Radiant Behavior

SENWiEco

First of all, thanks to those of you who responded to my questions back in Dec 2013 regarding radiant ceilings.

Well, the house in question is under construction and I am finally designing the radiant heating/cooling system. I am working with dsdesignconsultants.net to provide the actual mechanical design of the system, but am personally responsible for designing the radiant panels re placement, size, and construction.

I am utilizing 90% radiant ceilings with a little bit of wall and some floor. My current plan is to utilize 1/2" PEX-AL-PEX (to reduce thermal movement) snapped into ThermoFin U transfer plates, and building panels similar to the ones shown in John's Modern Hydronic Heating except at 6" spacings.


For some areas where I just do not have enough ceiling space for the expected Btu output of the system, I am looking at Ray Magic Panels

I have asked Daniel (DS Design) and Robert Bean (via his Radiant Based HVAC group) for information on how hydronic heating acts in a vaulted space. Both provided some good insight. Daniel provided an excellent report from Frenger Systems in the UK (attached) that discuss high ceiling radiant performance.

Prior to reading the Frenger report, I was concentrating all of the radiant cooling and heating required, at the 'pedestrian' level in the house so the people could 'feel' the radiant efforts. But after the report, I instead started thinking of the panels as just another type of 'insulation', in that they can be utilized to stop the heat loss or gain at the sources (see pg 19 & 27). SO for instance, this report talked about having the panels located where their rays can help keep the inside face of the window warmer which increases the occupants average experienced temperature.

I believe this makes a lot of sense (no pun intended), and I am wondering how many agree or disagree with this approach as I do not have ANY actual practical experience designing or living in a radiant dwelling (just a moderate amount of schooling).

With this new (for me) concept of how to treat radiant, I now think that for my three separate areas with vaulted ceilings



that instead of trying to overcome the loads, inserted by the surface areas and windows of those vaulted spaces, down at the pedestrian level, I should instead install wall and ceiling panels in the vaulted part of these spaces to handle the loads imparted by these spaces. I would still have lower panels to deal with the general losses and gains from those rooms, but they would be smaller panels now that they do not have the added loads from above.

Does this sound like a reasonable path forward?

Many thanks for all insight!

Cheers
Sean

hot_rod

I would suggest any info from Roberts site is very well researched and documented.

SENWiEco

Hi @hot_rod. Robert answered some simple questions I asked on his LinkedIN website that were generally unrelated to what I am discussing above. Thanks.

Gordy

I would agree with any of their research also.

Installing emitters to curtain high load areas is really nothing new. It’s usually where rads, forced air diffusers, and baseboards end up.

Radiant panel tube density can be increased in front of same given areas.

How high are the vaulted ceilings?

SENWiEco

Hi @Gordy Please see response to hot-rod. The topic above is different than what I discussed with Robert. But sounds like the above is on the right track. The ceilings start at 8ft and peak at 13'6"

hot_rod

You need enough emitter to cover the room load regardless. Radiators are usually located under windows for the same reason, that is where the load is and people congregate in rooms with windows.

Gordy

The biggest issue I see is a ceiling panel covering the wall area in the lowest part of the vaulted ceiling. And then up near the top the panels would be focused on the upper wall section more than the occupied space.

I know you gave me a height. So what is the roof pitch? 8/12?

SENWiEco

Roof pitch is 15º or 3.22/12.

Yes the top panels would be focused on the clerestory wall section and in doing so be addressing the heating and cooling loads that portion of the wall is introducing into the dwelling (in theory)

Gordy

That’s pretty shallow pitch. So the room is 20’6” long.

I don’t think it’s a big deal for heating. The cooling is another matter.

What is the 8’ walls orientation? South?

Gordy

Radiant cooling will be sub 10 btus a sf I believe.

SENWiEco

Thanks for replies @Gordy and @hot_rod.

8ft wall is facing north. Clerestory faces south and allows winter and shoulder season heat and light in but blocks summer sun due to overhang.

Running 50ºF cooling water (possible lower) should produce above 15 Btu cooling I believe. Still looking for firm specs on this. But comparing my planned panels with specs provided on the Ray Magic panels, this should not be too far out. Magic Rays provide 34.8 Btu/ft2/hr at 46ºF cooling water and 76ºF room set temp. And 22.3 Btu at 56ºF cooling water. I will have wider spacing (6" instead of 2-5/8") but will be using 1/2" tube instead of their 5/16". So I expect I will exceed the 15 Btu but am being conservative for now.

To give you an idea on loads, the middle of the rooms shown in photo (largest) has a total cooling load of 1400 Btu with about 600 of it coming from the vaulted portion of the room.

SENWiEco

Let me clarify the above post in not my idea or new. It is just a new way for me to think about how to 'use' radiant. Yes radiators (or better termed convectors) were placed in front of windows traditionally. But this was because they created a convection current that washes the window and draws the cold air coming off the surface up near the ceiling where it can first mix with the internal air and warm up before interacting with the occupants. But radiant does not have convective currents, so the mechanism is different. Just trying to ensure this all makes sense from a building science point of view.

hot_rod

Here is a good read on hydronic cooling, pay attention to the charts and limitations of surface temperatures, humidity, dew point, etc. pages 47-

Gordy

I went to bed, but I was going to say that Dew point control will be the biggest factor in being able to get the output for cooling.

SENWiEco

Thanks @Gordy - agreed re dew point control. I am also working on a dehumidifying coil that will install into the HRV intake. Targeting 45%RH for now (hoping for less) until I get results back from coil sizing. At 76ºF room setpoint and 45% RH, dewpoint is 53ºF. So could run 55ºF cooling water. If I can only get down to 50%RH, I would run 58ºF cooling water which will lower the panel efficiency. But still should get above 15Btu cooling.
The expansion tank filling was a great piece of info. Thanks

SENWiEco

@Gordy just happened to be reading the WaterWorks Small-Scale Hydronic Cooling Bulletin from SpacePak and the formulas for calculating cooling potential for ceiling panels are on Pg 15. With average ceiling surface temp of 65ºF and Room average at 75ºF, the btu output of 8" spacing 1/2" PEX with plates would be 18.6 Btu/ft2/hr

Gordy

Is your location in the south west? A more arid climate?

SENWiEco

I am in North Vancouver BC Canada.

SENWiEco

I have now finished my cooling load calcs and can generally handle the required cooling load with 66ºF cooling water. AT these temps I would need a humidity over 95% to reach dewpoint. So from a condensation point of view I should be fine.

Is there any other placement recommendations that you all can provide? Any concerns with locating panels whose sole purpose would be to heat or cool clerestory spaces (vaulted portion of ceiling) and not be able to 'see' the floor space or occupants.

Many thanks for any direction provided.

Voyager

SENWiEco said:

I have now finished my cooling load calcs and can generally handle the required cooling load with 66ºF cooling water. AT these temps I would need a humidity over 95% to reach dewpoint. So from a condensation point of view I should be fine.

Is there any other placement recommendations that you all can provide? Any concerns with locating panels whose sole purpose would be to heat or cool clerestory spaces (vaulted portion of ceiling) and not be able to 'see' the floor space or occupants.

Many thanks for any direction provided.

I am not folllowing you. You need to reach 100% RH for condensation to occur. That is also the point where the dew point and the temperature are equal. So, no matter what the temperature is, you need to be above 95% RH to have condensation occur, because you need to be at 100% RH to get the dew point and the temperature equal at which point condensation will commence.

And you don’t “reach” dewpoint, dewpoint is simply another way to measure relative humidity. You “reach” condensation when the temperature and dewpoint become one.

Are you referring to needing 95% RH outside the structure before you need to be worried about condensation inside?

Gordy

Well when I saw the initial load you posted I knew it would not take much

Gordy

Dew point is dependent on RH, and surface temp. That’s how windows get condensation in the winter time. You can get condensation with 35% RH if the window is cold enough.

The OP is doing radiant cooling using chilled water in the radiant panels. There for one has to closely monitor surface dew point so panel condensation does not occur.

Gordy

SENWiEco said:

I have now finished my cooling load calcs and can generally handle the required cooling load with 66ºF cooling water. AT these temps I would need a humidity over 95% to reach dewpoint. So from a condensation point of view I should be fine.

Is there any other placement recommendations that you all can provide? Any concerns with locating panels whose sole purpose would be to heat or cool clerestory spaces (vaulted portion of ceiling) and not be able to 'see' the floor space or occupants.

Many thanks for any direction provided.

Remember there will be back losses of the panel in both heating, and cooling. There is a bit of an off set there. How much I don’t have definitive information other than it is there.

SENWiEco

@Voyager - No I am saying with my very low cooling loads/flux and therefore warming temperature cooling water, the interior humidity would need to reach 95% before I would have condensation on surfaces at my 76ºF room setpoint. For this set point I will be running 66ºF cooling water

SENWiEco

Thanks @Gordy Understood re back losses. Majority will still be inside conditioned space so not really lost (benefit of ceiling in basement over floor). With R60effective insulation for top floor ceiling/roof + the 3/4" polyiso of the panel, and only running 66ºF cooling and 85ºF heating supply, I do not think I need to be too worried about transfer through the envelope. Internal floors will also be back insulated to R22 above the panels.

SENWiEco

To give everyone an idea on loads. This dwelling has 5000ft2 of conditioned space and my total heating load is 18,230 Btu/hr and Cooling 21,252 Btu/hr. My heating Flux range is 1.82-9.58 with an average of 5.19

Voyager

SENWiEco said:

@Voyager - No I am saying with my very low cooling loads/flux and therefore warming temperature cooling water, the interior humidity would need to reach 95% before I would have condensation on surfaces at my 76ºF room setpoint. For this set point I will be running 66ºF cooling water

I am curious how you came to that conclusion? That didn’t sound right based on my experience so I did a quick check and it seems to me that if you have 66 degree panel temps in a 76 degree room, you would risk condensation if the RH in the room was above about 65% rather than 95%. Here is one fairly easy to use calculator, for example. http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html

Maybe I am still not understanding your thinking...

SENWiEco

@Voyager You are totally correct. Thank you for making me take a step back and think. I had an error in my spreadsheet as was running the dew point of the cooled surface and not of the room air. But I am still good. As I am running 66ºF degree cooling water, my ceiling surface temp will be 70.84ºF meaning I would still need a humidity above 84%, @ 76ºF room set, to reach a dewpoint temp of 70.84. This will never happen (other than possibly in a bathroom after a shower - need to figure out what to do about that or if it is for too short of a time period to worry about). But again, thanks for making me take a step back.

Gordy

You have to put together the whole radiant cooling package. He will be using an HRV to control humidity, along with air exchange with that low load envelope. There will be no chance of condenapsation with proper sensors, and control strategy. The cooling is less than 2 tons for a 5k sf home. So the Average panel temp will not need to be that low.

Gordy

If you have 75% RH with 75 set point which would condense on a 66 degree panel it would be a pretty nasty in there

Gordy

One thing about it if you get a good handle on the dehumidification you’ll have plenty of cooling capacity.

SENWiEco

thanks @Gordy Agreed. I am still waiting for a mechanical vendor to quote a condensing hydronic air handler and see how much humidity reduction I can get on the incoming air stream and 8" duct. I will be bring in around 85cfm on a continuous basis and ramping up the bathrooms/kitchen to 140 cfm when in use. I doubt I will ever need to set the HRV to its 230 cfm high. The air leakage target through the envelope will be .5 ACH50 which is helping keep the latent load really low.

Voyager

SENWiEco said:

@Voyager You are totally correct. Thank you for making me take a step back and think. I had an error in my spreadsheet as was running the dew point of the cooled surface and not of the room air. But I am still good. As I am running 66ºF degree cooling water, my ceiling surface temp will be 70.84ºF meaning I would still need a humidity above 84%, @ 76ºF room set, to reach a dewpoint temp of 70.84. This will never happen (other than possibly in a bathroom after a shower - need to figure out what to do about that or if it is for too short of a time period to worry about). But again, thanks for making me take a step back.

Keep in mind that condensation on the room surface of the drywall ceiling isn’t the only consideration. There will also be air and water vapor inside the stud cavities and around the tubing and aluminum plates. If the RH of that cavity is such that the dewpoint gets near your cooling water temp, you could still get condensation and mold formation inside the stud cavities. Depending on the quality of your sealing and vapor barriers, you may get moisture infiltrating the stud cavities from the outside as well as the inside.

I am not trying to talk you out of radiant cooling, but I have read a number of cases studies where things did not go well. It takes very good control systems to keep water vapor levels low enough to avoid having problems somewhere at some time in a radiant cooling system. Heating is very simple and forgiving in comparison.

Gordy

I’m pretty confident in an envelope of 5 BTUS a sf. It’s a thermos bottle.

Also the back side of the panel will be considerably warmer than the room side.

SENWiEco

Well now that we are on the topic my envelope comprises -from exterior to interior- cladding (horizontal engineered T&G cedar), 3/4" rain-screen/capillary break (ventilated and drained), 6" of ROCKWOOL ComfortBoard 80, self adhering water/air barrier (Delta Vent SA), 1/2" Ply, 2x4 Structure infilled with ROCKWOOL ComfortBatt, Drywall, VB Paint. With the exterior insulation, the ply and studs will always be close to interior temperatures. Plus we are talking summer when the outside will be hotter than the inside, therefore the further outboard you get in the assembly the warmer it will be. With the exterior air barrier there will be NO air movement through the wall and VB paint will be more than adequate to prevent wet-up by diffusion.