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# Heat load - Its not a linear relation to outdoor temp is it ?

Member Posts: 239
If I try to calc my heat load using the BTU I've used , it seems a bit off.
I take my gallons used x BTU/gal x 0.81 (my boiler eff), I get a total BTU into the structure over a winter.
Then I find the HDD(65) over the same period to calc a BTU/HDD(65).
Divide that by 24 to get an avg per hour number.
Multiply that by the spread between the design temp (-21 in my case) and 65.
That should be the average BTU/h at design temp.
In my case I end up with 31k.
My attempts at structural calcs usually end up 65k or higher.

Clearly, it would be better to calc by fuel used and HDDs of the coldest period to get a better calculation. Im wondering when you only have full-season info, if there's a "k" factor that could be multiplied to the HDD numbers to allow for the accelerated loss due to the high delta temps ? For example, using 1.2x , an HDD=10 becomes 12, a HDD=30 becomes 36 and a HDD=75 becomes 90. Non-linear.
30+ yrs in telecom outside plant.
Currently in building maintenance.

• Member Posts: 19,135
The heat loss rate of the structure is actually remarkably close to being a linear function of the temperature difference between inside and outside. It's not quite exact, as wind speed is a factor (often not taken into account) in the calculation of the R values, and it is also a factor in infiltration. But it's not a bad first approximation.

I don't see anything fundamentally wrong with your sequence of operation -- so I'm not sure I can explain the discrepancy.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 5,717
edited April 23
you calculated average, the heat loss is close to worst case. at least i think i'm looking at your calculations right.

er. i'm not looking at it right, but i'm not quite following the units through your calculation.

the loss is very nearly a linear function of indoor and outdoor delta t
• Member Posts: 16,524
exactly what @Jamie Hall said. Which is why ODR can be such a great way to control systems. A curve adjustment of 1 is usually right on for many applications

Heat load calcs or predictions are not 100% spot on, too many variables. Most agree on a 10% or more fudge factor in the various load calcs

Perhaps the most accurate “best guess” is in the middle of your numbers😙

Starting on page 79 give you a good explanation of building loss and various control logics

https://www.caleffi.com/sites/default/files/file/idronics_12_na_2019.pdf
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 426
I am no expert, but I think the discrepancy is in your first calculation - boiler efficiency.
That number reflects how well the fuel is burned to create heat, basically the flame.
Combustion efficiency is not the same as AFUE (annual fuel usage efficiency).
The number you use needs to reflect all of the other losses.

You could have the most efficient boiler ever created, but if installed in an old drafty house, you will still burn a ton of oil.
• Member Posts: 984
The biggest variant is infiltration. This increases radically as the temperature difference between indoors and outdoors goes up. My old Mechanical Engineering Books show that for tight construction the infiltration rate is 0.4 Air Changes per Hour at 50F outdoor. At 0F outdoor it doubles to 0.8 Air Changes per Hour. Heat loss due to air infiltration is very much a non-linear number. When estimating heat losses, you really need to take an educated guess at your air leakage to see what impact it may have. I have found that when working off cold month fuel usage and factoring in the efficiency of the equipment at the average load for that month and then doing the math for the design day ( about 0F around here), that adding 15% to the number works well for typical older construction.
Tekmar Control long ago modified the linear outdoor reset curve of thier controls to a non-linear accelerated supply temperature increase at extreme temperatures. The other factor that comes into play is the "cold 70" effect when the walls and other exterior surfaces begin to pull a lot of heat off the surface of the body. The temperature in most older buildings needs to be set a bit higher in extreme cold to counteract this impact. Having radiant heat sources, like steam or hot water radiators, or radiant floor will probably help counteract this impact.

The acceleration of air leakage in extreme outdoor temperatures shows up all the time in the 3 story buildings we work with that have a single zone system heating multiple units. The building may heat very evenly in typical 20F days, but when it drops to zero, the first floor usually gets cold and the top floor overheats due to increased air entry at the bottom of the structure and then the movement of warm air up to the third floor.

Wind is the other impact, which the ASHRAE numbers assume is 15 mph outdoor wind in their design data.

I have been calculating hot water boiler sizing, and steam boilers where appropriate, from coldest month fuel usage for about 30 years and this methodolgy almost always ends up with a much smaller boiler than the standard loss calculations come up with. A big part of this is probably the impact of internal gains from people using the space, solar gains, thermal coupling by the foundations to the earth below ( especially in Masonry structures) that reduces the impact of extreme outdoor temperatures and, again in masonry structures, the thermal flywheel effect of delaying the impact on the interior load from the extreme nightly lows until the outdoor temperatures have increased in the daytime a day or 3 later.

• Member Posts: 5,724
What is your zip code and number of gallons you burned in the last 12 months?
steve
• Member Posts: 239
Steve - No Zips in Canada here, but Im about 30 mins north of 55605. 594 gallons since Oct 15 startup.

Steam Whisperer - Infiltration, I bet thats it ! Although it never trips any CO testing (even with a calibrated tester) and it's got good stack draft (gauged) , Im quite sure that the boiler has insufficient makeup air. It's sucking it in thru the structure. The boiler runs more when it's cold out, and creates a higher infiltration rate. I hadnt thought of that, it seems logical though.

30+ yrs in telecom outside plant.
Currently in building maintenance.
• Member Posts: 19,135
If you have enough data -- and it takes a lot, very granular -- you can approximate the overall effect of the linear loss and the infiltration with a second order polynomial (heat loss equals a contsant time delta T plus a constant time the square of delta T). Works quite well --but you pretty well have to have daily fuel use and temperature data to do it.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 13,056
edited April 23
I'm using an outdoor reset on a steam system and while it doesn't function the same as one for hot water, it did allow me to notice a few things.

Infiltration isn't linear. The colder it gets, the more infiltration a structure has. My house seems fairly linear until you get below 10 degrees F, then things go weird. Below 0F and infiltration is really a big issue for me.

The creator of the system added some settings that made it so I can change basically a multiplier below and above a set temperature. The upper one I have set to I think 40F or 50F where it actually tapers off from "normal". Below 10F or 12F I have it start to ramp up.

There's another issue I noticed and I'm betting it applies to most if not all systems but I have no way of knowing as I've only watched my own setup.

The behavior of the house isn't in sync with the outdoor temperature, there's a substantial delay but the outdoor reset doesn't have a delay. Meaning, the walls etc take time to cool down or warm up vs the outdoor temperature. So, as it cools off outside the heating system starts putting out more heat than it should and as it warms up outside the heating system produces less heat than it needs to. The air outside could have gone up 20 degrees but the mass of the house, walls, windows etc are still much cooler.

I've considered installing my outdoor temperature sensor inside a wall or have the thermistor connected to the clap board on the inside of the wall for this reason to see what would happen. I'm betting it would ultimately end up more accurate after some tweaking.

I wonder if just putting a Styrofoam sleeve around the temp sensor would improve it...
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
• Member Posts: 16,524
ChrisJ said:

I'm using an outdoor reset on a steam system and while it doesn't function the same as one for hot water, it did allow me to notice a few things.

Infiltration isn't linear. The colder it gets, the more infiltration a structure has. My house seems fairly linear until you get below 10 degrees F, then things go weird. Below 0F and infiltration is really a big issue for me.

The creator of the system added some settings that made it so I can change basically a multiplier below and above a set temperature. The upper one I have set to I think 40F or 50F where it actually tapers off from "normal". Below 10F or 12F I have it start to ramp up.

There's another issue I noticed and I'm betting it applies to most if not all systems but I have no way of knowing as I've only watched my own setup.

The behavior of the house isn't in sync with the outdoor temperature, there's a substantial delay but the outdoor reset doesn't have a delay. Meaning, the walls etc take time to cool down or warm up vs the outdoor temperature. So, as it cools off outside the heating system starts putting out more heat than it should and as it warms up outside the heating system produces less heat than it needs to. The air outside could have gone up 20 degrees but the mass of the house, walls, windows etc are still much cooler.

I've considered installing my outdoor temperature sensor inside a wall or have the thermistor connected to the clap board on the inside of the wall for this reason to see what would happen. I'm betting it would ultimately end up more accurate after some tweaking.

I wonder if just putting a Styrofoam sleeve around the temp sensor would improve it...

I think indoor feedback combined wit ODR will fine tune the system even more. Internal gains play a part, cooking, clothes dryer, lighting, computers, model railroads etc. So just because then outdoor temperature drops doesn't mean the system needs to quickly adjust SWT.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 13,056
edited April 23
hot_rod said:

ChrisJ said:

I'm using an outdoor reset on a steam system and while it doesn't function the same as one for hot water, it did allow me to notice a few things.

Infiltration isn't linear. The colder it gets, the more infiltration a structure has. My house seems fairly linear until you get below 10 degrees F, then things go weird. Below 0F and infiltration is really a big issue for me.

The creator of the system added some settings that made it so I can change basically a multiplier below and above a set temperature. The upper one I have set to I think 40F or 50F where it actually tapers off from "normal". Below 10F or 12F I have it start to ramp up.

There's another issue I noticed and I'm betting it applies to most if not all systems but I have no way of knowing as I've only watched my own setup.

The behavior of the house isn't in sync with the outdoor temperature, there's a substantial delay but the outdoor reset doesn't have a delay. Meaning, the walls etc take time to cool down or warm up vs the outdoor temperature. So, as it cools off outside the heating system starts putting out more heat than it should and as it warms up outside the heating system produces less heat than it needs to. The air outside could have gone up 20 degrees but the mass of the house, walls, windows etc are still much cooler.

I've considered installing my outdoor temperature sensor inside a wall or have the thermistor connected to the clap board on the inside of the wall for this reason to see what would happen. I'm betting it would ultimately end up more accurate after some tweaking.

I wonder if just putting a Styrofoam sleeve around the temp sensor would improve it...

I think indoor feedback combined wit ODR will fine tune the system even more. Internal gains play a part, cooking, clothes dryer, lighting, computers, model railroads etc. So just because then outdoor temperature drops doesn't mean the system needs to quickly adjust SWT.
My system actually had an indoor sensor as well but I ended up disabling it.
That may have been just the sensor it self having issues with being too sensitive, or the program not ignoring it I.E. averaging it out over a long period of time etc.

I find the system performs better in my case without it. But, I am using my thermostat in 2 stage heat where high stage bypasses the Ecosteam. This lets me do a larger recovery faster as without it, it'll literally take hours to raise 1 degree. Hours....and I'm not exaggerating.

The indoor sensor was setup to compensate for temp drop etc, so the Ecosteam would add extra time etc, but it just didn't work for me. I tried multiple locations, insulating the sensor from the wall etc and finally gave up.

That said.............the Ecosteam with just an outdoor sensor and a bimetal switch on the steam main by the main vents is probably the biggest improvement anyone could ever make to a residential steam system. I would say it literally makes a cast iron steam system invisible as far as feel. Over the years I've tried bypassing it and using the thermostat alone for a month or so at a time and every single time it seemed unbearable, even my wife complained that something seemed very different and wrong.

Now, in my case I'm using 5 TRV's to compensate for indoor things in the kitchen, livingroom, bedrooms etc. So those rooms simply don't get heat, or get less etc. There's many days the kitchen radiators stay cold all day long, and some times bedrooms get very little or no heat due to the sun.

ODR and TRVs............I wouldn't want to live without them.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
• Member Posts: 3,318
ChrisJ said:

I'm using an outdoor reset on a steam system and while it doesn't function the same as one for hot water, it did allow me to notice a few things...

I would love to know how ODR works on a steam system

Edward Young
Retired HVAC Contractor from So. Jersey.
Services first oil burner at age 16
P/T trainer for EH-CC.org
• Member Posts: 13,056
edited April 23

ChrisJ said:

I'm using an outdoor reset on a steam system and while it doesn't function the same as one for hot water, it did allow me to notice a few things...

I would love to know how ODR works on a steam system

It sets a run time based on outdoor temperature to compensate for the mass of the system and in my case, it interacts with a normal thermostat.

For example, thermostat fires the boiler, the Ecosteam waits until steam hits the main vents and then starts counting down based on outdoor temp. Once it feels enough heat has been produced it shuts the burner down and then there's a hold period where it won't let it fire again. Mine is set to 30 minute cycles, so if it heats for 10 minutes it holds for 20. During really cold weather I'll go to 20 minute cycles (3 CPH).

In normal use, the Ecosteam shuts the burner off and usually the thermostat becomes satisfied 5 to 10 minutes later. The result is far less overshoot, and since the house didn't heat as much it'll likely fire again sooner, so the deadband ends up very small. Instead of firing once every 3 hours like a typical thermostat on a steam system, mine will fire 6 times in 3 hours consistently.

This also helps TRVs work much better from what I've seen. It gives them a lot more control.

On windy days, the thermostat often never shuts off so the system runs completely by time and it's actually pretty interesting to watch.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
• Member Posts: 3,318
@ChrisJ , so more of a timer than a temperature thing. I like it!
Edward Young
Retired HVAC Contractor from So. Jersey.
Services first oil burner at age 16
P/T trainer for EH-CC.org
• Member Posts: 984

ChrisJ said:

I'm using an outdoor reset on a steam system and while it doesn't function the same as one for hot water, it did allow me to notice a few things...

I would love to know how ODR works on a steam system

The ODR setup I use is for steam pretty much the same as it is for hot water. The system has to be 2 pipe with supply valve orifices. The control monitors outdoor temperature and directly adjusts the boiler firing rate. In warm weather when the boiler is at its minimum firing rate, a thermostatic control will cycle the boiler on and off as needed. Once it gets cold enough (usually around 40F outdoor) the burner begins to move off minimum firing rate and the outdoor control begins to ramp up the boiler firing rate accordingly. From feedback of some living onsite building managers, the boiler will fire continuously for weeks at a time, just modulating up and down based on the outdoor temperature.
It's important that you set up the mod burner to respond linearly through most of its range or a little quicker than linearly when approaching high fire, so the control can operate the burner at the appropriate firing rates.
We've also found that some systems need a higher firing rate initially to "prime" the system on start up in warmer weather before you can drop to the ODR firing rate. An example of this type of system is one that has some mains fed off the top of the common header, while others are off the bottom. The steam naturally favors the stop supplied mains over the bottom supplied mains at start up. To deal with this we simply install and aquastat at the end of the slow main and have it trigger a manually adjustable "pot" to run the boiler at a fixed input above the minimum input to prime the system with steam. Once the aquastat is satisfied, control switches back to the ODR control.

We use this same concept when stage firing multiple smaller steam boilers. In this case we use a good multistage thermostat to call on the boilers as needed.
We also use the orifices on single rate boiler installations. The orifices allow the boiler to be downsized to heatloss and still heat evenly, improving comfort and boiler efficiency by providing long steady firing rates even in moderate weather.

These systems work beautifully and very economically, comfort is excellent and very little noise from the system ( expansion noises).