EcoRenovator - View Single Post - What kind of PEX tubing for hydronic heat?

AC_Hacker · 10-01-13, 10:43 AM

Daox,

The following is in regards to Low-Exergy radiant heating and is a departure from radiant heating conventional wisdom...

I'm not sure what your expectations are for your floor, but I did some research on Low-Exergy radiant heating, and there's not so much direct information here in North America. The design considerations of a Lo-Ex floor are substantially different from a typical install.

But, I did download a copy of the free Watts Radiant design software, and by pushing the software and trying lots of different combinations, I was able to get an inkling of what would be involved. They even have someone who will respond in a timely and helpful manner to oddball issues such as very high efficiency radiant floors (AKA: Low-Exergy).

Anyway, what I learned is the obvious, that the closer spaced the PEX, the lower the required feed temperature.

There is an unfortunate design limitation in the Watts Radiant software, in that it will not recognize spacings that are closer than 6". In my estimation, this is a real bummer. But it is possible to see what the trends are and if you happen to be obstinate and resourceful, you can record your data from various runs, and do a graph or a regression analysis on your data and get some good predictions that go beyond (closer) than 6" spacings.

Piwoslaw mentioned a book in Polish that he (thought) he had access to that described European approaches to radiant floors, and spacings of 6" and 4" were not uncommon. Unfortunately, that resource never made it to our forum.

BUT, one thing I learned from all this is that if you want to operate in the range of VERY low feed temperatures (very Low-Exergy), you need to increase your flow rate. To accommodate

this efficiently you need larger PEX.

I have heard mention of 5/8" PEX being used, close-spaced, in Low-Exergy installations.

But this all is in keeping with what I know about solar and heat pump systems for forced-air systems that are designed from scratch, rather than being a retrofit...

When doing a retrofit solar or heat pump installations, installers usually do not rebuild the existing ductwork because the extra expense may cause them to be underbid and lose the job.

The delta-T of heat exchange surfaces is solar or heat pumps is smaller, so to get the required BTU rate, extra air velocity is required. When you push extra air through existing ducts there is most often an increase in ventilation noise.

My girlfriend recently changed her house over from oil to ASHP forced air.

Her house is now comfortable, but there is a nearly constant background sound of ventilation noise.

So, for Low-Exergy installations, keep in mind that higher volumes could be required.

Something to consider...

-AC

10-01-13, 10:43 AM	#12
AC_Hacker Supreme EcoRenovator Join Date: Mar 2009 Location: Portland, OR Posts: 4,004 Thanks: 303 Thanked 723 Times in 534 Posts	PEX diameter... Daox, The following is in regards to Low-Exergy radiant heating and is a departure from radiant heating conventional wisdom... I'm not sure what your expectations are for your floor, but I did some research on Low-Exergy radiant heating, and there's not so much direct information here in North America. The design considerations of a Lo-Ex floor are substantially different from a typical install. But, I did download a copy of the free Watts Radiant design software, and by pushing the software and trying lots of different combinations, I was able to get an inkling of what would be involved. They even have someone who will respond in a timely and helpful manner to oddball issues such as very high efficiency radiant floors (AKA: Low-Exergy). Anyway, what I learned is the obvious, that the closer spaced the PEX, the lower the required feed temperature. There is an unfortunate design limitation in the Watts Radiant software, in that it will not recognize spacings that are closer than 6". In my estimation, this is a real bummer. But it is possible to see what the trends are and if you happen to be obstinate and resourceful, you can record your data from various runs, and do a graph or a regression analysis on your data and get some good predictions that go beyond (closer) than 6" spacings. Piwoslaw mentioned a book in Polish that he (thought) he had access to that described European approaches to radiant floors, and spacings of 6" and 4" were not uncommon. Unfortunately, that resource never made it to our forum. BUT, one thing I learned from all this is that if you want to operate in the range of VERY low feed temperatures (very Low-Exergy), you need to increase your flow rate. To accommodate this efficiently you need larger PEX. I have heard mention of 5/8" PEX being used, close-spaced, in Low-Exergy installations. But this all is in keeping with what I know about solar and heat pump systems for forced-air systems that are designed from scratch, rather than being a retrofit... When doing a retrofit solar or heat pump installations, installers usually do not rebuild the existing ductwork because the extra expense may cause them to be underbid and lose the job. The delta-T of heat exchange surfaces is solar or heat pumps is smaller, so to get the required BTU rate, extra air velocity is required. When you push extra air through existing ducts there is most often an increase in ventilation noise. My girlfriend recently changed her house over from oil to ASHP forced air. Her house is now comfortable, but there is a nearly constant background sound of ventilation noise. So, for Low-Exergy installations, keep in mind that higher volumes could be required. Something to consider... -AC __________________ I'm not an HVAC technician. In fact, I'm barely even a hacker...