Two weeks ago we looked at two solar thermal and radiant heating systems by Radiantec. One of the options that Radiantec offers is a very simple diverter valve that allows the homeowner to run the incoming cold water through the radiant floor prior to reaching the hot water heater and fixtures. The diagram below details this setup that advertises free additional cooling in the summer for homes with radiant slabs such as ours.
I’ve had some discussions about this theory with some professionals in the past two weeks and thought we might benefit from a closer look at this setup. First, Radiantec offers test data based on a case study house they built in the 80′s in a DOE Report (pdf). The cooling option is discussed on pages 22-23.
Radiantec Test Case Data & Assumptions
- 2 story, 1,400 sf home with a 720 sf 5.5″ thick concrete slab on grade foundation
- Water supply temperature of 55 degrees and a design temperature of 78 degrees
- Household water consumption of 300 gallons per day
- Heat exchange efficiency of 90% in the radiant slab
- 51,667 BTUs extracted per day
100K Adjusted Figures
- 2 story, 1,050 sf house with a 648 sf 5.5″ thick slab on grade foundation (0.9 Case Study)
- Household water consumption of 100 gallons per day (0.33 Case Study)
- 15,345 BTUs extracted per day (0.9 * 0.33 * 51,667)
OK great, but what does this mean. Well I dug up an estimated cooling requirement calculated by a mechanical contractor a few months ago that said we needed just over 22K BTUs of cooling. This was a previous version of the house that had less insulation, poorer windows, over double the lighting and took into account none of the passive cooling strategies. So if we used this old estimate, the free radiant cooling feature would cover 70% of our cooling requirements. I can’t quantify the effects of the passive elements on our cooling load to get a better figure without spending a lot of money on more energy modeling, so for now we’ll have to be happy with these figures.
OK this is all great, but what about the main concern with radiant cooling – condensation? Well the lovely people at Radiantec (I like them more and more as I delve deeper into their websites) have created an FAQ to address this issue and other questions on the cooling feature. They address the condensation issue in two parts:
- Since the radiant tubing is embedded in the concrete and not exposed to the air, you will not get condensation on the tubing. Makes sense, but what about the slab itself?
- They claim that the system will not create enough cooling to present a whole house moisture problem and go on to say that in very hot and humid climates an air conditioner will be needed and will help dehumidify the air.
This is nice but does not entirely quell my fears of a slimy floor syndrome in the summer. I happened to run into a Mechanical Engineer, Joe, from Bruce Brooks & Associates at a local pub last night and decided to get his thoughts on this issue. He has had some experience using radiant cooling in commercial applications, but like most cool HVAC techniques, it is not widely used in residential applications. I first thought that the fluid in radiant cooling systems would be cooled to a temperature much lower than the 55 degrees we have in our calculations but he said that it is actually only 48 degrees. This hurt my assumption that we would not get condensation due to the higher temperature of our cooling liquid.
Next I asked him if active dehumidification would help in the house as needed. He said it would but was unsure of what degree. Joe then mentioned that they actually simply create an air flow over the radiant cooling surface with fans. I mentioned that we will have multiples ceiling fans in the house and he said that would certainly help and might just be enough.
So in summary, I am now about 95% confident that our combined low-cost cooling and dehumidification techniques will be sufficient enough to keep the home comfortable on all but possibly the very hottest and longest heat waves of the summer. I especially love the fact that we can achieve up to 15K BTUs of free cooling daily but utilizing the radiant cooling detail from Radiantec.
Let us know below if you think any of this sounds crazy…