Design-Day Water Temp for Climate Panel

Ted_9

Constantin, why don't you have the design work from your heating contractor? I know that you would have wanted to look that over before they started the project.


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Constantin

So I'm having fun in the basement...

... with my Vitola and Vitotronic 200... but there is one question that is nagging me... the supply water temp I should be choosing.

According to HVAC-Calc and Wright-Soft calculations, I should be losing about 12BTU/(ft² x hr) on a design day. The tubing is in sub-floor mounted Climate Panels that have 3/4" of Oak above and R13 of Icyenene below. Allegedly, my climate panel tubing is larger than the usual 5/16" tubing but I'll have to verify that.

So where would you start to set the curve, and what is the maximum water temperature that the RFH circuits should ever require, considering that the most you can expect over the oak is a throw-rug (if that). I'm inclined to believe that 120°F water would be the hottest we'd ever need, with a curve of 0.8. Any thoughts?

Unknown

According to my charts, at that heat loss and that flooring you're at more like 105 degrees under design conditions IF you are using climate panel. the curve question I'd have to throw to my partner.

I would question how your climate panel tubing would be larger. If it's larger, it's not in climate panel, which can only accept 5/16" tubing unless viega has started making a larger version of climate panel when I wasn't looking.

Constantin

Thanks Rob!

Well, at least I think I measured it in this picture... does the 5/16" ID tubing have an OD of about 0.510" or 1.3cm? I think it very well may be as I recall 0.1" wall thicknesses being commonplace for PEX that small. Oh well!

Anyway, the other picture shows some of the writing on the tubing that I was able to dig out... looks like PEXc.

Again, many thanks for the information! I will set my curve even more aggressively at 0.6 or 0.5, which is what Jim advocated for PEX in concrete.

Time to break out HDS and see what the effect of 5/16" ID circuits has on the flow rates / heat transfer.

Constantin

Hi Ted!

Actually, I don't think I reviewed the system design in detail other than going over the house with him to detail out where things like manifolds, AH, etc. ought to go. I learned way back when I helped lay out a production facility that "monument" placement is very important, and tried to accomodate the various trades by setting aside space in walls, utility spaces, etc. for them.

It is entirely likely that I simply misunderstood my contractor when he stated that the climate-panel used in our house was different from the normal stuff and that it had a larger ID than 5/16. It's not a big deal, as the house heated just fine, even in the dead of last winter, with just the storm windows on the outside.

Unknown

Good question.. sounds like you might have 3/8" pipe. I don't have exact enough OD measurements handy to say for sure, but that would be my guess. If it's different than climate panel, I wonder if your contractor made his own?

Mike T., Swampeast MO

Ratio sounds rather high as well.

What's your BTU/sqft loss at about 50° F?

Mike T., Swampeast MO

Reset Ratio

Am working a touch blind, but here's an example:

For 12 btu/hr/sqft output you need a surface temperature of about 78 degrees. With a panel R-value of 2 (should be estimating high here for climate panel with 3/4" oak above) you need an average supply temp of only 95 degrees. If you're using a 30 degree delta-t that equates with 110 degree supply at design.

Now let your heat loss drop drastically to 5 btu/hr/sqft. For that output you need a surface temp of about 75 degrees (note only 3 degrees less) with an average supply requirement (at R-2) of 80 degrees average. With a 30 degree delta-t again, supply requirement is 90 degrees.

Say your design requirement of 12 btu/hr/sqft occurs at zero degrees outside and the 5 btu/hr/sqft occurs at 40 degrees.

40 - 0 = 40 degree temp change outside

110 - 90 = 20 degree supply temp change.

20 / 40 = 0.5 degree change in supply for each degree of outdoor temp.

Plug in your actual numbers and see if 0.8 reset ratio doesn't seem a bit high.

Your heat loss is so low that your floors will be significantly cooler than many? most? RFH systems. With such a low surface temp requirement your reset ratio must get quite flat because there is so little difference in required surface temp between design conditions and moderate weather.

My guess is that you'll have an exceptionally efficient system with no apparent source of heat. Guests will wonder where the heat is coming from.

In that regard, you [might] have some problem as exposed flesh will still be loosing heat to every part of the structure at all times. Your abundant original windows--even well restored and with storms--will become by far the coldest objects in the space. You [might] wind up with the "Euro-cave" feeling unless you warm the floors (and eventually the air) to a somewhat higher degree than possible with floor temps in the 80s but with MUCH higher heat loss per square foot.

I have a feeling that you'll have quite a bit of experimentation ahead with your thermostats. Move in TINY increments and give DAYS to stabilize. Find your comfort level but remember that men in particular typically want more heat as they age. You can add clothing, but your wife can only remove so much...

Once you find your comfort setting I cannot imagine that you would want any but the slightest daily setback.

IF, however large areas of the house are typically unused and you want decent temperature response, you'll want to up your design loss supply temp to the 120 degrees (or possibly a bit more) you mentioned.

Constantin

Interesting...

Well, I thought my contractor told me that the tubing in our home was larger than the regular climate-panel tubing at 5/16"... Presumeably, the measurement measures ID, as the ½" pipe in the basement has an OD of about ¾", IIRC.

Then again, 3/8 vs. 5/16 is not that much of a difference. It's not like warmboard with its ½" tubing. Also, as all the stuff came emblazoned with the happy polar bear, my guess is that it's "official", not home made.

Constantin

Some answers...

Calculated Heat Loss (HVAC-Calc) at

• 50°F: 3.5656 BTU/(ft² x hr)
  
• 9°F (ASHRAE 1%): 11.68 BTU/(ft² x hr)
  
• -15°F: 16.43BTU/(ft² x hr)
  
  Does this help? No solar gains, gains by the presence of humans, etc. are used by HVAC-Calc for the purpose of estimating the heat loss.

Constantin

Here you go...

... at least I think. I experimented with HVAC-Calc until I could get 5BTU and 12BTU outputs per sq ft per hour. Does the calculation look about right?

Mike T., Swampeast MO

You Be One Smart Cookie Constantin!

No need to mention to you that delta-t is almost certain to drop with increasing outdoor temp and decreasing heat flux.

Looks VERY good with the exception of a simple error. Your supply temperatures and vectors are reversed as related to heat output.

Constantin

Thanks, Mike!

I've corrected the picture above... 0.6 it is, then! Thanks!

gasfolk

Constantin...

Are your actual fuel use and HVAC-Cal and Wrigtsoft predictions close? Why might your home be better modeled than ours? Can you point us to a thread describing your home and installation? Interestingly, from our actual fuel use, we get 14 BTU/sq.ft.xhr as well. Any thoughts?

Also, do you think you would have gone geothermal if you had reached the ECR rep, or did geo also no longer appear cost effective.

Thanks,

gf

Constantin

Good Question...

... Unfortunately, we have yet had a season in the structure, so it's hard to tell just how close the predictions are to reality. Our last winter had the house full of contractors, who like open windows and a warm house at the same time. I hope our usual modus operandi will involve closed windows in the wintertime!

The house is an 1872 Mansard that has received a gut job to retrofit Corbond in the true 4" old walls and Icynene in the new ones (6" nominal, 5.5" actual). We kept all the old windows and put Harvey Tru-Channel storms in front of them. All old sashes were removed, stripped, and remanufactured. The frames were retrofit with spring bronze and other weatherstripping. The new windows are Marvin Ultimate Double-Hungs. Oh, and then there is the new foundation...

I would not be surprised if the actual fuel usage will be significantly lower than what the initial heat-loss calculations predict. For one, there is significant solar gain in this climate and the calculators only use it for heat gain in the summertime. Never mind the heat gain from humans, which is also calculated only for the heat gain in the summer, not the winter. With a full house, my heating needs on a design day would almost be covered 10% by humans alone.

As for the justification of geothermal, I thought about it long and hard. There certainly is a neighborly appeal to being able to heat and cool your home w/o noise or noxious flue gases. Plus, the technology is very elegant. On the other hand, the intallation cost is very high and the payback makes the most sense in areas where power is derived from low-cost power sources like hydro, nuke, or coal energy.

In the Boston area, much of our power is now generated by turbines running on natural gas. I suspect that as EPA and other requirements climb, equipment is replaced, etc. that the share will increase. It follows that electricity prices will become increasingly dependent on the cost of natural gas in our area. So, while the GSHP may have a COP of 4, if the underlying price of the raw energy is high, where will the savings be?

In other words, the money you save may be better invested in the stock market, particularly if you do not intend on installing AC in the home. If I had been able to find the company in NH that I met at ISH (they do actual ECR installs), I may have still gone for the GSHP out of concern for my neighbors and the ability to run the home on very little energy. However, by the time ISH rolled around, my heating equipment was already installed.