats go to just keep it at whatever temperature you want but I have some questions about this. 

In transition seasons with a traditional boiler if I don’t turn on the heat then (hot water aside) nothing is telling the boiler to turn on. Since with this setup the thermostat isn’t calling for heat to the boiler is there any way to not call for heat? 

Would I need to change the temperature on the Logamatic (or put it in summer mode or something)?

Basically it’s fall and starting to get cold but I didn’t turn the heat on at the thermostats yet and I was just realizing that I may be using the same amount of oil regardless of having the circulators/blowers on — is that right?

Jamie Hall

Um... shouldn't be. If the system isn't calling for heat and the fans aren't running, the only thing that might happen is that the ODR might be asking for a higher water temperature, and the boiler might run now and then to maintain that.

HeatingHelpMePlease

Okay thank you.

I know people have said a basic thermostat is better than a programmable one (esp those “smart” thermostats) so maybe I was just reading too much into that. 

I’m still a little fuzzy on how it works. 

The boiler is set I think for 70° and the outside temp is let’s say 40°. If the thermostats are off the boiler still runs sometimes for the hot water but will run less frequently than if the thermostats were on?

I guess from there I wonder what’s wrong with programmable thermostats like having the heat lower if I’m not at home.

Sorry if I’m being dense about this!

Jamie Hall

It can get confusing. The main problem is that there are two very different control strategies involved. The traditional approach is to have a space thermostat of some kind. When the space thermostat senses the temperature is below whatever it's set for, it turns the boiler (or whatever) on, and the heat is delivered to the space in a variety of ways -- in your case, by the hydronic air handlers. When the space temperature rises a bit, the thermostat turns off and so does the boiler (or whatever). Works fine.

However, in the quest for higher efficiency, it would be nice if the boiler could run at as low a water temperature as possible, so that the water vapour in the flue gas could condense and add that heat to the system. But to do that, the boiler would have to run all the time, and so would the heat delivery mechanism --and there has to be some means to tell the boiler what temperature to run at. That is the purpose of the outdoor reset. It senses the outdoor temperature and adjusts the boiler water temperature to provide just the right amount of heat to the structure to keep it at a set temperature. I might note that getting this relationship right can be a bit tricky...

Under this control strategy, the best operation is achieved when the boiler never turns off, but just varies its output as needed -- and in these arrangements, the space thermostat really only serves to turn the system off if the relationship isn't quite right and it gets too warm. In the better systems, it will also boost the boiler temperature if the space gets too cold.

You can't mix the two control strategies. They will fight with each other, and the result will be unwanted temperature swings and excess fuel usage.

Similarly, setbacks don't mix with outdoor reset control, or don't mix well (the can be mixed, but that requires more sophisticated matching than most systems have). Also, setbacks don't mix well with hydronic systems, never mind steam or radiant floors, as all those systems respond much more slowly than forced air (your system is sort of midway between forced air and hydronic, but more like hydronic).

With outdoor reset, there should be an outside air temperature above which the boiler won't run at all, and for some temperatures below that when it will only run once in a while, as it simply can't reduce its heat output enough to match the demand.

HeatingHelpMePlease

Thanks a lot for taking the time to give such a detailed reply. I appreciate it. I’m starting to understand it better, but I have an oddly specific question. 

I’m addition to my central heat system my house also has a nice efficient fireplace. If the fireplace is cranking out heat, am I correct in thinking this has no impact on the boiler?

What I mean is, the boiler knows its internal temperature and it knows the outside temperature, but it doesn’t know the inside temperature and its not really directly impacted by the thermostats? So basically if the fireplace is running and the thermostats say it’s hot enough so the central blowers and circulators are off — none of this impacts the boiler so it will just keep running whenever it feels like it based on its temperature and the ODR? Is that correct??? Thanks again!

Jamie Hall

Fireplaces do weird things to systems with outdoor reset since, as you point out, the boiler doesn't know about it.

However, one of two things should happen, and it depends on the boiler control logic. Either the space thermostat should sense overheat in the space, and keep the boiler off, or the boiler might fire from time to time, sense that the air handlers aren't taking much heat out of the water, and shut down.

I say... should happen. What actually will happen depends on the specific setup.

HeatingHelpMePlease

That makes a lot of sense. Okay, thanks so much for taking the time to explain things!