Technically Exotic
Very efficient house with 36" Rumford


I just read your piece [on Green Fireplaces] and while overall I like it and agree with the sentiments I think you have made some technical errors that, while minor, undermine your position.

"Mass inside the house complements the way fireplaces heat - radiantly. The masonry fireplace and chimney wall gets warm. The fireplace also heats up the other surfaces in the room - the walls, ceiling and floor - bringing up the "mean radiant temperature" so that people are more comfortable at cooler air temperatures and with good ventilation."

I don't think this is quite correct. Radiant heating and mass are not really interrelated at all. In fact a radiant system will perform identically regardless of its mass for any given set of conditions (surface temperatures).

What, as I'm sure you know, mass provides is a flywheel. So if the heat source varies the mass can smooth out the effect of the changing energy input. This is not always a good thing. In fact it can be a very bad thing when the load varies and you would like to change the input but can't due to the mass.

I think what you are trying to say is that mass can be a good thing in fireplaces because generally there is a desire to smooth out the energy flow to a more even tempered flow regardless of the instantaneous output of the fire. However I would not say that mass in the structure is necessarily good nor does the presence or absence of a fireplace change the decision as to whether one chooses to go high or low mass in a structure. That is driven by other issues which I won't discuss here.

In my house we have a high mass fireplace and a low mass structure. The reason is that we get wild swings in both temperature and solar gain on many days. The result can be a heating load in the morning and a cooling load in the afternoon. High mass can be a real bear under these conditions particularly where the house is heavily glazed.

"Mean radiant temperature - the average temperature of all the surfaces in a room"

This certainly isn't the proper definition nor do I believe it necessarily represents a good lay one either. I suggest you do some research and come up with better wording.

"When the air in the room isn't so warm and there is less differential between indoor and outside air temperature, the insulation works better and good ventilation doesn't cost so much."

Not true. The insulation works exactly the same way, its just there are less losses. If the temperature inside and outside were the same there wouldn't be any losses at all regardless of the amount of insulation. Again, I just think you need to word this better.

As for the comment about ventilation I don't think the matter is so simple. While it is true that if the air temperature can be kept cooler because of the radiant component that is contributing to comfort, I'm not sure there aren't other losses that you haven't accounted for. You might be right, but I think you should double check this.

Also it isn't clear that the amount you can drop the air temperature because of the radiant effect is material. I would venture that it is far smaller than just sealing the house against leaks so while it might be true it is misleading if you represent it to be the driving parameter when it is not.

"With lower indoor air temperatures, the home's insulation works more efficiently and there is less heat loss through the building envelope. Adequate ventilation is also less expensive in a radiantly heated space because the fresh outside make-up air doesn't have to be heated up as much."

Same words, same objection. The insulations efficiency is unchanged. The reduced heat loss is a result of a decrease in the temperature differential only.

Overall these are minor quibbles except that when there are factual errors it can lead an informed reader to question the validity of the entire piece. I think what is fair to say is that if a well designed Rumford exhibits efficiencies in the range a stove or glass fronted fireplace there is no reason to treat them as unacceptable.

Since the major concern of those unfamiliar with the details of Rumfords seems to be the amount of heat lost due to air up the flue I think you should address that concern with data. I encourage you to add a section with real numbers in cfm for various sizes of fireplaces (perhaps with a couple of burn rates ie roaring, middle and smoldering). Since the worst case is where all that air is coming from the room with no attempt to have make up air that is supplied locally (and therefore might be subject to less preheating and therefore less loss) I suggest you use that scenario to calculate losses due to exhaust air.

Since these losses only occur when the fireplace is operating and in most energy efficient homes the fireplace is an occasional indulgence rather than the primary means of heating I am sure the losses will be trivial compared to the leakage of even a very tight house like mine let alone a normal tight house. In fact even if the house were completely airtight with zero leakage the energy loss from the fireplace would be trivial compared to the energy loss in the best ERV. Since these run only about 70% (amount of energy recovered less the energy to run the fan) and they run more or less continuously it isn't even a contest.

I assume the point you are trying to make is that fireplaces are not fundamentally incompatible with an extremely tight house (because they pose no danger) nor do they materially increase energy consumption (at least as generally used ). The way to prove that is to put up actual numbers that people can see (and argue over) and to show that any losses are tiny compared to any number of other loss mechanisms of the structure. In fact it is all but certain that even in the best of structures any one of these, let alone in aggregate, these other loss mechanisms are many times the losses in the fireplace. And as a practical matter since it is all but certain that these other loss paths have not been fully minimized the situation is much worst.

So, in summary, if you want a fireplace don't feel guilty about it. If you want to reduce your energy use/impact upon the world there are things you can do that will have a large effect. But eliminating the fireplace isn't one of them.

My two cents worth.



On Aug 21, 2008, at 1:50 PM, Jim Buckley wrote:


Your "two cents worth", of course, is worth a lot more than two cents. Thank you.

Your point is well taken about being more careful with language and not mixing up thermal mass with radiant heat and the real way insulation works.

I will have to do some research on "mean radiant temperature". I thought I knew what it meant - that it was a simple concept - "the average temperature of all the surfaces in a room" - and I probably never double checked. Now I will.

The "real numbers" about air loss can fall out of upcoming testing. What we now have is a rule of thumb - one cubic foot per minute per square inch of flue area - based on very limited testing.

I'll see what I can do. And thanks again. Your perspective about what the problem is is especially helpful.

So are you going to write an article about your house?




The Mean Radiant Temperature of an environment is defined as that uniform temperature of an imaginary black enclosure which would result in the same heat loss by radiation from the person as the actual enclosure.

Wikipedia entry is here...

A more useful explanation can be found here...

The difference is three fold from yours. First it relates to energy, not temperature. Second it includes all the other characteristics of the room such as color and specific heat that might affect the flow of energy. Third, it accounts for the amount of each material. More precisely the angle subtended upon you by the surrounding material (which is why the MRT varies dramatically depending where you are in a room).

Just measuring the temperature of the walls and averaging won't work. You need to calculate the amount of energy that the room is going to absorb in each direction going out from the person and then the MRT is the temperature of an imaginary black box that would absorb the equivalent amount.

Just by example think of a large room with one wall of glass. If you stand way in one of the corners where the solid walls come together your body "see"s mostly solid wall. The angle subtended by the glass is small and the MRT will be higher compared to if you are right up near the glass and your body is experiencing the much higher losses to the windows (assuming winter).

Pretty simple actually.

As for your numbers for air, they are close enough. Think of a 2000 sf house, 8' ceilings (this is small for most energy efficient homes). 16,000 cu ft of enclosed space. Assume .25 ACH. Assume there is zero leakage anywhere in the house. None at all. You still have 4,000 cu ft/hr through the ERV. That is 66 cfm. Assuming you recover net of motor losses 70% that is the equivalent of 20 cfm of loss.

I should add in real losses for the window, doors etc. but that 20 cfm is every minute of every day. So assume Portland which is pretty mild. Average temp 50 F for half the year. Assume and indoor temp of 65 F. So over a year you are heating up 180 x 24 x60 x 20 or 5 million cu ft of air by 15 F.

Now do the fireplace. Wasn't it 10% of the face area for the flue? So 3' x4' is about 200 sq inches more or less (if I'm wrong you can correct this). Say 250. That yields 259 cfm when the fireplace is on.

Assume twice a week for 5 hours for half the year. 26 x 2 x 5 so 260 hours a year. or about 67,000 cfm per year for the fireplace.

It ain't even a contest. I was generous with the time the fireplace is on and way low on the leakage number for everything else and still (unless I made a mistake) the fireplace losses are less than 2% of the total losses of the house.

See if I'm right. If so then the exact number don't matter. The fireplace is irrelevant even if the house is 100% airtight.

And its still irrelevant if you do what I would do which is to run the ERV a lot less because .25 ACH while below Washington law is probably way more than people need on a continuous basis (because houses are so big in the US we can actually get enough air just off the leakage of the best house you can build except when you are cooking or having a party or something).

There is simply no case for saying a fireplace is a major source of energy loss because it just isn't used enough to be so.

There is a case to say that a fireplace can't be used in a tight home unless you provide the appropriate amount of make up air. However the exact same thing can be said for an exhaust fan. Those babies pull 1200 cfm so if someone is concerned about people backdrafting (and they should be) in tight homes they should be freaking out about the lack of make up air for hoods.

All it takes is one of those in a tight home, any sort of unsealed appliance and you are dead. At least if they screw up the make up air and there is a fireplace the soot and smell will come flying out the first time you turn up the fan on the hood, the owner will complain and maybe someone will realize what the problem is before he dies of CO poisoning.

Better the air come down down that 200 sq in hole in the house you call a flue that get pulled out of the firebox of some furnace or water heater.

Isn't that a hoot. The fireplace becomes the Canary in the mine against builders who forget about make up air. Its not incompatible, it's a potential lifesaver.



On Aug 21, 2008, at 6:47 PM, Jim Buckley wrote:


I love your logic. May I add your comments to the website - no name of course?



Sure. Just double check my math.

And once you have edited it, since it was just off the top of my head and isn't very concise, I should review it for accuracy.

Of course you are also welcome to extract any of the ideas and fold them into your own writing. I think that would work better. But up to you.




Kind of you, and I agree that making the article better and more accurate is better than inaccurate with your corrective comments.


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