Ratio of radiant to convective heat transfer

justinwebster

I'm currently trying to decide on a radiator vs blown air central heating system and it seems difficult to get real data. I hear a lot of emotive phrases like "radiators are a much nicer heat" and "it warms the things in your room, not just the air"... and usually the person says the word "science" once to back up their position.

But what I want to know is:
What percentage of the heat emitted by radiators is due to IR radiation vs convection?

I'm talking about heating an insulated home with modern water heated panel radiators vs modern blown air heating.

I had one radiator installer suggest theirs were only 10% radiation, and obviously much less if hidden behind a sofa. If that's true then any argument about "directly heating objects" is clearly just marketing babble which architects seem to have latched onto.

Any recommendations or links would be greatly appreciated.
Justin

HVACNUT

Is this new construction or major renovation?
As long as there is a proper heat loss calculation done, to me, it's s matter or preference and budget.
I prefer hydronic heat.
I currently have a hot water boiler with an indirect water heater, 3 fin tube zone, and radiant in the kitchen (my favorite. Warm tootsies).
I have CAC, A/H in the attic, with a steam humidifier. Central return in the hall, one in the master, and one in the basement. Also a MERV 11 air filter.
If you go FHA, definitely install a humidifier.
Another option is hydronic coil(s).
Cheaper than installing rads.
Or a high efficiency heat pump system with a FHA furnace and a 2 stage thermostat.
If I were to start from scratch, I'd do a mod con boiler, indirect water heater, radiant floor heat throughout, with a heat pump back up.

Jamie Hall

I honestly doubt that you will be able to find hard data, of the sort which I think you may be looking for, regarding the actual amount of heat -- in BTUh -- emitted by a radiator. It is significant, that I can assure you of, but only if you can see it. Most of the heat transfer from a radiator, or baseboard, will be by convection.

There are, however, other significant differences between forced hot air (blown) central heating and hot water (or steam) heating. Some of these have to do with comfort; most people who have experienced hot water or steam heating seem to prefer it to forced air from the comfort standpoint. For me, at least, there are three major comfort differences. First, no draughts. I dislike draughts in my rooms, and hot water or steam doesn't have them. Second, evenness. Forced air systems turn on and off at frequent intervals; now you have a hot draught, now you have nothing. Steam and hot water are nice and uniform heat. Third, noise. Steam and hot water are silent. Forced air isn't.

A second difference is space. While a baseboard system does take some space, and radiators are bits of furniture, the actual piping for either one is usually quite easily disposed of in the walls. The duct work for a forced air system usually takes a good deal of space. The forced air furnace is significantly larger than the boiler for hot water.

Forced air does have some advantages. It is easier to add humidity control (although it is also pretty much required), and it is easier to add air conditioning.

Then there is construction. In a new build, there isn't that much difference. Retrofitting forced air into an existing building can be remarkably difficult, unless it is single story.

There is yet another option, which is hot water radiant heat, usually in the floors but it can be in walls or ceilings. If the heat loads are low enough, this is an excellent option for a new build, and can be done as a retrofit although it is somewhat harder to do.

mikeg2015

Forced air get a bad rap in part because the same attention to detail that REQUIRED in hydronic systems gets ignored. It because popular because it's cheaper and required minimal skill to make it work OK.

A well installed forced air system heats very evenly, is silent, and has no drafts. My "rule of thumb" on all the homes I've owned and installed new systems on (too many... long story) is to downsize any AC and furnace. Suddenly the ductwork isn't undersized and you can add balancing dampers without penalty and dial it in. Mainly I'll look at utility bills and work backwards using CDD/HDD. Never had a system that ended up too small.

A properly sized 100k BTU furnace for example could easily heat a 4000sqft home in a northern climate. a 4 ton AC if it's well shaded and has a lot of mass, as in a old home, is adequate as well. Most contractors would install 2x80k and 2x 3 tons in that scenerio. Costs go up, comfort goes down, noise or ductwork size goes up.

Jamie Hall

@mikeg2015 's comments are quite correct -- particularly the one regarding design and installation. A really well designed and installed forced air system can indeed work very well. Unfortunately, perhaps even more than for hot water (but less than steam!) finding someone willing to do the design and installation correctly seems to be rather difficult -- and doesn't come cheap.

I might add two other benefits for forced air, though, while I'm playing devil's advocate,

One, it is rather easy to add whole house filtration to such a system. I am not referring to the ordinary filters found on any forced air system, but the real thing (always HEPA, often electrostatic; sometimes with UV or other additions as well). One must remember that whole house filtration will add to the installation cost and maintenance, but if there is a need for it for health reasons or just being in a horrible climate, it does have its points.

The other is that it is also relatively easy to add controlled outside air exchange with sensible heat exchangers to otherwise very tight envelopes.

I'll still take steam anytime, though!

jumper

Starting from scratch, panels mounted high to minimize convection. New construction,ceiling radiant. High radiant temperature means that one is comfortable at relatively low sensible temperature.

Harvey Ramer

A radiator follows a nonlinear curve between convection and radiation, based on the surface temp/water temp it is supplied with. A person can increase or decrease the radiation to convection ratio by sizing the rad to the right specs. Going with a thinner rad and more face surface area increases the radiation. Sizing rads to deliver the btu's at lower water temps lowers convection.

Steam humidifiers can be installed in a house in the absence of ductwork.

Hydronics are much easier to zone and create a more comfortable healthier environment.

Gordy

A Radiators output is 80% convection, and 20% radiant energy. However if you took that same radiator, and put it in the ceiling its output ends up being 70% radiant energy, and 30% convective.

Gordy

Obviously a forced air system is 0 radiant energy unless you want to count the hot diffuser after a cycle. Minimal.

When you are heating the objects in a room directly with radiant output there is more comfort with an elevated MRT "mean radiate temperature". All mass in a room is absorbing, and then giving up radiant energy to the space.

ChrisJ

80 20?
I seem to recall, at least steam radiators were 60 40, no?

Jamie Hall

Too many variables, folks, to give a single value. The radiant energy from something -- such as a radiator -- is proportional to the fourth power of the absolute surface temperature and to the emissivity. The convection will be more or less linearly proportional to the temperature difference between the lower parts of the object and the upper parts, but is also affected by the geometry both of the object and any nearby enclosures or obstacles.

Good luck coming up with a single percentage!

Gordy

Not sure about steam rads which have a higher operating temp than hot water. The ratio can vary wildly with supply temps which as Jamie said.

JUGHNE

Justin, is this new construction or remodel?

Gordy

http://www.engineeringtoolbox.com/heat-emission-radiators-d_272.html

Quite sure a formula exists here if you can nail down some values. Many formulas browse the topics.

HVACNUT

> @Jamie Hall said:
> Too many variables, folks, to give a single value. The radiant energy from something -- such as a radiator -- is proportional to the fourth power of the absolute surface temperature and to the emissivity. The convection will be more or less linearly proportional to the temperature difference between the lower parts of the object and the upper parts, but is also affected by the geometry both of the object and any nearby enclosures or obstacles.
>
> Good luck coming up with a single percentage!
>
>
>> You just blew my mind!
Is that what the scarecrow said when he got a brain?

Larry Weingarten

Hello, We shall now refer to Jamie Hall as, Professor Sir!! That proportional fourth power stuff has my head needing it's bolts tightened, so it doesn't fly off!

Yours, Larry

justinwebster

Thanks for the many detailed answers.
I'm surprised though - even here people are stating that radiators provide more "comfort" - what does this mean?
If 80% - 90% of the heat is due to convection then where does this extra "comfort" come from compared to ducted air?

And I think (correct me if I'm wrong) the arguments about ducted being ON/OFF blasts, being noisy, drafty etc are resolved in modern systems.

This is a renovation, but a very invasive one, with new structures added - so we have access to every wall/floor and there will be lots of insulation in walls/ceiling. Single story timber house. Temp extremes here range from 5C to 32C. Cold air-con not required.

delta T

Here is my take on the comfort issue. Whether the convective output of a radiator is 20% or 80% of the whole, the main difference as compared to FA is the velocity of the air. If you use a fan to push air through a duct, then out a through some kind of diffuser, it has much more turbulence as it enters the room than the convective air currents set up from a radiator or baseboard. The less turbulent the air the less it will feel 'drafty' and in my experience at least, the less it will tend to stratify and set up the hot head, cold feet situation of FA. The other side is that whether the radiant output of a radiator is 20% or 80%, its significantly more than 0% (FA).

Even a little radiant heat makes a room more comfortable in my opinion!

justinwebster

yeah but if the real world value is 10% it's not significant:
by the time you've got the air in the room to a comfortable temp (via the 90% convection) you've done very little radiant heating of the few objects within range of a radiator (remember the inverse square law!)

Seems to me like forced air is going to result in a much more consistent distribution and do it much faster.

And stratification should be greatly reduced if the air is being circulated right? It's like stirring cold milk into hot coffee.

Jamie Hall

justinwebster said:

yeah but if the real world value is 10% it's not significant:
by the time you've got the air in the room to a comfortable temp (via the 90% convection) you've done very little radiant heating of the few objects within range of a radiator (remember the inverse square law!)

Seems to me like forced air is going to result in a much more consistent distribution and do it much faster.

And stratification should be greatly reduced if the air is being circulated right? It's like stirring cold milk into hot coffee.

Well now... first of all, the radiant to which we refer so lovingly isn't from the objects in range of the radiator. It's from the radiator itself (hence the name), and unless you have experienced it, you can't possibly realise what it means. Note that baseboard heat does not do this. Has to be a radiator.

Second, it is quite true that forced air is faster. This is why it is possible to run bigger setbacks with forced air, for one thing.

Third, stratification could be reduced -- if there is one of three things: sufficient air velocity in jets to force the air in the room to circulate, or overhead fans to push warm air down to the floor, or really brilliant location and sizing of feed and return registers. The first two of these options are not, usually, tolerable in most houses -- the first, because of the resulting draughts; the second, because there simply isn't enough ceiling height to make the fans really effective (they can work wonders in spaces with more than a story and a half -- cathedral ceiling "great rooms" for instance). The third option is quite viable -- and almost never found in residences, as it takes a good deal more thought and skill in design and installation that in is usually present.

Last, I might point out that a thorough understanding of the advantages, limitations, and disadvantages of the various ways of providing space heating is absolutely essential to recommending, never mind designing and installing, the optimum system for a give space and usage. There is no such thing as a one size fits all solution, and I don't think that any of the professionals would say otherwise.

Gordy

MRT (mean radiant emperature) provides more significant comfort than MAT (mean air temperature). If the air is warm, and objects are cold guess what wins. hot goes to cold. Mother natures battle is achieving balance.

jumper

Here's some real world experience. In the seventies some single family home subdivisions were all "all electric" . Ductwork served both A/C and electric furnace. Some people installed heat pump after A/C wore out. Then A/C was over sized.

When it was major renovation time I know an engineer who installed ceiling electric radiant. He downsized A/C including lower CFM blower. Cooling was more consistent.

Winter time comfort improved while energy consumption decreased. So I believe that radiant heating is superior to convection heating because we enjoy being warmed at lower sensible temperature.

Ecorad

I have to precise that the 80% convection 20% radiation is for cast iron radiator (hot water), it is not applicable for " modern water heated panel"; that are not made of cast iron

jumper

Ecorad said:

I have to precise that the 80% convection 20% radiation is for cast iron radiator (hot water), it is not applicable for " modern water heated panel"; that are not made of cast iron

Modern hydronic panels are much lighter than cast iron radiators. So with steam heat one can mount them high and thereby reduce convection.

ChrisJ

When it's below 0 here and my radiators are 3/4+ full you feel them, a lot. And I don't mean the air coming off of them.

When I walk into the bathroom at night it feels like the sun shining on you, it's HOT from across the room.

I find it hard to believe, no, in fact I don't believe that I'm only feeling 10% of it's output, that's insane.




40% I could believe but honestly, even that number seems small on those super cold nights.

EBEBRATT-Ed

This doesn't apply to most residential construction but old, leaky drafty building especially those with high mass construction (brick, stone and concrete) cannot be heated sucsesfully with warm air.

an old brick building with steam heat in Hartford, CT was gutted, turned into offices. they installed gas fired roof top units.

The results were awful. The large mass of cold brick made the space uncomfortable although the air temp was above 70 degrees.
Couldn't heat it without hot water or steam perimeter heat.

With normal construction in a house I agree with @mikeg2015 that warm air can work well if installed properly

Gordy

^depends on the residential construction. Non radiant slab construction types are brutal in the winter. Especially if you have lived with radiant.

DanInNaperville

It varies from spot to spot and moment to moment, within the room. But you can make some guesses based on your own experience.
Think back to a very cold, sunny day with zero wind. It doesn't "feel" cold at first. The air near your skin warms up almost immediately and forms a boundary layer. That air will drive a slow convection current, but much of the heat loss will still be due to radiation, not conduction, unless/until the wind picks up - the heat radiated by the sun may come close to balancing the heat lost through conduction to the air.
If it's a bright, sunny day, and there really isn't any wind, you may not feel cold for quite a while, even when it's really cold out. If nothing's blowing the air around, a good part of the heat is transferred through radiation. (If it's night time, you'll get cold pretty quickly).
Last fall we moved from a house with forced air heat to a house with a mix of baseboard and in-floor radiant heat and I am stunned at how much more comfortable the baseboard/radiant heat is. Most impressive in the rooms with radiant heat, but the lack of a breeze that starts cold, becomes warm, then returns to being cold in an endless cycle is very nice.
There is a sun room with radiant floor heat that stays comfortable even on cold days and with the temp set lower than I would have thought it could be (while remaining very comfortable).
It's a lot more expensive to install, and more complex to maintain, but there is a significant comfort benefit. You'll also probably save a few dollars in fuel costs, but compared to adding a gas burner to the air handler and ducts you needed for A/C, regardless, hot water heat is far more expensive. OTOH, a cardboard box under a viaduct would be even cheaper and easier, but lots less comfortable - it's a matter of where you want to spend your money. Forced air heat may be the perfect balance for what you're trying to do.