What is the correct temp setting for a hot water boiler?

Al Roethlisberger

Per my earlier post about a gravity hot water conversion to a sealed circulated system.... I have another question about the correct temperature setting for boiler itself.

I have read that around 180 degrees may be a typical setting for the boiler control, but during my investigations into this system, I noticed that they had it set at 200 degrees.

I have also seen recommendations anywhere from 165-180 as a typical setting.

So my question: What is a normal boiler control temperature setting for a circulated hot water system?

I'm sure that the prior 200 degree setting must have contributed somewhat to their $500 a month heating bill!

Thanks
Al

Brad White_9

It depends...

The ideal method is to know your calculated heat loss then know how much radiation you have in EDR. Often HW radiation is sized for 180 degree supply/160 degree return with an (obvious) 170 degree average. This will give you a radiator output of 150 BTUH per SF of EDR in a 70 degree room.

If you have a surplus, the maximum temperature you need will go down.

200 degrees is plenty high given our recent warmish weather. Even if you use the boiler to make domestic hot water, that can be a switched priority; it need not be kept there!

You should have outdoor reset, it will save money and increase comfort.

Remember though, what we are talking about here by knowing the radiation to heat loss ratio is to establish the maximum temperature you will need on the coldest day. It goes downward from there.

Hope this helps.

ALH_4

Outdoor Reset

I recommend purchasing a Tekmar 256 (w/o water heater) or Tekmar 260 (with water heater) outdoor reset control. Many times these can be installed with no piping changes. These will control the boiler temperature to optimize the operation and efficiency of the current system.

Al Roethlisberger

Tekmar questions

So, will this interact or interfere with my indoor programmable thermostat? Or are they independent functions?

Also, would this completely replace my current Honeywell boiler controls that also controls the circulator pump, or does this just piggyback onto the existing boiler control module?

Thanks
Al

Brad White_9

Independent Control

The outdoor reset is a limit on how hot the water temperature will be at a given outdoor temperature.

Your space thermostat is just a switch that says, "I need heat". OD Reset says, "how much/at what rate you will get it".

The specifics of your system are unknown to me. For example do you cycle the boiler and pump or run the pump only when the boiler is up to temperature? Lots of variables.

Keep in mind that OD reset if on a conventional cast iron boiler will have a low limit of about 140-145 degrees. Only when de-coupled Primary-Secondary can the radiation loop enjoy deeper reset temperatures. That is where the most energy is saved.

Ragu_5

Ah, Brad....

You serve words of wisdom, and I listen to your every word. Well put, my friend.

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Al Roethlisberger

Unfortunately I don't know enough about this to understand the Primary, Secondary, decoupled comments.... but this is a cast iron boiler.

The circulator pump only runs when the boiler is on. Is this best? Should it circulate at time without the boiler on?

I don't have any kind of bypass loop, so I have one ciculator, and the return and output are the only connections into and out of the boiler into ths sytem.

I am very interested in this Tekmar device though. Is this something one can "do it yourself" if typically handy with most construction, plumbing, and electrical?

Thanks
Al

Brad White_9

A little lifting..

Al, I thought it might get to this, not that I was holding back or anything. Here goes:

Your boiler is a block of iron that wants to remain warmer than the spent flue gasses (140F for gas, maybe 130F for oil). Let's call it 140 to be safe.

If you set your boiler operating temperature below that for extended periods the flue gasses will condense, form acidic deposits and eat the boiler. So one must spend energy to maintain temperatures higher than that. By this it means that your range of setting the boiler down when it gets warmer out is between say, 180 and 140. (Yawn.) Better than keeping it at 180 forever but you will be sending 140F water to your radiators despite their real needs at a given time.

Now, suppose your radiator were on their own circuit. They do not care about "too cool" water, so long as the water is warmer than the space in order to heat the space.

So picture this: Your boiler has a circuit, water out and back in a continuous circulation pattern. The radiators have a similar situation. Water from the boiler is injected into the radiator circuit at a rate designed to precisely maintain the radiator temperature relative to the outdoors. It can vary from 70 degrees (room temperature) up to 180F if you need it.

The boiler is maintained at a different scale, the 180 to 140 range because you have to. Oh well. But the radiator will "sip" this water and return and equal amount back to the boiler. The boiler water not sent out keeps the boiler warm.

This way you get a full range of radiator modulation, practically tepid on warm-ish days and reserved capacity for the cold ones. Done all the time. And you can too.