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Brad Georgal
Radiant Heating questions

I have just purcahsed an outdoor wood furnace and am installing radiant floor heat in my 1820's farm house.I think I have a decent handle on it but in a continuing effort to keep within budget I am in a dlimemma of how many loops I should run. I have 6 rooms on the first floor and was initially going to run 6 loops of 1/2 pex in the basement joists. Each run from send manifold to return manifold is between 190 and 280 feet of pex. The manifold I was going to purchase has flow and temp gauges. I now see a good deal on a 5 port manifold with the same specs and could combine two areas on one loop. so my question is : what length of loop of 1/2 pex is efficient. Can I run 2 loops of 280 feet, one loop of 213 feet, one loop at 196 feet and one loop at 376 feet (originally 2 loops at 190 and 180 feet). this would save me about $200-$250 if I could buy a 5 loop manifold and combine the 2 short loops as described above. Is this still efficient or should I go back to the original plan of 6 loops??? any help would be greatly appreciated. thanks Brad

Re: Radiant Heating questions

Hi Brad,

Sorry I didn't answer sooner.

I have a limited knowledge of designing and installing under-floor radiant, but there is lots of help on the internet for you.

The maximum length of a 1/2" coil of pex has been 300' or less for some time.

The reason for this has to do with the limited number of gallons of hot water that can flow thru 1/2" pex, as well as the amount of heat loss that takes place in longer lengths.

Have you gotten the help you need in designing this radiant system ???

Staple-up under floor radiant requires that a heat loss calculation be done, and also a calculation of the amount of heat output that the total pex loops you install will put out.

The hot water that goes thru the PEX piping in
Radiant systems is designed to operate at ~120 degrees
F., so that the flooring will not be damaged by the higher temps coming out of the boiler.

Since boilers usually put out HW at 180 degrees, a 3-way mixing valve has to be installed to get the lower temps going thru the PEX loops

It is also strongly recommended that the PEX loops be roughly the same length as each other, as much as possible.

Another big factor is the amount of insulation inside the exterior walls, and the condition of all the windows (double pane, storm windows, etc).

If you need further help, Google "radiant heat forum" to get on-line forums that concentrate on this specialized subject.

The "house needs" site offers some preliminary tips on how to proceed; there are also initial heat loss calculations you can use below.

The manufacturers of radiant manifolds all have a technical help line, and most of them have an 800 number you can call to run by with them the basics of your system.

Sites for Wirsbo (Uponor), Stadler (Viega) and Slant/fin are good starting points.

The Slant/Fin site also has an excellent free heat loss calculation pgm.

You should also have some thin aluminum sheeting that you can staple over the oxygen barrier pex so the heat is reflected upwards.

"Modern Hydronic Heating, vol 2.", by John Siegenthaler is in most public libraries, and has several good chapters on radiant.


Brad Georgal
Re: Radiant Heating questions

Thanks, this will help tremendously. Since I have posted this I have been to some similiar sites and have learned alot. You're right,300 feet seems to be the max for 1/2 pex. I will need a 7 or 8 port manilfod. I have thought about ordering the foil and will do so after reading your post. I have attached a couple diagrams I drew up last week for preparing my order for pipe and electronics. I copied exactly a central boiler diagram of what parts I will need but when I placed my order through another vendor I was told I can remove the "bypass" on the attached diagram and the 3 way zoning valve. Does this diagram of parts look to be efficient and effective????

I will keep this simple at first and have one thermostat and 1 zone. I have also attached my joist layout in my basement and each run will be between 200 to 280 feet of 1/2 pex. I will need to play around with the layout of pipe so I can have the send and return where I want them. Any more suggestions on how to run this pipe and if my home made schematic looks good.

Oh, you mentione temperature, I have received t other opionons. to run it at 140 degrees and even 170 degrees????

Thanks again,

Re: Radiant Heating questions


I had some difficulty reading your plan layout and diagram due to small size, but I assume it will work ok.

This looks like a lot of work and expense to do 5 rooms in one shot; I hesitate to recommend you plunge ahead now on such a large project.

Radiant can be done as a diy project, but in a first time attempt, a lot can go wrong.

There are a lot of fittings, valves, purge stations to eliminate trapped air, etc. that you usually don't see on a straightfoward baseboard or radiator install.

It may be a better stratgegy to focus on doing one room at a time, say a kitchen/bathroom; then iron all the bugs out of it during the coming heating season, then proceed to do the rest of the rooms based on the experience you've picked up.

What do you have there now??

Do you have radiators or baseboard from the previous winter that you can hook up while you climb the learning curve???

Baseboards or radiators are infinitely more simple to install and run, while you're gathering more info on your radiant install.

In addition to the physical task of stringing ~1500 ft. of PEX, and installing the reflectors, the manifold stations, you will need to install access template blocks, etc. that will allow you to force-bleed trapped air in the system.

There are many controls that have to be installed, the brass fitting connectors to the pex lines have to be leak-proof.

You may be able to find a radiant installer in your area (Yellow Pages, Heating Contractors) who will act as a consultant for a small fee, and will run you through the basic ideas a layout so that you avoid wasting time and money on improper installs on parts of your system.

The Siegenthaler book, mentioned previously is invaluable, as well as any other books you can get at your local technical school (there should be a technical school in your area that trains contractors on radiant installations & has relevant books on the subject).

The sites below have forums and technicians who frequent the sites; they may be able to recommend specific books or web sites that are helpful.

the radiant company site offers a free installation booklet (no obligation) that is helpful.

At heatinghelp.com, click onto "The Wall"; at HVAC-talk, click onto residential heating.

I've heard of some installations that allow high temp 140-170 degrees installs; as long as the high temp doesn't damage the floor and the PEX is rated well above 200 degrees, it probably can be done; higher temps usually waste fuel in the long run.

The basic concept of radiant is to pump as many heat btu's into a floor as possible over a long period to raise the temp of the ENTIRE floor structure up to 65 degrees.

Once this is achieved, the entire floor acts as a giant radiator, constantly emitting 65 degree warmth to the rooms.

Believe it or not, people feel very comfortably warm in rooms so heated.

According to the Siegenthaler book, 1/2" pex put out ~20 btu/sq.ft of floor space @ 120 degrees water temp.

When you do your heat loss calculation, you can use this info to do a rough calculation of how much heat output will be produced by the pex inside the floor, and how many btu's/hour are bleeding out of the house on a cold day.


Brad Georgal
Re: Radiant Heating questions

Thanks for all the helpful info. your links and knowledge has definitely helped me re think my plans and take my time.

I have an oil furnace with only 1 3'x3' forced air duct straight into the dining room to heat the entire house. I burn about 200+ gallons of fuel oil a month at $3/gallon so I am eager to fire this project up since wood is essentially free for me living on a 76 acre farm.

One of the most helpful things I learned is that I will place enough valves and bypasses so that if I have a problem with a certain area I can keep the furnace going and redirect the water back to the furnace while I correct the problem.

I'll start one room at a time and let you know how I make out.

Thanks very much for your help!


Re: Radiant Heating questions

I am new to this forum and also have a question on radiant heat. We live in CA and just purchased a 50 year old home in Pennsylvania. We are currently having work done before we move in next year. It has radiant heat baseboards on almost all the walls with a large standing register in the kitchen. Being from California we are used to forced air heat and air conditioning systems. We have had several heating contractors in to give us bids on new systems for this house. Several have suggested we keep the radiant heat.

My question is: What is the opinion of you folks on the advantages and disadvantages of radiant heat over forced air?

Thanks in advance for any and all responses.


Re: Radiant Heating questions


The responses you will find here are from regular respondents who are part of a forum, and have an interest in residential HVAC, or home heating and AC systems.

Heating systems in the U.S. are mostly divided between hot water systems (which includes HW baseboard, HW cast iron radiators, embedded pipe radiant, under floor pipe radiant, etc.) and forced hot air ducts, heat pumps, etc.

I think you may be confusing what you describe in your post as "radiant baseboard", which sounds like copper fin-tube baseboard convectors, and "free standing register", which sounds like a cast iron radiator, with radiant heat, which is technically pipes embedded out of sight in a floor that are more strictly defined as radiant floor heat.

The resident TOH Plumbing and Heating Expert, Richard Trethewey, has a strong preference for hot water/radiant systems.

This may be partially because he is based in the Northeast, where HW heating is predominant and a growing industry, but also, according to a book he wrote on the subject, because he feels it is much more efficient than forced hot air in the amount of heat that can be delivered to a home at the lowest fuel costs.

The theory states that it is much more effective to heat 10-15 gallons of water to 200 degrees and circulate it thru pipes and convectors than to heat large volumes of air in a furnace then blow it thru ducts and registers to heat the rooms of a house.

This has more importance in the northern, colder regions of the U.S. than milder areas of the West or South, where forced air heat pumps popularly double as AC systems.

Northern areas experience zero and sometimes sub-zero temps in winter, and that is no doubt why the heating contractor encouraged you to keep your present system.

I would agree that you should keep your present system if it is not too old and is working efficiently.

If you have no AC, many people in the northern U.S. are putting in mini-split AC systems; they require no ducting, so installation costs are low, and are very effective as AC systems.

Fuel costs are the dominant issue and topic of conversation in these days of unprecedented increases in natural gas and fuel oil (you will find both types of fuel used in Penn.).

Thus the focus is now on maintaining high-efficient heating equipment and increasing exterior wall and ceiling insulation to its maximum.

Please post back to advise if you have copper fin baseboard convectors, cast-iron baseboard convectors, and a cast-iron radiator in the kitchen.

For further info on forced hot water and radiant heat, Google "Richard Trethewey radiant hydronic", "Mark Eatherton radiant hydronic", "John Siegenthaler radiant hydronic", "Dan Holohan radiant hydronic" (google these phrases without the quotes).

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