Floor Covering (Engineered Wood) Question

jb9

Hello,

I have a question about wood floors over radiant floor heat. I am looking over all the literature that show the resistance levels (Rv) of floor coverings and how this value affects the required water temperature of the system. Anyhow, I am assuming that the floor covering is actually the “emitter” that design professionals refer to. Is this correct? Obviously, the choice of a wood floor is informed by one’s aesthetic, budget and long-term durability. In my case, however, I want to be sure that my choice of flooring is going to work well with my system. Does anyone have any suggestions or can anyone share any pitfalls they have seen in the field with clients’ floor covering choices (materials, sizing, installation)? What I have settled on is that I am going to use an engineered “floating” floor that can be installed without using flooring nails (to hopefully eliminate the risk of a puncture to the Pex-Al-Pex). I know sometimes these decisions probably fall outside a designer’s jurisdiction, but I thought it would be helpful to ask folks’ thoughts.

Thanks.

SWEI

Start with this.

hot_rod

This company has done a lot of hardwood flooring testing also, a longtime RPA supporter.

A top quality engineered flooring with a baked on factory finish should last a long time over radiant.

The key is to not overheat it, try for constant circulation or outdoor reset to lessen temperature swings.

Humidity control has a lot to do with flooring performance also.

http://www.launstein.com/launstein_radiant.html

Mark Eatherton

Here's a link to an old Radiant Flooring guide (2013) but recommendations as it pertains to wood floors hasn't changed.

http://digital.bnpmedia.com/publication/?i=96350

As HR said, low even temperatures (use good plates, or an engineered floor system, use as low an operating temp as you can, place insulation below the radiant floor) and humidity control (addition). More the addition of humidity, than the removal, unless you want complete control of wood in general, in which case you MAY need a de-humdifier depending upon where you are.

Check with the manufacturer of the flooring product. Not all manufacturers will warrant their product over a floor heating system. They've had a LOT of problems with DIYer who don't know what they are doing, and they point their finger of blame at the wood floor manufacture when in reality, it is the homeowners problem. Too hot for too long, and no humidity control will cause wood to change dimensionally, and not usually evenly (warping and cupping) and incorrectly acclimated wood (installed wet,unevenly dried out by the RFH system) can cause checking, crazing, panelization, shrinking, these are all the results of improperly installed, maintained and operated radiant floors. Oh yeah, a lack of insulation below the floor has a negative impact as well. Causes longer run times, baking the wood.

Done right, these floors will last forever. Done wrong, and there is likely to be a lot of problems, and no one goes away happy...

ME

Mark Eatherton

Here is the 2015 version. It has been renamed to the Radiant Comfort Guide.

http://digital.bnpmedia.com/publication/?i=242283

Contractors can order additional copies for placement with potential customers by contacting the IAPMO bookstore.

ME

jb9

Thanks for these links! One other floor related question... I have been reading several WB charts that describe the relationship between the desired ambient temperature of a room and the required temperature of a radiant floor. In one article, I have read that this is derived from the flux value calculated during the design process. Can someone explain in basic terms how the floor temperature and desired room air temperature are related?

Thanks again for all the knowledgeable folks here!

jonny88

@ME to funny I just ordered that copy a few weeks back and paid $20 or close to overnight it.Nice link thanks for sharing.Still saving up to join.......

Mark Eatherton

jb9 said:

Thanks for these links! One other floor related question... I have been reading several WB charts that describe the relationship between the desired ambient temperature of a room and the required temperature of a radiant floor. In one article, I have read that this is derived from the flux value calculated during the design process. Can someone explain in basic terms how the floor temperature and desired room air temperature are related?

Thanks again for all the knowledgeable folks here!

As with any hydronic question, the only correct answer is "It depends..." And if you go to www.healthyheating.com there is a comfort calculator there that you can use to adjust the variables, like mean radiant temperature, air temperature, humidity etc and see the effects on comfort. The best advice I can give you or anyone else transitioning from a conventional system to a radiant system is to start low and go UP slow... Start out at 65 degrees F.,, and then change the temperature by nor more than 2 degrees per day, and allow the system to "soak" for at least 24 hours before making any adjustments. Once you hit your comfort zone, turn the temperature back one degree F, and again let things soak. You will eventually find your comfort zone based on your needs.

Radiant heating and cooling systems in general affect a factor called the Mean Radiant Temperature (MRT for short). MRT is the primary determining factor in human comfort. Its definition is "The average surface temperature of those surfaces surrounding your body" Too high, and you will break out in a sweat when you get up to cross the room. Too low, and you will be cold regardless of what you are doing or where you are doing it. No one has developed a good "MRT" sensor, yet. As soon as they do, we will be able to set an MRT for a given exposure. Air temperature is a secondary effect.

So, for the time being, we have to look at the "operative" temperature. The operative temperature is the average of the MRT and the rooms air temp.

I do hope you enjoy your new found comfort. Done right, it will be incomparable to any system you've ever experienced, and has to be experienced to be understood. Done wrong, you cold be more miserable than you've ever been before. Pay attention to the critical details.

Start low, and go up slow...

ME

SWEI

Flux = flow of heat, and probably refers to the heat loss of the structure. The goal is to replace what is lost as it is being lost. Doing so is far easier with radiant than it is using other heating methods, especially at a residential scale.

hot_rod

jb9 said:

Thanks for these links! One other floor related question... I have been reading several WB charts that describe the relationship between the desired ambient temperature of a room and the required temperature of a radiant floor. In one article, I have read that this is derived from the flux value calculated during the design process. Can someone explain in basic terms how the floor temperature and desired room air temperature are related?

Thanks again for all the knowledgeable folks here!

Heat Flux is generally expressed as BTU/ hr./sq. ft

in a formula Q= heat flux under design load.
So design heat load of room divided by area of panel.

Take a 20X20' room with an 8,800 BTU/ hr. calculated load.
Divide that by the room area of 400 square feet

So 22 BTU/sq.ft/ hr

Where you need be careful with heat flux is the "available" floor surface. Take a kitchen for example, and use the above example, but put cabinets inside that room that block the output of the radiant panel.

You need to subtract out that square footage of the cabinet footprint.

Use that 20X20 room, add cabinets along two walls, and a 8X2 island cabinet = 92sq. ft.

400-92= 308 sq. ft
8,800 BTU/hr. load divided by 308 "available" surface area = 28.5 BTU/ sq. ft. required.

Same would apply to a living area with sofas or overstuffed chairs in the room, they limit the available radiant panel area.

A common rule of thumb is 2 BTU/ hr./ sq. ft for every 1° difference between floor surface and ambient temperature.

85°F should be the max. floor surface temperature for residential applications. Some suggest 82- 83° max.

So 85°floor temperature - 68° ambient= 17 X 2 = 34 BTU/ square foot output. This is where most of the 35 BTU/ sq.ft maximum output number that you hear, from a residential radiant floor comes from.

Of course some folks prefer a 70 or higher ambient for the elderly especially. That further limits the radiant panel output.
If the occupants desire a 72°F ambient, it looks like this:

85°- 72°=13 X 2 = 26 BTU/sq. ft. output.

So always run a load calc, then pay attention to how m much actual floor surface will be the heat emitter, and crunch the numbers.

What commonly happens when a design is missed is the supply water temperature gets cranked up to compensate. This was a very common "fix" attempt for staple up tube systems that were unable to get that floor surface a consistent 85F due to limited tube contact and the related temperature striping.

Here is a look at WB under power. The striping does go away quite a bit when floor covering is added, as the back 1/2 has carpet. This is a good visual of how the tighter 6, 8 or 9" on center could even out surface temperature and give overall better output as the entire surface area warms.

Notice where I router through the aluminum surface and how it completely stops the heat transfer.

Mark Eatherton

Hr, you said "A common rule of thumb is 2 BTU/ hr./ sq. ft for every 2° difference between floor surface and ambient temperature."

I think you meant A common rule of thumb is 2 BTU/ hr./ sq. ft for every 1° difference between floor surface and ambient temperature.

Thanks for contributing.

ME

hot_rod

Mark Eatherton said:

Hr, you said "A common rule of thumb is 2 BTU/ hr./ sq. ft for every 2° difference between floor surface and ambient temperature."

I think you meant A common rule of thumb is 2 BTU/ hr./ sq. ft for every 1° difference between floor surface and ambient temperature.

Thanks for contributing.

ME

Correct, thanks for catching that. I'll serif I can edit.

Sorry we could not connect in Colorado this week.

Mark Eatherton

Me too. Hope your trip went well. I guarantee I will see you in Orlando Thanks!

ME

jb9

Thanks Hot Rod, Mark, and SWEI. I have been reviewing all of your detailed responses! I think I am close to understanding the flux concept. Looking at the math, I don't think I fully understand how the lower ambient temperature requires more emitter (floor) output... The example really helps. I "almost" have it... Jus t didn't want you guys to misinterperet my radio silence. So, to hone my question, why does the lower ambient room temp require more output and the higher ambient temp (70 as you used) reduce the floor/emitter output?

Thanks again. You guys really know your stuff.

SWEI

The ambient room temp paired with the average fluid temp in the loops is what "drives" the output. The greater the difference between the two, the more heat flows into the room. Assuming the fluid temp remains constant, as the room warms up, the flow of heat (flux) decreases. When we vary the fluid temp using outdoor reset, the goal is to put the same amount of heat into the room (from the emitters) as is leaving the room (to the cold outdoors.) When those two amounts are close to the same, the room temp and comfort level stabilize in a way that is almost magic. Once people have experienced it, they find it hard to go back to whatever they had before.

jb9

Ok. So using the max output value above, let's say that the kitchen space presented (308 sq.ft. of emitter area) is running at that 35 BTU/sq ft maximum. Then the system could drive 10472 BTUs into the space. Assuming it was a tight envelope house, that would clearly be too much heat compared with the heat loss of the space, right? So basically, this flux calculation works in tandem with the room by room heat loss calculation to determine ideal fluid temps, right? And heat loss calc gives you the load to sustain the heat content and the flux further refines the design by giving you a fluid temp to match the heat loss, right? I might have a room heat loss calculation value to try this part of the design with soon... Each phase of the design is starting to make sense as I see how they are inter-related.

bmwpowere36m3

Correct... but, you won't know whether 10k BTUs/hr is too much till you do a heat loss. For example my kitchen is 290sqft and the heatloss is 6k at 7* (design day). Assuming 240 sq ft of emitter space, I need about 25 BTUs/sq ft (reasonable for radiant flooring).

I wouldn't design the system in the hopes it'll output 35 BTU/sq ft (maybe a shop or structure where you don't walk barefoot). I think 20-25 is more reasonable (at least that's what I've read).