Best solution for radiant heating fluid 1

[Hadrianswalls]

I have a question that has been brought to my attention regarding water quality applications in a closed loop radiant floor heating using PEX tubing and a tankless boiler in connection with a solar water heater.

Specifically, the discussion has been; what is the best unreactive solution that could be run in the system without potential leaching of platicizers in the PEX tubing and thus wanting a long lifespan of the tubing and boiler without corrosion.

The possible fluids that have been suggested are;

1. Tap water
2. Distilled Water
3. Propylene glycol and tap water
4. Propylene glycol and distilled water
5. Other unknown combinations.

Some have stated that distilled water causes leaching and is not good in PEX tubing. I do not know. I know that tap water has minerals that may build up but to what extent I do not know.

With longevity of the system in mind, may I ask your expert advice on what path I should take or what solutions may be best suited for use in this system?

 

[ccfowler]

Unless you can be thoroughly assured that freezing conditions can never occur in the system, a suitable "anti-freeze" solution should be used. Sorry, I can't advise you on the chemistry. Have you consulted the tubing manufacturer?

 

[RMDPE]

A typical radiant heating/boiler system uses tap water as it must have a connection to a water supply for make up water. Also a propylene glycol solution is used for freeze protection.

As far as protecting the boiler a rust inhibitor chemical can be used to guard against corrosion

Another thing that must be considered is the water temperatures used. A boiler typically produces 180F water. However the radiant floor system should only have a maximum temperature of 90F. A 4-way mixing valve must be used to obtain this temperature as well as to keep the boiler return water temperature at approx 160F otherwise you will overheat the floor slab and shock the boiler.

You should check with a manufacturer to find a local sales engineer in your area, they are very helpful in specifying and laying out the entire system.

 

[TBP]

The two main problems from a water treatment point of view are scale, and oxygen corrosion. In the vast majority of cases, the little bit of hardness and oxygen in the initial fill water aren't a problem. You'll have a tiny bit of scale formation, and an equally tiny bit of oxygen corrosion, but too small to measure. Problems arise if the system isn't tight, and make-up water is added regularly. This water brings in hardness which causes scale, and dissolved oxygen which will corrode ferrous components. Make sure that you use PEX tubing designed for heating systems (it has an oxygen barrier), as opposed to the kind made for domestic water supplies (no oxygen barrier). Regular plastic tubing lets atmospheric oxygen diffuse through the tube wall (even when buried in concrete), and will allow your system to corrode to death in short order.

If the system is exposed to freezing temperatures, then glycol is required. Bear in mind that glyol needs to be analyzed annually. It can degrade to a corrosive level and cause real problems. It's not a big deal to check and put in some additives to correct things, but someone needs to remember to DO IT. Propylene glycol costs more, but is non-toxic. Ethylene glycol is a lot cheaper, but is poisonous. If you're considering heating domestic water with the system, stay away from ethylene.

Most radiant floor systems run about 120*F max. This will give you a floor temp of about 85* to 90*F, which is the temp of your clothed body. Any hotter that that, and your feet will be too hot. You need to watch return water temps to the boiler, as you can condense flue gas in the boiler, which will be acidic. Unless you've got a boiler specifically designed to deal with relatively cold return water temps, you will require a blending arrangement to keep the water temp in the boiler up. I understand Viessmann have a model that will take -30*F glycol return temps from snow melt systems.

 

[davefitz]

To add to prior messages:

a) if no additive is used and it is connected to potable system ( so called combi system), the normal setpoint temp of 90-110F is the ideal range for nurturing legionella bacteria. Such systems are suspect by many local health authorities, and generally require complete flushing at least once a week, and must have the oxygen barrier on the PEX tubing, and generally should avoid iron or ferritic components as the iron oxide can be a nutrient to the bacteria.

b) if potable water is only for refill purposes, a code approved reverse backflow preventor or air gap is required to isolate the potable water from the heating fluid.

 

[Popeye45]

Another option for insuring a high enough return water temp to you boiler is to use "Primary/Secondary Pumping". You can get some really good information on this from Bell & Gossett's Bulletin No. TEH-775. This is a unique application which uses less operating energy than traditional methods with very good temperature control. It also gives you the ability to have different zone temeratures than would be possilbe with traditional piping and control.