The Cruise Control / Car Analogy

Sean_17

With the cruise control analogy, I think of heat loss as being a variable akin to the hills that you encounter while driving in cruise control. The speed you set cruise control to maintain is like your thermostat setpoint temperature. When you come upon a hill with a steep grade, as the car decelerates, cruise control inputs more throttle to maintain speed, kind of like how a modulating boiler with outdoor reset responds to increased heat loss on a colder day by inputting more heat energy into the equation. Then when you get over the crest of the hill, cruise control "sees" the acceleration and lets off on the throttle. Similarly, the modulating boiler lowers temp on a warmer day. That make sense?

Best regards,

Sean

gasfolk

This could really clear up my confusion.

So, is "heat loss" more like miles-per-gallon, zero-to-60, towing capacity, range-per-tank, or what?

Do Manual-J numbers reflect some combination of MPG, acceleration, or what?

Thanks.

gf

Kevin O. Pulver

another view

Consider heat loss this way: You are trying to fill a barrel with water. As the barrel fills, you notice various little holes spurting water from all over the barrel. The higher the water level, the more holes spurting water and eventually you can't get the level any higher. And the colder it is, the faster the leaks (due to the physics of delta T).
So you either: A: get a bigger hose to overcome the leaking holes. Or B: start plugging holes and use the same hose you've been using.
Remember though, that every structure will have a minumum heat loss even if you,"plug all the holes". Water will "seep" if you will, through your bucket even though there aren't any visible leaks. For that reason, you must do a heat loss analysis, and figure out how big a hose you will need before you start. Some people think any size heat source will eventually "fill the building" but it won't if it's not big enough to overcome the seeping and the holes. Kevin

Unknown

BTUs are "miles".

Your efficiency at "covering those miles" (providing BTUs, driving mileage) is measured by fuel usage in both cases.

So you end up measuring miles per gallon... or heating fuel usage.

Your loss per degree day or whole-house U-factor would be one factor in that efficiency... the envelope efficiency. This is like the weight of the car, aerodynamics... the basic factors that govern the boundaries of your fuel usage. Nothing we do outside of this will change it.

Then what you're left with is combustion and operational efficiency. This is either how well you draw power from combustion (in a car) or heat from it (for heating). Also, how efficiently that power or heat is utilized... such as the "stomp the gas" vs "cruise control" methods of driving

Sean_17

Kevin,

That's a great analogy. I think when I was talking about "hilly terrain" as it applies to the cruise control analogy, I was referring to the rate of heat loss of the envelope that varies depending on outside conditions. But you've also included the inherent heat loss in a structure - the number and size of the holes in that barrel.

Best regards,

Sean

Ron Schroeder

Hi GF,

Unless it is modulating, a car is not a good analogy for a boiler. The longer a burner runs EACH TIME THAT IT RUNS, the closer it is to it's steady state efficiency. I have seen short cycling reduce a boiler's efficiency to less than 1/3 of it's steady state or AFUE.

Ron

Tom_35

Kevin---really good analogy

I will remember that analogy---as good as any I've ever heard.

Tom Atchley

gasfolk_2

Translation?

Thanks for your good comments. I have more familiarity with cars, and since you folks speak boilerese, could you further indulge the car analogy translator? There is lots wrong below, so how would you correct the following?:

House = Car
Boiler = Engine
Weather = Terrain
Gas Valve = Carbeurator
Modulation = Transmission & Clutch
Water or Steam = Drivetrain
Condensing = Exhaust Manifold Heat Recovery
Radiation = Tires
Btus = Miles
MBH = MPH
MPG = AFUE
Acceleration = Oversizing (OK, not necessarily a bad thing)
Towing Capacity = Oversizing
Clutch Slippage = Inverse of Efficiency
Cold Engine Performance = Part-Load (In)Efficiency

gasfolk_2

Translation?

Thanks for your good comments. You folks speak boilerese, and since I have more familiarity with cars, could further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and corrections:

House = Car
Boiler = Engine
Weather = Terrain
Gas Valve = Carbeurator
Modulation = Transmission & Clutch
Water or Steam = Drivetrain
Condensing = Exhaust Manifold Heat Recovery
Radiation = Tires
Btus = Miles
MBH = MPH
MPG = AFUE
Acceleration = Oversizing (OK, not necessarily a bad thing)
Towing Capacity = Oversizing
Clutch Slippage = Inverse of Efficiency
Cold Engine Performance = Part-Load (In)Efficiency

gasfolk_2

Translation?

Thanks for your good comments. You folks speak boilerese, and since I have more familiarity with cars, could further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and corrections:

House = Car
Boiler = Engine
Weather = Terrain
Gas Valve = Carbeurator
Modulation = Transmission & Clutch
Water or Steam = Drivetrain
Condensing = Exhaust Manifold Heat Recovery
Radiation = Tires
Btus = Miles
MBH = MPH
MPG = AFUE
Acceleration = Oversizing (OK, not necessarily a bad thing)
Towing Capacity = Oversizing
Clutch Slippage = Inverse of Efficiency
Cold Engine Performance = Part-Load (In)Efficiency

gasfolk_2

Translation?

Thanks for your good comments. You folks speak boilerese, and since I have more familiarity with cars, could you further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and clarifications:

House = Car
Boiler = Engine
Weather = Terrain
Gas Valve = Carbeurator
Modulation = Transmission & Clutch
Water or Steam = Drivetrain
Condensing = Exhaust Manifold Heat Recovery
Radiation = Tires
Btus = Miles
MBH = MPH
MPG = AFUE
Acceleration = Oversizing (OK, not necessarily a bad thing)
Towing Capacity = Oversizing
Clutch Slippage = Inverse of Efficiency
Cold Engine Performance = Part-Load (In)Efficiency

gasfolk_2

Translation?

Thanks for your good comments. You folks speak boilerese, and since I have more familiarity with cars, could you further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and clarifications:

House = Car

Boiler = Engine

Weather = Terrain

Gas Valve = Carbeurator

Modulation = Transmission & Clutch

Water or Steam = Drivetrain

Condensing = Exhaust Manifold Heat Recovery

Radiation = Tires

Btus = Miles

MBH = MPH

MPG = AFUE

Acceleration = Oversizing (so, not necessarily a bad thing?)

Towing Capacity = Oversizing

Clutch Slippage = Inverse of Efficiency

Cold Engine Performance = Part-Load (In)Efficiency

gf

gasfolk_2

Translation?

Thanks for your good comments. Since you folks speak boilerese (and since I have a bit more familiarity with cars), could you further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and clarifications:

House = Car

Boiler = Engine

Weather = Terrain

Gas Valve = Carburetor

? = Fuel Injection

Modulation = Transmission & Clutch

Water or Steam = Drivetrain

Condensing = Exhaust Manifold Heat Recovery

Radiation = Tires

Btus = Miles

MBH = MPH

MPG = AFUE

Acceleration = Oversizing (so, not necessarily a bad thing?)

Towing Capacity = Oversizing

Clutch Slippage = Inverse of Efficiency

Cold Engine Performance = Part-Load (In)Efficiency

gf

gasfolk_2

Translation?

Thanks for your good comments. Since you folks speak boilerese (and since I have a bit more familiarity with cars), could you further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and clarifications:

House = Car

Boiler = Engine

Weather = Terrain

Gas Valve = Carburetor

(? = Fuel Injection)

Modulation = Transmission & Clutch

Water or Steam = Drivetrain

Condensing = Exhaust Manifold Heat Recovery

Radiation = Tires

Btus = Miles

MBH = MPH

MPG = AFUE

Acceleration = Oversizing (so, not necessarily a bad thing?)

Towing Capacity = Oversizing

Clutch Slippage = Inverse of Efficiency

Cold Engine Performance = Part-Load (In)Efficiency

gf

gasfolk_2

Translation?

Thanks for your good comments. Since you folks speak boilerese (and since I have a bit more familiarity with cars), could you further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and clarifications:

House = Car

Boiler = Engine

Weather = Terrain

Gas Valve = Carburetor

(? = Fuel Injection)

Modulation = Transmission & Clutch

Water or Steam = Drivetrain

Condensing = Exhaust Manifold Heat Recovery

Radiation = Tires

Btus = Miles

MBH = MPH

MPG = AFUE

Acceleration = Oversizing (so, not necessarily a bad thing?)

Towing Capacity = Oversizing

Clutch Slippage = Inverse of Efficiency

Cold Engine Performance = Part-Load (In)Efficiency

Thanks,

gf

gasfolk_2

Translation?

Thanks for your good comments. Since you folks speak boilerese (and since I have a bit more familiarity with cars), could you further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and clarifications:

House = Car

Boiler = Engine

Weather = Terrain

Gas Valve = Carburetor

(? = Fuel Injection)

Modulation = Transmission & Clutch

Water or Steam = Drivetrain

Condensing = Exhaust Manifold Heat Recovery

Radiation = Tires

Btus = Miles

Btu/hour = Miles/hour

AFUE = Miles/gallon

Oversizing = Acceleration (so, not necessarily a bad thing?)

Oversizing = Towing Capacity

Efficiency = Inverse of Clutch Slippage

Part-Load (In)Efficiency = Cold Engine Performance

???

Thanks,

gf

gasfolk_2

Translation?

Thanks for your good comments. Since you folks speak boilerese (and since I have a bit more familiarity with cars), could you further indulge the car analogy for a time? Probably there is lots wrong below, but I hope to learn from your criticisms and clarifications:

House = Car

(Comfort = ?)

Boiler = Engine

Weather = Terrain

Gas Valve = Carburetor

(? = Fuel Injection)

Modulation = Transmission & Clutch

Water or Steam = Drivetrain

Condensing = Exhaust Manifold Heat Recovery

Radiation = Tires

Btus = Miles

Btu/hour = Miles/hour

AFUE = Miles/gallon

Oversizing = Acceleration (so, not necessarily a bad thing?)

Oversizing = Towing Capacity

Efficiency = Inverse of Clutch Slippage

Part-Load (In)Efficiency = Cold Engine Performance

???

Thanks,

gf

Tom R.

Car v. Boiler

Compare your house to a hybrid (gas/electric) vehicle. View the total heat loss from the house as the variants in driving: wind resistance, rolling resistance, friction in moving parts. Look at these as the outside temperature, condition and amount of insulation, air infiltration. Add to that the attitude of the vehicle (terrain) and the speed you require. You can smooth the airflow, use better lubricants, and drive on a flat surface at 30 MPH and get great gas mileage with a very small engine. However, you will not be able to climb hills, accellerate except very slowly, and it will take a long time to reach your destination. Unless you have a large bank of storage batteries (in heating, compare to a buffer tank) you are limited. So you compromise with a larger engine that is not as efficient, but allows for times when you call for more power. The overcapacity of the heating system is needed to overcome the lowest OA temperature expected and how fast you want to recover from a setback temp. Not an exact comparison, but you can get the idea.

gasfolk_2

OK, that is very clear. Thanks.

Now, can you measure a functioning system to determine the smallest "engine" that will still perform?

For example, if I let my house equalize at a steady 60 deg F and then measure how much fuel and time are needed during recovery to 70 deg F, could those measurements (BTUs and time from 60-70), or others measurements, help to size a replacement boiler?

If one never uses setback (and if boiler failure is rare), could one accept a somewhat slower recovery from idle in exchange for much better efficiency at the *measured* steady-state design load? Is there a rational way to plan reserve capacity from *actual measurements* of system response?

Most folks don't use software to calculate their Miles-Per_Gallon and acceleration. Besides estimates from Manual-J, are there "automotive-type" measurements of my existing boiler that could help size the replacement?

Thanks,

gf

gasfolk_2

OK, that is very clear. Thanks.

Now, can you use *actual measurements* of an existing system to determine the smallest replacement "engine" that will still perform?

For example, if I let my house equalize at a steady 60 deg F and then measure how much fuel and time are needed during recovery to 70 deg F, could those measurements (BTUs and time from 60-70), or others measurements, help to size a replacement boiler? If one never uses setback (and if boiler failure is rare), could one accept a somewhat slower recovery from idle in exchange for much better efficiency at the *measured* steady-state design load?

Most folks don't use software to calculate their Miles-Per_Gallon and acceleration. Besides estimates from Manual-J, are there "automotive-type" measurements of my existing boiler that could help size the replacement?

Thanks,

gf

gasfolk_2

OK, that is very clear. Thanks.

Now, can you recommend *actual measurements* of an existing system to determine the smallest replacement "engine" that will still perform?

For example, if I let my house equalize at a steady 60 deg F and then measure how much fuel and time are needed during recovery to 70 deg F, could those measurements (BTUs and time from 60-70), or others measurements, help to size a replacement boiler? If one never uses setback (and if boiler failure is rare), could one accept a somewhat slower recovery from idle in exchange for much better efficiency at the *measured* steady-state design load?

Most folks don't use software to calculate their Miles-Per_Gallon and acceleration. Besides estimates from Manual-J, are there "automotive-type" measurements of my existing boiler that could help size the replacement?

Thanks,

gf

gasfolk_2

OK, that is very clear. Thanks.

Now, can you recommend *actual measurements* of an existing system to determine the smallest replacement "engine" that will still perform?

For example, if I let my house equalize at a steady 60 deg F and then measure how much fuel and time are needed during recovery to 70 deg F, could those measurements (BTUs and time from 60-70), or others measurements, help to size a replacement boiler? If one never uses setback (and if boiler failure is rare), could one accept a somewhat slower recovery from idle in exchange for much better efficiency at the *measured* steady-state design load?

Most folks don't use software to calculate their Miles-Per-Gallon and acceleration. Besides estimates from Manual-J, are there "automotive-type" measurements of my existing boiler that could help size the replacement?

Thanks,

gf

Ron Schroeder

Hi gf,

Knowing the recovery time won't help you calculate boiler size needed but you can estamate from knowing what your output rate and percentage of run time on a design day but there will be inaccuracies due to partial load factors of the existing oversized boiler. You can also make rough estamates on any cold day by scaling to the ratio of Delta T between inside and outside temperatures to the ratio of the delta T between the inside temperature and the design temperature.

For example, if your boiler has a rated output of 100,000 btu and it runs 50% of the time on a design day, then a 50,000 btu output boiler could replace it. In actuality, an even smaller one could do it since the output of the 100.000 btu boiler is really less than 100,000 btu when it is running at less than steady state.

Tom R.

Boiler sizing

You really have to use standard heat loss calculation sheets - measurements and conditions of the structure, and design temperature - to find the actual BTU requirement for any given building. Using the existing boiler as a reference is probably the last method you want to use. In all likelyhood, when you reach the correct BTU's for your circumstance, you will almost never find a boiler rated exactly to match. In that case, you will settle for the next larger size. And that brand new boiler will never again be as good as it is the day it is installed, so you have to allow something for deterioration. Assuming that you are going to need some professional help to install, or at least set up the boiler, why not let the Pro size it from day one?

jerry scharf_3

couldn't resist taking a stab at this

I was trained as a phyicist, so humor me as I go. Try to go along with this, as it shows where analogies go.

First, heat is energy. Joules, BTUs, gallons of fuel, etc. Heat flow is power, energy per time. Watts, BTUH, gallons per minute.

So to take the car analogy, you burn fuel with hopes of going somewhere. All the energy you release by burning the fuel goes somewhere. Some of it is lost out the exhaust, some of it heats the tires/road/air via friction. Some of it is resisting gravity when you go up a hill. Some of it goes into the brakes when you slow down. Virtually all of it is taken away rather than being stored in you as you move. the faster the car moves, the faster the energy is used and discipated. MPG is not a physics measure, it's an overall system estimate of energy required to accomplish something useful.

Fortunately, heating is much simpler, because the goal is the energy, so things work more simply. The temperature in a room is a reflection of the stored energy. When you have a difference in temperature, energy flows from the warmer to the cooler, power. So the heat loss is the power of the enegy flow from hot to cold. To keep the temperature the same, you need to add exactly the same power as is flowing out from loss. rememeber, we're looking at the room and not he boiler right now.

But it's not quite like a car, we have the water acting as a secondary energy storage system. It would be a bit like adding a flywheel to the car. There is a switch on the dash to a clutch between the flywheel and the wheels. There is an automatic system (black box) that turns the engine on and off to keep the flywheel up to speed.

So now let's start building the analogy. The switch is the same as a zone valve or circulator turning on and off to supply heat to a zone. The black box is like the boiler/zone control panel. Its not going to be a smooth ride, and you're not going to run the car near it's efficeincy point.

Time for better controls. One thing to do is to adjust the flywheel speed to approximate the load. You still have to turn the clutch on and off, but it will happen less often and be much smoother. This is what outdoor reset does to the room. Because the flywheel is slowed down, the efficiency of the freewheel and wheels increase as well.

But we want to improve the engine side as well. The black box started the engine each time the flywheel got to slow, and burned max power to spin it back up to a max setting. So we replace the black box with another that acts like a governor to adjust the fuel to keep the flywheel spinning at the desired speed. Since the enginer runs in a more efficient way and doesn't turn on and off, it is much more efficient. This is burner modulation.

The final change is going to a variable transmission between the flywheel and the wheels. This makes things even smoother and can further reduce the demand on the flywheel. This is adding TRVs or other proportional valve technology.

worth what you paid,

jerry

Cosmo_3

Overanalyzation!!!!

My simple analogy was to try to explain in as simple a way as possible, the difference between a non-modulating, non-outdoor reset controlled boiler, and a fully modulating burner with an outdoor reset control. Of course technically the outdoor reset is not technically required for analogies sake as a simple thermostat is the simplest of outdoor reset controls... as it gets colder outside the thermostat calls for heat more.

Please do NOT over analyze, and of course it does not apply to a boiler equipped w/ a non-modulating burner.

Kevin's barrel analogy is a good one. Thanks Kevin

However in my honest opinion, anyone who wants to really learn how a boiler operates should learn more about how different boiler controls and burners work and then analogies are not needed. The problem w/ using an analogy sometimes is that the layperson mistakenly may think that a simple analogy describing a specific boiler/burner combination's operation is a blanket statement that can be used for ALL boiler systems, this is simply not true and I apologize if I confused anyone.


Cosmo Valavanis

Dependable P.H.C. Inc.

gasfolk

Excellent. Thank you.

Could you indulge a few followup questions?

1) Without creating a mini research lab, could a homeowner measure a few simple parameters and get a reasonable number for boiler replacement (or other system modification such as potential efficiency benefit from control retrofit)?

2) Blower Door Testing shows air-leakage, so couldn't "Home Performance" contractors use dataloggers to determine the thermal time constant of a house? A ballpark empirical heat loss?

3) During a cold snap, the local weatherman could encourage homeowners to "Time your boiler tonight," allowing homeowners to collect data perhaps useful to their heating contractor?

Possible?

Thanks,

gf

gasfolk

Excellent. Thank you.

A few followup questions?

1) Without creating a mini research lab, could a homeowner measure a few simple "automotive-type" parameters from this model and get a ballpark number for boiler replacement (or other system modification such as potential efficiency benefit from control retrofit)?

2) Blower Door Testing shows air-leakage, so couldn't "Home Performance" contractors use dataloggers to determine the thermal time constant of a house? A ballpark empirical heat loss?

3) During a cold snap, perhpas the local weather channel could encourage homeowners to "Time your boiler tonight," allowing homeowners to collect data perhaps useful to themselves and their heating contractor? "If your boiler runs less than X minutes, you may need a heating tuneup (or replacement or whatever)." Could quickly identify the community's least efficient systems.

Possible?

Thanks,

gf