Mixing Radiation

DC123

I've heard that common wisdom that you don't mix different types of radiation on the same zone. My understanding was that that was due to lag times differences between fin-tube (fast) and panels (medium) and cast iron (slow). Does that still hold true with modulating boilers that are intended to run for hours or even continuously? It seems like the temperatures would reach a steady state and the type of emitter would matter less (in maintenance of temperature at least - not accounting for convection vs direct irradiance).

Mark Eatherton

The reason comes down to differences in final delivery. A true radiator (pick one) controls and manipulates the mean radiant temperature within a given space. A convector (pick one) influences the convective air temperatures only. You can have two identical rooms, one with convective base boards, and one with radiators, and both rooms can have an air temperature of 70 degrees F, but as you walk from the convective room into the radiant room, your body WILL feel the difference, and will want you to go back to the radiant room.

Our skin has something like 150,000 temperature sensors spread among 20 square feet of skin surface. It is one of the most sensitive sensors known to man kind.

A person can do mixed means of delivery, so long as the end user is made aware of the differences BEFORE the installation is done. Otherwise, you will find yourself explaining the above to the consumer, who really doesn't care. They just want to be comfortable ALL the time EVERYWHERE in their house.

HTH

ME

DC123

But overall, it sounds like replacing certain sections of fin-tube with panels should not be a problem and could only improve things? I'd also planned to replace those fin-tube sections with panels that had double or triple the output both for improved comfort (those rooms can be bit chilly even with fin-tube running at 180F) and to increase the likelihood of heating with lower water temps (house is a fairly open floor plan, so don't anticipate that those rooms would over-heat) - any issues you'd foresee there? Thanks for your help!

SWEI

In addition to what Mark said above, the performance of the two will change dramatically as water temperatures drop. The convectors will lose output much faster than the radiators will, which makes ODR tuning a bit of a challenge.

DC123

Is that true, though? My impression had been that the temperature curves in terms of BTU/ft at 180 vs 140F were fairly similar between the two? E.g. Runtal UF-2 at 180F is 600, and at 140F is 350 where as for Slant/Fin 30-75, output at 180F is 580F and at 140F is 320.

Mark Eatherton

DC123 said:

But overall, it sounds like replacing certain sections of fin-tube with panels should not be a problem and could only improve things? I'd also planned to replace those fin-tube sections with panels that had double or triple the output both for improved comfort (those rooms can be bit chilly even with fin-tube running at 180F) and to increase the likelihood of heating with lower water temps (house is a fairly open floor plan, so don't anticipate that those rooms would over-heat) - any issues you'd foresee there? Thanks for your help!

I think you will be fine. Just remember to explain the difference to the consumer, unless YOU are the consumer. When dealing with people who are used to radiant, and them adding space and going convective is where it really becomes a problem. Going from convective to radiant doesn't generate complaints.

Radiant has two components of its output, radiant and convective. As the water temperature gets lower, its convective contribution becomes less, and its radiant component becomes dominant. Radiant is the ONLY correct way to manipulate the MRT in a given space. Attempting to do that with a convective system will require air temperatures so high that discomfort is guaranteed.

Robert Bean has a comfort calculator on his web site at www.healthyheating.com Plan on spending a few hours, or days there. Lots of great information.

ME

SWEI

DC123 said:

My impression had been that the temperature curves in terms of BTU/ft at 180 vs 140F were fairly similar between the two? E.g. Runtal UF-2 at 180F is 600, and at 140F is 350 where as for Slant/Fin 30-75, output at 180F is 580F and at 140F is 320.

Outputs fall faster below 140°F. UF-2 (like the Myson Contractor Series) is sort of a cusp species. More radiant than baseboard, but less than a CI or panel radiator. Don't forget to remove the "15% Heating Effect" from the published baseboard numbers.

DC123

What distinguishes the UF-2 from a "panel" radiator? Are you referring to the vertical dimension?

Re: fin-tube output below 140F, do you think that 200 btu/ft is still a reasonable approximation for 120F water?