Outdoor reset on fin baseboard system

RascalOrnery

In short, is there significant advantage to installing an outdoor reset mixing valve system on the heating system when not possible on the boiler?

For example, two story house, fin baseboard, two zones. Tankless coil for hot water. Boiler set for LL of 165. For mild winter days, 165 circulation temperature (or higher depending on HL settings) meets heat call quickly and boiler temp ends up higher than necessary.

So a mixing valve to the baseboard loops instead of an outdoor reset on the entire boiler and a removal/reduction of the HL setting - is there really much advantage to that much trouble?

Is this improvement a matter of comfort or matter of reduced fuel usage?

Jamie Hall

Before we decide what, actual benefits there might be from a mixing/tempering valve for the secondary loop, consider what the valve will enable. If the outdoor reset curve is set correctly, what will happen is that the secondary loop will flow continuously, or very nearly so.

Now unless the boiler can modulate, the boiler will still cycle on and off, controlled by its aquastat, to provide -- on the average -- just as much power as the secondary loop moves into the structure and hence what the structure demands. Perversely, the boiler will actually see a higher return temperature at low loads; if it is ever going to condense it will be at maximum load.

So as I see it the only real benefit is possibly increased comfort from the radiation being always on at just warm enough to keep the house warm... If there is an expansion noise problem, that would be reduced, too.

LRCCBJ

Before I would go to the expense of piping a mixing valve to allow outdoor reset for just the CH zones (while leaving the boiler at 165/185), I would install an insulated electric water heater. Continue to use the tankless coil to provide HW for the new water heater. Use an aquastat on the water heater to control a circulator through the coil. Set the aquastat to 135F and set the LL to 145F. Now you have reduced the cycling for HW and you have reduced the losses from the boiler jacket to the surrounding air. You also might be pleasantly surprised that the structure MAY be able to maintain temperature with a setting of 145/165. If not, just go down to the boiler and raise it slightly in the bitter cold. There is nothing magical about outdoor reset on a CI boiler if you don't mind making a trip to the basement a couple of times per year.

You may also find that, if you wire the HW heater, you can shut the boiler down for six months. The cost of using the HW heater is likely to be less than the cost to run the boiler for six months at LL. The boiler will likely consume $945. in oil for six months. Even with fairly high electric rates ($.30/kwh) the KWH consumed for 75g/day HW will be $705. YMMV depending on number of people in the building and how conservative they are with showers.

The side benefit of this approach is that you get a steady HW temperature at the fixtures. No cold water followed by a slug of 165F water followed by something around 125-130F depending on the condition of the coil. Coils are dangerous in many situations when there is no mixing valve utilized. If you have a mixing valve, you can eliminate it with this approach.

Ironman

Getting rid of the tankless setup which requires the boiler to be kept hot all the time would save far more on fuel than adding ODR to that system.

LRCCBJ

Ironman said:

Getting rid of the tankless setup which requires the boiler to be kept hot all the time would save far more on fuel than adding ODR to that system.

I did a quick calculation regarding the losses that a small CI boiler suffers to the basement with a tankless coil. The result is 7K BTU/HR to maintain 165F. 61M BTU/year. 548 Gallons of oil just to maintain temperature in the boiler!

NONE of it is "saved" in the building if the basement is unheated.

RascalOrnery

Thanks all for your input, very helpful to me.

LRCCBJ said:

Ironman said:

Getting rid of the tankless setup which requires the boiler to be kept hot all the time would save far more on fuel than adding ODR to that system.

I did a quick calculation regarding the losses that a small CI boiler suffers to the basement with a tankless coil. The result is 7K BTU/HR to maintain 165F. 61M BTU/year. 548 Gallons of oil just to maintain temperature in the boiler!

NONE of it is "saved" in the building if the basement is unheated.

This means that in a day 168,000 BTUs is 'lost' in the basement, this seems rather high to me? I can see on an uninsulated boiler but that's over a gallon of oil and at 140,000 BTU at 80% efficiency that pretty much means over a gallon, if not a gallon and half of oil just to maintain temperature, not even keep up with usage?? How did you calculate this? My house averaged about 1000 gallons one year when I asked the oil company, that would mean over half of it was standby...there's no way, because we get snow here. And if I lost more heat to my basement than to heating my house then I've discovered a magical thing...

Ironman

There’s a tremendous amount of standby heat lost up the chimney on a boiler with a tankless coil.

I’ll be replacing an old oil fired boiler with a Burnham NPO this week. Burnham claims an average savings of 49%; 20% of that is from adding the ODR module, the rest from getting rid of the tankless setup.

My Buderus rep has Guaranteed for years that a G115 with a Logimatic control would save at least 40% over an old tankless. I’ve put quite a few in and I’ve never had him proven wrong.

LRCCBJ

RascalOrnery said:

This means that in a day 168,000 BTUs is 'lost' in the basement, this seems rather high to me? I can see on an uninsulated boiler but that's over a gallon of oil and at 140,000 BTU at 80% efficiency that pretty much means over a gallon, if not a gallon and half of oil just to maintain temperature, not even keep up with usage?? How did you calculate this?

The heat transfer is governed by this formula:

q=U A dt

U is the constant for cast iron to air. There is some variability here but 5 is a reasonable number.

A is the area. A small boiler is 1.5 x 1.5' x 2.5' tall. Approx. 15 square feet.

dt is the temperature difference between the CI and the surrounding air. Figure 100 on a CI boiler that maintains an AWT of 150F.


The result is 7500 BTU/hr. 65M BTU/yr.

Think of it as a stovetop gas burner running constantly. You can generally feel the basement getting a bit warmer in most cases. Just about all of this energy goes out the basement walls.

The above example does NOT include the typical 1/2" of insulation on the inside of the jacket. Probably saves a bit as the air temperature between the insulation and the CI is higher than 50F. If you run the numbers at a DT of 70 you'll get 5250 BTU/h. 46M BTU/yr.

You really have to ask the question why Buderas is the only manufacturer putting 3" of insulation around their boilers. The answer is that most folks do not understand the massive losses from CI boilers and simply look at the combustion efficiency for comparison. The fact that Buderas effectively guarantees a 40% fuel saving over a CI with a tankless coil pretty much confirms the above.

RascalOrnery

Ironman said:

There’s a tremendous amount of standby heat lost up the chimney on a boiler with a tankless coil.

LRCCBJ said:

Just about all of this energy goes out the basement walls....

You really have to ask the question why Buderas is the only manufacturer putting 3" of insulation around their boilers.

I agree, and am quite confused as to why all boilers aren't insulated to the hilt, considering exactly what was mentioned and I think few people would argue against. But I was questioning the math.

However, I do have to wonder how a 50+ year old chimney life will be prolonged by being held dry by a little 'heat loss' from the boiler. Heat calls only every few hours or even once or twice a day assuredly allow for buildup of moisture which create other problems?

Not making an argument for the tankless situation but saying for the guy who can't afford the indirect or HWH there might be a small benefit from that 'heat loss'

Thanks for all the input

LRCCBJ

RascalOrnery said:

I agree, and am quite confused as to why all boilers aren't insulated to the hilt, considering exactly what was mentioned and I think few people would argue against. But I was questioning the math.

You are not the only one who is confused. There are respected individuals on this site who simply use the combustion efficiency as a measure of boiler performance. They completely ignore the system efficiency (which includes the losses from the boiler to the basement, the stack losses, the cycling losses, and the temperature losses).

It costs the manufacturer quite a bit more money to put 3" of insulation on the boiler. But, that effort does not affect the combustion efficiency numbers one whit.

If certain people on HH cannot understand the concept of system efficiency, how can any homeowner manage to do it?

How is it possible for a Buderus G115 to save 40% in fuel when the combustion efficiency is 84% and the combustion efficiency of the older, maintained CI is 81%? The homeowner only sees 3% in the literature!!!!!

pecmsg

Heat loss into the basement is still INSIDE the envelope. 

RascalOrnery

pecmsg said:

Heat loss into the basement is still INSIDE the envelope. 

Don't you agree that the basement walls and floor are practically a 'radiator to the earth'? I know people say all the time how a heated basement is a heated house, but that's only partially true, it's still making a large portion of it's way out through the walls, I mean...global warming at it's finest!

LRCCBJ said:

RascalOrnery said:

I agree, and am quite confused as to why all boilers aren't insulated to the hilt, considering exactly what was mentioned and I think few people would argue against. But I was questioning the math.

It costs the manufacturer quite a bit more money to put 3" of insulation on the boiler. But, that effort does not affect the combustion efficiency numbers one whit.

If certain people on HH cannot understand the concept of system efficiency, how can any homeowner manage to do it?

How is it possible for a Buderus G115 to save 40% in fuel when the combustion efficiency is 84% and the combustion efficiency of the older, maintained CI is 81%? The homeowner only sees 3% in the literature!!!!!

Pretty annoying how that works, quality is often looked at through a single lense. In this case, combustion efficiency. However when you introduce other lenses like, a car that is paid off is more efficient than one that's not, or longevity vs initial price...people just seem to take offense. Like getting the 'cheapest contractor' to do the work...you really think that's going to be a win every time? We get what we pay for don't we!!

hot_rod

Hot goes to cold always. The rate of exchange is based on temperature difference

The  cold basement will see some heat transfer from the heated space above.  Insulation will lessen that transfer

A 180 degree chunk of cast iron in the cold basement will transfer its temperature based on the surface area of the boiler, insulation around it, and the ambient air around it

Not to forget and combustion air grill open to the outside for boiler air🥴

LRCCBJ

pecmsg said:

Heat loss into the basement is still INSIDE the envelope. 

What is the temperature of the basement?
What is the temperature of the first floor?
What is the R value of the basement walls above the ground level and below the first floor?
What is the DT between the basement walls above the ground level and below the first floor?

Where would the energy lost from the boiler go?

LRCCBJ

hot_rod said:

Hot goes to cold always. The rate of exchange is based on temperature difference