Weather Responsive Boiler Controls

bk14x32

Will Weather Responsive Boiler Controls cause the following boiler problems during mild weather due to the cooler return water:

1. Boiler Thermal Shock

2. Condensation

3. Boiler running too long

Should a boiler bypass be used with such controls to temper the return water?

Thank you,

bk14x32

Jean-David Beyer

As usual, It Depends.

Will Weather Responsive Boiler Controls cause the following boiler problems during mild weather due to the cooler return water:



1. Boiler Thermal Shock



Depends on design of the boiler. A mod|con boiler would presumably not be troubled by this.



2. Condensation



Depends on design of the boiler .A mod|con boiler would presumably not be troubled by this.



3. Boiler running too long



Does it matter? I would assume it would run long enough to get back up past the set point of the thermostat. Some people recommend running the circulators 24/7 and varying the output temperature of the boiler to match the current heat loss of the building. I do not think I would go that far.





Should a boiler bypass be used with such controls to temper the return water?



If the boiler cannot tolerate low return temperature water, just modify the reset curve to remove reset when the return water gets too cool.

bk14x32

As Usual, it Depends (reply)

Thank you for the interesting and detailed reply to my question. I am not familiar with the boiler design you suggested but will do some research. Yes 24/7 for the circulator, I have been hearing a lot about that mode. Perhaps the topic of a future question.

JD thank you,

bk14x32

Bobby K 

Unknown

Look around "Off the Wall"

Hi- You might want to look around in what is know as  "Off the Wall" under "Resources " and "Systems" tabs above. There is a lot of good info there. I would also browse through past postings in the Main Wall and Control sections as discussion dealing with Outdoor Reset comes up quite often.

- Rod

Jean-David Beyer

Mod|Con boiler

I just got my first mod|con boiler this May, so I have not been through a winter with it yet.

I have two heating zones and one zone for indirect hot water heater. One heating zone is finned tube baseboard. I put in rather more baseboard than calculated because I do not want to run at the normal 180F, since I want the return water as cool as possible to promote condensing in the boiler. I would not wish to run that zone 24/7. Downstairs is copper tubing in an on-grade slab that I could run almost 24/7 since I have outdoor reset on the boiler. I have set the reset curve so that at 70F outdoors, the boiler would deliver 70F water through the slab. (Of course under those conditions, the thermostat would not call for heat, so nothing would happen.) At the design day, when it would be 10F outside, the boiler would supply 120F water to the slab. I think I would like to set it up at first so it would run 12/7 so if it got colder than my design temperature, the the system could just run longer to keep up. Only if that did not work would I need to diddle the reset curve. Since this is my first year with the boiler, I imagine I will learn a lot. ;-)

bk14x32

Reply to JD and Rod

JD sound like an interesting system you are experimenting with. As you learn, especially about the 12/7, please keep us informed. Running a bit longer may do the trick without redesigning for a lower temperature. Apparently that new boiler can handle the lower return water temperature.

Rod I will investigate the area of this site that you recommend.

Thank you both,

bk14x32

Bobby K 

Jean-David Beyer

about the 12/7, please keep us informed

The 12/7 is not on stone tablets. There is no such setting anyway. I can set the endpoints of the reset curve (both the temperature outside and the temperature of the water) anyway I want. If I knew the exact heat loss of the building (and I do not), presumably I could set it "just right" and run 24/7 with the thermostat at max, letting the reset control the temperature. I have seen this suggested. If I set the reset curve too hot, then it will not run 24/7, and presumably I could get whatever proportion I want, at least close enough. But I do not know, at this point, what I really want.



But this is tricky, even if I were to set it "just right." This is because the upstairs zone can go to 140F on the design day and the downstairs zone is set to go to 120F. The downstairs zone has priority over the upstairs one. So you could imagine the downstairs running all the time would prevent the upstairs getting any. But there are two mitigating factors.



1.) If both zones ask for heat, both circulators will run, but the boiler will put out the lower temperature. This will help the upstairs some, but may not be enough.



2.) The other is that the  controller for the boiler will relinquish control to the lower priority zone after 30 minutes and allow that zone to run for 15 minutes, so that all the zones will get some time. The 30 and 15 are adjustable numbers, but those are the defaults.



As I said, I expect this to be an interesting learning experience.

bk14x32

12/7 Clarification

Thank you for the clarification regarding the "12/7". I have thought about the term since reading your post and have never seen it used as such before. Nevertheless, your system appears unique and would be interesting to learn how it passed the winter.

bk14x32

Bobby K

Jean-David Beyer

I have been playing around with the idea in my mind, but...

... I think it will be more productive when it actually gets cold enough that the thermostats call for heat. It goes down to the mid 40s many nights, but inside the house it has remained above 70, and hit 74 today, just from the heat load of my being here, running two computers and a new refrigerator. And whatever radiant heat I get from the sun.



One thing I would like to see is if it gets cold enough that both zones and the DHW heater all call for heat at the same time, since each is at a different priority, and each priority calls for a different temperature water from the boiler. I wish there were a computer connection from the logic board in the boiler; read-only would be good enough.



Maybe 18/7 would be good enough for downstairs. ;-)



I am not sure how unique my system is. Surely others have purchased the same boiler. I may be taking more advantage of the capabilities of its control board than others, but I am not even sure of that.