Boiler Temp

It depends on the ratio

of radiation to heat loss. That determines your lowest maximum temperature on a design day. God help you if you need 220 degree water...

The upside of that is you will not complain day to day about too little heat; rather you will complain and loudly only one day of the month

If your heat loss is, say, 50,000 and you have about 335 SF of radiation or the equivalent if fin-tube, you should get by with 180 degree water (170 average assuming a 20 degree drop) as one example.

Do you know your heat loss and do you know your radiation potential? Those two together tell the story.

As Brad W says above, it depends on the ratio of radiation to heat loss.

For instance, in my house I have finned baseboard. On design day (-4) according to the heat loss calculation I can get by with 150 degree water. It's actually -8 currently and I am running an average water temperature of 160 degrees. And some zones are cycling off. (The rooms on the west side of the house are about 1 degree below setpoint because they are dealing with the 20 mph winds , while the eastern rooms are getting all the prewarmed air moving through them and those zones are cycling off.)

You will need to do some experimenting to see how low you can go with the water temps in your particular system. Turn the temperature down in 10 degree increments and give it a couple days to see if it can keep the room temperatures up. You will need higher water temperatures when it is colder outside, and can use lower water temperatures on the warmer days during the rest of winter. Don't go so low that the return temperatures are below 140-150 degrees for an extended period of time. (brief dips when a zone first starts calling is ok) This can lead to condensation and boiler damage.

They do make outdoor reset controls that will change the boiler water temperature automatically as the outdoor temperature changes. The Tekmar 256 is the most basic contol. (others available for DHW / multiple stages / etc)

Michael

A word of caution, Phil

Outdoor reset is a fantastic way to both increase comfort and to save energy at the same time. Be advised though, unless you have a Modulating Condensing (ModCon) boiler (meaning you have a cast iron conventional boiler), your reset options are limited.

Specifically, you have to protect your CI boiler from thermal shock and from flue gas condensation regardless of gas or oil. This means keeping the boiler return water temperature at least 135 degrees.

Ideally, I would want to see you decouple the radiation loop from the boiler loop. Keep the boiler between 140 degrees at the low end and whatever you need at the high end. The radiation loop can be separated from the boiler by a 4-way mixing valve or an injection setup, two of several ways to do it.

By doing so, your reset to the radiation can be deeper and proportional to the heat loss, enhancing comfort.

If you have a ModCon, forget what I just wrote.

At those high temperatures

and add to this if the circulator pumps toward the expansion tank (rather than away from it which is the correct way), cavitation could have been occurring.

High temperatures and low pressures promote what is essentially boiling which is not only noisy but damaging to your pump impeller...

Something for the list...