Open Radiant Heating Systems

[bmxdarcy]

I already know this is a hotly debated topic, so maybe I'm wasting my time starting this thread but after all of the reading from closed contractor forums I've noticed they never discuss how an open radiant heating system prevents stagnating hot water.

For those that don't know this is done by always feeding fresh water from the street into the radiant loops before they reach the hot water heater where it then proceeds to the fixture, (shower, sink, dishwasher, etc.). This happens regardless of the season, regardless if the radiant heat is running.

So my question is, how is this all that different from a normal DHW tank? Or the popular circulator loops for instant hot water at your fixtures? The water will cycle hot and cold but it is always heated before it reaches your fixture.

Here are a couple links to open designs if I haven't described the design very well.

http://www.radiantec.com/about-radiant-heating/open-direct-system/
http://www.radiantcompany.com/system/opensystem/

 

[roscoe2000]

I would would stay away from that type of design., with the biggest issue being cross contamination, follow be no freeze protection in event of power loss. The safest approach would be the addition of a water to water heat exchange thereby keeping the two system separate.

 

[Radiantec Guy]

This is a good question and one that we get often. Roscoe brings up some good points that I want to quickly address. In regards to cross contamination, all of the components are rated for potable water. With our system design (which has been evaluated by the ICC) the water in the system remains fresh assuming domestic hot water is consumed in the house. This happens even in the summer when you're not using the heating system. The installation of a water/water heat exchanger actually increases the chance of cross contamination because you are connecting a non potable and potable system together. Granted, the likelihood of cross contamination in this arrangement is very low.

Some will argue, "but what happens if you don't use hot water for awhile?" This is another good question. It is common in any domestic water system to have periods of little or no use when people go on vacations or go away for the weekend. This is fine. If your system will be in a shop where domestic hot water usage will be sporadic and limited then we recommend that you go with a different system.

As for the lack of freeze protection, our question is always about how the system will be used. If this is a shop or garage where you may want to shut the heat off from time to time then you should go with a closed type system where you can add glycol. But if the system will be used daily during the heating months then it is rare you will encounter a scenario that freezing will be a concern. With a radiant heating system you are increasing the surface temperature of all of the objects in the building as well as heating the entire mass of the floor. It takes a long time to heat up but it also takes a long time to cool down. I've personally experienced power outages of several days and still had a slab temperature of 55 degrees!

When people ask me about the lack of freeze protection I often reply with a question of my own: "What do you do now when the power goes out in your home? Do you drain your entire plumbing system? Your well pressure tank?" The answer is invariably, no.

The open system is not for everyone and you will also want to check your local codes to make sure it's allowed. Contrary to popular opinion, many states DO allow this type of system but others will want you to keep the heating and domestic separate. This is a design that Radiantec has used for 25+ years. We know it's safe and we've never had a single issue with any health issues associated with the use of this type of system.

 

[bmxdarcy]

Thank you for the reply.

I would like to heat both the house and the attached garage with this means, so freeze and stagnation issues should be null.

It seems in practice these systems are in fact safe, but I wonder if those that are arguing these systems are not safe propose that the water will be moving slow enough through the floor to allow for bacterial growth? The floor is sometimes divided into zones and the piping is typically larger than the inlet from the street, correct?

 

[Jackfre]

Sure they are safe...until they are not! You are looking for "cheap". I have done plenty of open loop Hydro-air systems, but I just don't like open loop with a slab...to much volume. Can it bear done? Sure! Is it as good or safe as a a closed loop system? No! I would suggest that you look up Legionella Disease and read a bit. Also, how is your water quality? If you have pristine water quality your system will last. If not you have to treat the water. Value is a good engineering determinant of system design. Cheap is not.

 

[Radiantec Guy]

Jack makes some good points. A new home should always have the water quality checked and rectified if there are issues regardless if this type of system will be used. Hard water will impact your plumbing, fixtures, make it harder to clean your dishes, etc. Also, all water heater manufacturers will void the tank/heat exchanger warranty if you have hard water.

Legionnaires disease is a reportable illness meaning all cases have to be reported to the Center for Disease Control. If there was a single incident of illness associated with this system, we would know about it and would have been sued out of business long ago. 25 years and 10,000+ systems are a pretty good track record. But, like anything, proper set-up and installation are critical.

 

[bmxdarcy]

Thanks all for the replies.

I've read about Legionnaire's and open radiant systems in the closed contractor forums I mentioned. This is why I posted, because the evidence all seems to be hearsay. The one story I saw cited multiple times was actually about a contractor who got sick visiting his cabin without draining his hot water tank. He didn't even have an radiant system!

Depending on the source, (CDC, OSHA, etc.) Legionella grows in waters between ~55°F and 105°F. Some other sources say it can survive above 120°F, in which case if this is true I can't imagine an electric hot water tank is any less stagnant than an open system as described. Also, what about homes with 'Home-run' manabloc plumbing and hot water recirc loops? That water has been heated and cooled similar to an open system, except the water on the return line in a recirc loop is not necessarily flushed. How does Legionella not grow in those sorts of systems?

 

[roscoe2000]

That is a good point about the Legionnaires disease, since heating loop would be stagnant over the summer and early fall.

I'm puzzled at the use of 180 deg water to heat the radiant floor system since the mast of the floor is being used as a heat sink. 180 deg water would provide 160 deg floor temp using a delta T of 20 deg. That would risk over heating the slab and wasting energy. There needs to be a secondary loop which would allow a lower water temp for the heating, say between 50 to 70 deg.

 

[brewchief]

That is a good point about the Legionnaires disease, since heating loop would be stagnant over the summer and early fall.

I'm puzzled at the use of 180 deg water to heat the radiant floor system since the mast of the floor is being used as a heat sink. 180 deg water would provide 160 deg floor temp using a delta T of 20 deg. That would risk over heating the slab and wasting energy. There needs to be a secondary loop which would allow a lower water temp for the heating, say between 50 to 70 deg.

If you look at the pics in the links they show a tempering valve that the piping diagram does not. I would also want a tempering valve for the domestic hot water.

 

[Radiantec Guy]

Roscoe, with the ICC reviewed design, the radiant system doesn't stagnate in the summer. Every time domestic hot water is consumed in the house, the cold replacement water enters into the tubing to flush the system. This is a very important safety feature. See this schematic. Also, I'm not sure where the 180 degree water came from. Since we're using a water heater, the temperature will be somewhere between 110 and 130 depending on the heat loss of the home.

Brewchief, many state codes require that an anti-scalding valve is installed on the domestic hot water going to the fixtures. Even if it isn't required by code, it's something that should be installed in my opinion.

 

[roscoe2000]

Roscoe, with the ICC reviewed design, the radiant system doesn't stagnate in the summer. Every time domestic hot water is consumed in the house, the cold replacement water enters into the tubing to flush the system. This is a very important safety feature. See this schematic. Also, I'm not sure where the 180 degree water came from. Since we're using a water heater, the temperature will be somewhere between 110 and 130 depending on the heat loss of the home.

So you system is primary for DHW and floor heating is secondary. Meaning in theory the floor slab would be be cooled by the domestic water supply at all times until there is a need for Slab heating.



Or is something I'm missing

 

[Radiantec Guy]

Roscoe, with the ICC reviewed design, the radiant system doesn't stagnate in the summer. Every time domestic hot water is consumed in the house, the cold replacement water enters into the tubing to flush the system. This is a very important safety feature. See this schematic. Also, I'm not sure where the 180 degree water came from. Since we're using a water heater, the temperature will be somewhere between 110 and 130 depending on the heat loss of the home.

So you system is primary for DHW and floor heating is secondary. Meaning in theory the floor slab would be be cooled by the domestic water supply at all times until there is a need for Slab heating.



Or is something I'm missing

Roscoe, that is a good question. Any cold water for household use goes directly to the fixtures and bypasses the radiant system. Any time domestic hot water is used, the cold makeup water feeds into the tubing. This happens summer or winter and ONLY when domestic hot water is consumed. This is a very important safety feature.

Many will ask if this cools the floor. Keep in mind how many hours it takes to initially heat up the mass of a floor, especially a concrete slab. It can take as many as 8-12 hours to heat up a slab in some cases. Knowing that the average shower uses about 17 gallons of hot water, a washer about 20, running this little bit of cold water through the floor will have 0 impact on cooling it. What little heat that is taken away gets put back into your water heater so you're essentially pre-heating your incoming water. Other than a little bit of loss through the heat exchange process, there is virtually no net heat loss on the system.

There is one exception. If you're doing a very small area (such as a 6x8 bathroom) where all of the cold water is channeled into that area instead of being spread through the entire house, then in very high hot water demands, cooling of the floor may occur. If someone wants to use this type of system for a small area we usually have them go with a different system altogether or set-up a different way for the water to refreshed.

 

[Outback2013]

I am building a 26 x 30 garage with 9' walls. Installing Pex tubing and will heat the floor with hot water via a hot water tank. Set point will be 110F. I have had a similar system in my basement which was installed in 1991 and worked perfect all these years.

Question now is how much pex do I require. Building will have R23 walls, R50 ceiling, with 2" styrofoam SM under the floor with 4" around the 4' perimeter of the slab. Also will have SM between the slab and footing. Installing very good doors and windows and going the extra mile with proper vapor barrier sealing. Will installing the pex at 12" centers along the mesh be adequate?

I live in New Brunswick, Canada. Can anyone shed some light on this?

 

[Radiantec Guy]

I am building a 26 x 30 garage with 9' walls. Installing Pex tubing and will heat the floor with hot water via a hot water tank. Set point will be 110F. I have had a similar system in my basement which was installed in 1991 and worked perfect all these years.

Question now is how much pex do I require. Building will have R23 walls, R50 ceiling, with 2" styrofoam SM under the floor with 4" around the 4' perimeter of the slab. Also will have SM between the slab and footing. Installing very good doors and windows and going the extra mile with proper vapor barrier sealing. Will installing the pex at 12" centers along the mesh be adequate?

I live in New Brunswick, Canada. Can anyone shed some light on this?

Hi Outback,

For a project like that I would go with 3-300' runs of 1/2" Pex tubing with Oxygen Barrier. Make sure you do a pressure test before you pour. I estimate your heat loss to be around 21,000 btu so a fairly standard water heater could be a good fit for you.

You'll want to make sure you insulate the side and under the slab with rigid foam, either XPS or EPS. 2" is the norm but you may want to check your codes to make sure they don't have other recommendations. Some locations are now requiring 3" (R-15) below a heated slab. Pagees 5-6 of this manual will give you some ideas about how to insulate.

Good luck with your project!

 

[Outback2013]

Thanks Radiantec Guy.
What spacing would you recommend with the pex? I seem to have read that tying the tubing to the 6" mesh at 12" centers is the common design. What are your thoughts on this?
Also, I see some tubing tied to the mesh with the total depth of the slab on top, and others show a spacer between the mesh and tubing which would place the tubing closer to the floor surface. With this approach, I wondered about the cement guys trying to do the pour. It would seem to be much harder for the guys to step on the tubing?? Or maybe the spacers are under the mesh/tubing??

I just viewed pages 5-6 as you mentioned. Actually, the diagram on the middle of page 6 shows exactly what the proposed floor will look like. I will have 2" of the styrofoam SM around the perimeter, under the floor, there will be 4".

 

[Radix2]

Thanks Radiantec Guy.
What spacing would you recommend with the pex? I seem to have read that tying the tubing to the 6" mesh at 12" centers is the common design. What are your thoughts on this?
Also, I see some tubing tied to the mesh with the total depth of the slab on top, and others show a spacer between the mesh and tubing which would place the tubing closer to the floor surface. With this approach, I wondered about the cement guys trying to do the pour. It would seem to be much harder for the guys to step on the tubing?? Or maybe the spacers are under the mesh/tubing??

I just viewed pages 5-6 as you mentioned. Actually, the diagram on the middle of page 6 shows exactly what the proposed floor will look like. I will have 2" of the styrofoam SM around the perimeter, under the floor, there will be 4".

I think the best method is to staple the tubing directly to the foam, when complete lay the mesh on top. It keeps it a bit into the concrete, keeps the tubes down. My installer did 5000 feet of tube in a half day.

I did the same 2" 4" perimeter insulation. 1 foot spacing except in a room with a ton of windows where it was closer. Also critical is the vertical insulation to below the frost line. More important than under the floor or in the walls. I think you have it all covered and it will work well.

 

[Radiantec Guy]

Thanks Radiantec Guy.
What spacing would you recommend with the pex? I seem to have read that tying the tubing to the 6" mesh at 12" centers is the common design. What are your thoughts on this?
Also, I see some tubing tied to the mesh with the total depth of the slab on top, and others show a spacer between the mesh and tubing which would place the tubing closer to the floor surface. With this approach, I wondered about the cement guys trying to do the pour. It would seem to be much harder for the guys to step on the tubing?? Or maybe the spacers are under the mesh/tubing??

I just viewed pages 5-6 as you mentioned. Actually, the diagram on the middle of page 6 shows exactly what the proposed floor will look like. I will have 2" of the styrofoam SM around the perimeter, under the floor, there will be 4".

For what you're proposing, 12" spacing will be fine. I personally like to err on the side of caution and put in more tubing than is required because the tubing is relatively cheap. If you're doing a 5,000 square foot shop then cost becomes more of a concern.

Stapling the tubing as Radix2 suggested is a good practice and many people do it. If you decide that you don't want to go that route then go ahead and attach the tubing to mesh using zip ties. The mesh usually gets pulled into the center of the slab during the pour by the concrete workers. Because of this, I typically like to add another layer of mesh over the top to add a layer of protection for the tubing.

 

[Outback2013]

Gents, thanks for your help with my previous questions.

I am going to try to post this again as I think I just lost my pump inquiry.

I am now trying to come up with a piping plan for two floors:

1. Present basement floor consisting of two 400 sq. ft. zones, each controlled by a zone valve with one old Grundfos pump.
2. The next floor will be my garage which will be 780 sq. ft.

My question is should I have a second pump for the garage along with the existing pump both being fed from one common header or one larger Grundfos pump feeding both the basement and garage. I see Grundfos has a 3 speed pump which could be run on single speed for one floor then bump up to 2nd or 3rd speed if both areas are calling for heat at the same time.

With all the information out there, I can't seem to find exactly what I am looking for. Can you shed any thoughts on this?

Thanks