Is heat loss linear?

sunlight33

I sent my boiler max. input to 40%, it's 0F outside and the house is at a comfortable 70F, does this mean my boiler can handle temperature down to -70F?

KC_Jones

It's more a function of the amount of emitters you have.

So let's say you have a 100k btu boiler, but the emitters only output 40k btu, you have 60k btu of boiler that can't ever be used.

We would need to know how much emitter you have to make a judgement on what your system can do.

sunlight33

Gotcha, that makes perfect sense.

sunlight33

Without changing baseboards in my house, if I add a bunch of computer fans on top/bottom of my existing baseboards, would this be a good (if not the best) way to increase emitter capacity?

EBEBRATT-Ed

in theory the fans would increase the baseboard output

SuperJ

There are some European rads that have fan options to significantly increase output. But with baseboards there isn't enough btu per foot to warrant fans. Rads have significantly more BTU per foot which allows a fan to be more effective.

You need more BTU than your baseboard can provide, adding a panel rads in parallel or series to your baseboard might be a better way to go, if you're already optimized for water temperature and flow rates.

Solid_Fuel_Man

There are few cases that adding more emitter is detrimental. Only time is if adding in one part of a space unbalanced the original installation's lack of radiation on other parts of the same zone.

ChrisJ

Also, I don't believe losses via infiltration to be linear.

hot_rod

all things being equal a 1:1 reset curve would track. Wind, sun, occupancy gain and use are all transient conditions.

A control with indoor and outdoor input could bet match the load and energy input.

KC_Jones

Has anyone thought about the impact of wind and adding an input for that? I know it seems crazy and over the top, but I am seeing issues on my personal system that I feel could be resolved if the wind speed was being monitored and added to the calculations. Basically track the infiltration rate and factor in the winds influence into the reset curve.

hot_rod

Wind speed and wind direction would need to be considered, I feel. I suppose it comes down to how tight a number you want or need. Heat load is a quick snapshot in time, could change every second

djackman

KC_Jones said:

Has anyone thought about the impact of wind and adding an input for that? I know it seems crazy and over the top, .

Yes! Have thought exactly the same thing so if it's crazy you're not alone. For all the improvements we've done to the building envelope 20mph of northerly winds makes a big difference in heat loss.

Gordy

I think there is to many variables to develope some sort of reliable wind curve. Not only direction, and speed, but every envelope has Different infiltration characteristics. In the end we know wind speed creates more infiltration increasing the load. The big variable is how much.

ChrisJ

Gordy said:

I think there is to many variables to develope some sort of reliable wind curve. Not only direction, and speed, but every envelope has Different infiltration characteristics. In the end we know wind speed creates more infiltration increasing the load. The big variable is how much.

If that's the case, then an outdoor reset in general has too many variables to be considered.

Also, why does direction matter? The building the system is controlling is 3D, no? The boiler is heating all sides of it.

Jamie Hall

I think it depends a great deal on the building, @ChrisJ . The two big variables that ouotdoor reset doesn't take into account are sun load and wind, which really translates to infiltration. The question, for any specific building, is: how important are those two variables in relation to just plain conductive heat loss? If you take a really tight, well insulated building, with "normal" amounts of glass, I would expect those two variables would be a relatively small fraction of the straight conductive loss (and no, I'm not going to take the time to figure it out -- go for it!). On the other hand, if that otherwise similar building had a lot of glass, then solar could be quite significant. Or, if like Cedric's home, you have great wind exposure and an old building with a great deal of infiltration, then wind load could (and does, based on Cedric's home) have a major impact (my own studies suggest that Cedric could maintain an 80 degree differential in no wind conditions -- but struggle to maintain a 60 degree differential in a 20 mph wind).

ChrisJ

@Jamie Hall What do you know about it? Your system only knows "balls to the wall" from what I've heard.

pecmsg

ChrisJ said:

Gordy said:

I think there is to many variables to develope some sort of reliable wind curve. Not only direction, and speed, but every envelope has Different infiltration characteristics. In the end we know wind speed creates more infiltration increasing the load. The big variable is how much.

If that's the case, then an outdoor reset in general has too many variables to be considered.

Also, why does direction matter? The building the system is controlling is 3D, no? The boiler is heating all sides of it.

A outdoor reset is tracking the average recovery time along with much more, so on a windy night it will be slow to recover but still do it. Enough windy nights and it will be spot on now get a calm night and it will recover too fast but once again learn and adjust.

Gordy

Wind direction can play huge roles on infiltration.maybe one side has more windows, and or doors than another. Maybe natural wind breaks protect another direction. The control would need to have site, and structure sensitive inputs to be real useful. A simple wind speed, and direction device is not enough.

ODR is not perfect, but does an excellent job so long as wind, solar gains, setbacks do not interfere with its logic. But it’s basic logic supplies a given water temp needed to correct for the load. At a real time outdoor temperature.

ChrisJ

Gordy said:

Wind direction can play huge roles on infiltration.maybe one side has more windows, and or doors than another. Maybe natural wind breaks protect another direction. The control would need to have site, and structure sensitive inputs to be real useful. A simple wind speed, and direction device is not enough.



ODR is not perfect, but does an excellent job so long as wind, solar gains, setbacks do not interfere with its logic. But it’s basic logic supplies a given water temp needed to correct for the load. At a real time outdoor temperature.

While that's true, the boiler doesn't need to know which room is colder. That's up to whatever is controlling zoning etc, isn't it?

Gordy

If it’s zoned. Not all systems are zoned. Zoning can easily dictate which room, or zone is colder, and call for heat. That’s a simple command. It’s the water temperature to be decided by the ODR curve, which is decided by the outdoor temperature only. The boiler still needs to know what water temperature is needed to supply to the emitter.

A -10 degree day with no wind will require a very different SWT than a -10 day with a 30 mph wind. Even that -10 with a 30mph wind would be a different SWT if it’s out of the north with no wind break, and lots of glazing verses out of the West with no glazing, or very little, and a nice row of spruces blocking the wind.
My point is there is more unpredictable variables in the wind equation to determine a useful reset curve. Most of which is the envelope itself, and how tight it is. No two homes are exactly alike.

The wind logic would need inputs for various infiltration rates, at different wind speeds, even direction, and be coupled to the normal outdoor temperature method.

I’d have to say this is where indoor feed back would trump any type of wind reset alogorithum. “I’m still cold raise the water temperature”. I just don’t think there is demand for a wind based type of reset for what it’s worth, and the complexity of it.

hot_rod

Trying to predict Mother Nature is a fools errand. Weather experts have been one upped for generations, still are even with all the technology available.

The Steam Whisperer

Yes, infiltration is very much non linear, even when there is no wind load....just look at the ASHRAE estimated infiltration rates as the temperature goes down. A building that may have only 1/3 an air change per hour at 40F outdoor AT 0F that same building will have an infiltration rate 3 times higher. This is is why air tightening is so much more important than insulation. And finally energy saving programs are starting to focus on air leakage.

Leonard

Seems at no-wind, infiltration would be temperature difference based---- sounds like chimney effect ( bouyency of warm VS cold air)

When add wind, infiltration is also based on wind speed (stagnation pressure) blowing air thru building cracks. would guess you can superimpose (add) this effect to above effect. Stagnation pressure is proportional to wind speed squared...so non-linear

When you look at heat transfer thru the wall, you have natural convection warming inside wall, conduction thru the wall , and forced wind convection outside

Conduction thru walls.... heat trasnfer is linear with temp difference between each WALL SURFACE's temp (not air temp)

Wind thins boundary layer outside the house , so that increases the forced convectional cooling from outside.
Been a while ....but I believe this heat transfer term is non-linear with wind speed

---------------------------------------------------------

I lived in apt once, over years weather stripping on windows fell apart and had LOT of leaks. One cold windy night inside temps got cold , but finned hot water baseboards were HOT. So I cut up large peice of cardboard and set it up to direct air from a window box fan over the baseboad fins. Room warmed up fast. Was a LARGE apt complex so furnace had LOT of capacity. (my load was small compared to supply)

Gordy

There is always “stack effect” acting on the envelope. In the summer it’s “reverse stack effect”. Wind also creates low pressure areas on the leeward side.this can really have an adverse effect on outdoor air combustion kits for wood fireplaces.

Harvey Ramer

As has been stated, heat loss from a building is not strictly a function of the drybuld delta-t between the interior and exterior.

When you look at a chart showing an ODR curve, it does appear as a linear representation between those two temperature points. The chart should perhaps have a bar that covers a 15° swath rather than a line targeting a specific temperature.

I believe this is where indoor feedback and ODR should meet. The ODR curve being set at the (highest temp) upper end of the bar and indoor feedback refining the boiler output to actual conditions. Remember, the indoor conditions are always more important than the outdoor conditions, and far easier to measure.

If you have a single zone system, it is easy to refine the boiler output by lowering the boiler target temperature with a single temp sensor. Tekmar has controls that will do this and hopefully boiler manufacturers will start adding this capability to their controls.

In a multi-zone system, I feel the most economic way to do indoor feedback and control the boiler output, is by modulating the flow to the individual zones based on the zones' demand. This can be accomplished with TRV's or modulating zone valves.

Leonard

Don't know much about ODR.

Are ODR just a simple on/off temp switch, or are they more like a thermocouple that given temperature input to a controller?

Maybe a porportional/integral type control? To minimize room temp overshoot on warm days.

Jamie Hall

They are thermistors -- variable resistance depending on temperature. It sends a proportional signal to the controller, which in turn modulates the target temperature of the boiler.