Insulating pipes

[Astro]

I've been insulating hot water pipes in my boiler room and wanted to find what the approved method is. I've done several hours of googling with mixed results.

First off, it seems that the suggested type of insulation is an elastomeric rubber - Rubatex, Insultube or Armaflex.

Now, I want to add a radiant barrier to the mix since it seems that quite a bit of the loss is radiant. The information that I've seen indicates that you need an air space to avoid conductance of heat through the barrier.

However, there's radiant barrier built on bubble wrap that you're supposed to be able to wrap pipes, heaters, etc. without the air gap.

So a question is, would you add a radiant barrier under or on top of the foam pipe insulation? Intuitively, I would think that you'd foam it first, then wrap the foam in radiant barrier. Since the foam would act (roughly) as a black-body radiator, any heat that got through it could be reflected by the radiant barrier.

On the other hand, if you put the radiant barrier inside, you get the radiant losses reflected directly back into the hot water pipes. Then the foam insulation on the outside simply reduces further losses.

Does anybody understand the thermodynamics of this and have any sort of quantitative handle on the savings that might be involved?

Let's try a real example. Suppose you've got 3/4" copper pipe with 180F hot water. The radiant barrier is coated two sides and is double thick bubble wrap, say 1/4". Barrier is spec'd at 97% reflective for "heat". The insulative foam is rated at R4.6. Assume a 1' section of pipe. Air temperature is 70F.

Nick Pine - you out there?

The emissivity of coper is said to run from .023 to .6 depending on the shinyness of the surface. Let's say that it's 0.1.

Using H = esAT^4, we get: Pipe area = pi * .75 / 12 square feet = .1963 convert to square meters = .196 * 0.0929 = .018 sq m

0.1 x 5.68e-8 x .018 x 355^4 = 1.65 J/s (watts) So radiant heat per hour = 0.77*3600 ~= 5900J = 5.6 BTU/hour

So my hot water is losing ~5.6 BTU/h per foot of pipe due to radiation.

The net heat flow is given by Hnet = esA [T^4 - T0^4]

So net heat loss is 1.23 watts or 4.2 BTU/hour?

Is this right? That seems totally counter to experience. Hot pipes seem to leak heat like crazy!

I suppose the emissivity could be 5x that estimate, then the pipes would be leaking 20BTU/h per foot of pipe. If we figure 20' of pipe in the boiler room, then we're up to 400BTU/h of losses or about the same as a

125 watt bulb.

Anybody? These numbers don't seem right.

Any more comments on insulating boiler room pipes?

[Astro]

ok, as for the math, I found this:

They compute more like 50BTU/hr/ft, roughly an order of magnitude greater than the calcs below. That sounds more like it. so ignore that part of my question!

[Astro]

yet another follow-up. Sorry.

So, according to them, with a 100F temperature difference, they spec 45 BTU / H/ft heat loss for a 3/4" pipe.

And, they say an "insulated" pipe with 1" of insulation will lose 8 btu/h/ft given insulation with a k of .043 W/mC

So a good factor of 5 less loss for the insulated pipe. Total of 160 btu/h loss for 20' of pipe.

[Joseph Meehan]

Keep in mind that the heat loss is not lost. It ends up where? In your home. I do recommend insulation so that you can control where the heat is going, but usually you do really loose the heat.

[m Ransley]

Get an IR thermometer and see for yourself. Boiler room or basement, you may end up with a cold basement. I removed the basement insulation on the pipes so it was a useable area. Remember heat rises. If it is to hot that is different, but you may be colder in the room above if it is comfortable now.

[Astro]

good points guys. There's enough standby losses on the boiler itself to keep the house pretty warm! (ok, minor exaggeration).

Unfortunately too, my boiler room is below my computer room, which is the hottest room in the house.

By the way, I read about 100 messages in a math group on the mathematical derivations for insulating pipes. Pretty interesting discussion but you have to read to the end to get the right answer....