Radiant Heat and Pex decision

[rotortuner]

Starting a shop build 68x60. Have looked over these forums for years and have always wanted the radiant heat. Im having the system designed and I have been talking to radiant tech. They are advising 7/8 Pex on 16" center. Ill be starting with R8 walls and ceiling. Coastal climate, Seattle area, most of our winter is in the mid upper 30's at night. Looking to keep it upper 50's during the winter. Any reason to go with 1/2" on 12" center? am I going to be wrestling the 7/8? Using electric. planning on ridged foam under the whole slab and sides. Pointers?

 

[bzinsky]

I think 7/8ths is normally used for snow melting outdoors.

Seems like more 1/2" would have higher surface area contact, for about the same amount of money or less, without the hassles of working with the thick stuff.

If you're using electric heat it doesn't matter, unless you're using a heat pump.

 

[rlitman]

If you're worried about wrestling with the PEX, get PEX-A, instead of PEX-B. The A type is more flexible, and less likely to kink (it is also the only type that can be repaired if kinked).

I used 1/2" PEX on a snow-melt system on my heated front steps. Aside from steps, with their tight loops, and perhaps really small rooms, 1/2" is not generally recommended for heating slabs.

 

[finn]

Smaller diameter will have more flow restriction, which means shorter loops and more pumps.

Make sure your tech is qualified to spec out large systems.

You don't get do overs once the slab is poured.

 

[rotortuner]

Thanks for the replies.

From all my research it seams like the likely place people mess up is with making too long of loops and too much restriction. he is saying 7/8 with 300ft loops which from my research seams quit conservative, I think a lot of guys do 300ft in 1/2" and 7/8 can go out to 400. Not advertising for the sight but I submitted the info on the radiantech website and got a call. I guess they just provide a material list and hope you order from them. I may order some of the stuff but shop some of it as well. Has anyone used them for specing out a system?

A lot of the threads on here guys use 1/2" on one foot center but their slabs are smaller than mine.

Thanks for the heads up on the different Pex types. will look into those.

 

[seanc_mt]

1/2" on 6" center for two loops near exterior walls. then 1 foot centers for the rest. keep loops under 250' ordered all my pex A from supply house. cant beat their prices anywhere. i did the foil/bubble insulation under the entire thing. there is a huge debate about under insulation. my system works fine and Im in NW montana where the outside air gets to -40 F several times during the winter.

 

[rotortuner]

1/2" on 6" center for two loops near exterior walls. then 1 foot centers for the rest. keep loops under 250' ordered all my pex A from supply house. cant beat their prices anywhere. i did the foil/bubble insulation under the entire thing. there is a huge debate about under insulation. my system works fine and Im in NW montana where the outside air gets to -40 F several times during the winter.

Whats your square footage and about how much $ a month to heat?

 

[Firebrick43]

Starting a shop build 68x60. Have looked over these forums for years and have always wanted the radiant heat. Im having the system designed and I have been talking to radiant tech. They are advising 7/8 Pex on 16" center. Ill be starting with R8 walls and ceiling. Coastal climate, Seattle area, most of our winter is in the mid upper 30's at night. Looking to keep it upper 50's during the winter. Any reason to go with 1/2" on 12" center? am I going to be wrestling the 7/8? Using electric. planning on ridged foam under the whole slab and sides. Pointers?

1/2vs5/8vs7/8 debate is moot. If you have 1/2 inch keep the loops under 250-300 and around 12" spacing. 7/8 can go larger spacing and lengths. Really no real advantage to one or the other just different ways to achieve the same results.

R8 is not enough insulation for a continuously heated structure, even in a mild climate. Insulation is the cheapest part of a building, why skimp and pay the oil/electric companies. Also high rate btu electric tankless boiler/water heaters don't have a good track record on durability. Many cases of constantly burning out elements and cracking element housings. Don't think a tank type water heater would keep up with your size of building of lack of insulation.

Radiant tech is one company that when doing reasearch for my floor I found a lot of unhappy reviews. They provide nothing a good book won't provide and have inflated prices.

 

[theoldwizard1]

You need more than R8 in the ceiling.

How much insulation under the floor ? I would go at least 2".

"You pay for insulation once. You pay for fuel every time the thermostat calls for heating or cooling."

 

[theoldwizard1]

Also high rate btu electric tankless boiler/water heaters don't have a good track record on durability. Many cases of constantly burning out elements and cracking element housings. Don't think a tank type water heater would keep up with your size of building of lack of insulation.

There is a BIG difference between a water heater and a boiler !

 

[yeldogt]

I have always used 1/2 PEX 6" on center using the 6" wire as the guide -- keep loops under 300 ...... supply to the outside. I don't know anyone who uses the larger pipe for indoor residential radiant.

The more loops (closer spacing of the PEX) -- the faster the slab will respond to the heat input. Think about it .. what heats the middle between the 12 on center? Answer -- nothing. That's why low temp systems can be even tighter 4" -- the PEX can only deliver so much heat per foot of tubing .. the greater the spacing the higher the temps required to get the same BUT's to the slab.

There is a difference --

Where there is less of a difference is when you do outside reset -- in that situation the system is never off and so the slab has a long time to equalize -- the still make hot spots in very cold temps as the water temp in the tubes must increase with fewer loops.

The OP is using electric -- so a condensing boiler is not providing heat. If he was using the loop's higher temp water would not be allowing the boiler to condense .. so would cost more.

The only place 12" spacing works is when something else is helping out --- like the aluminum in Warmboard -- in that case you are transferring more heat. Still have to watch the load on the building.

 

[Randy in Maine]

To be honest I would be hard pressed to come up with a better set up than my 1/2" O2 pex at 12" on center. I don't have any cold spots or hot spots and I am pumping in just warm water (120º water) through 4 x 300' loops in my 1200 square foot shop. My SIPS insulation is better than yours at about R-40 or better and I can almost heat it with a fart using a propane fired condensing boiler.

 

[Jackfre]

The size of your slab matters not a lot. What does matter is how you pipe the system to supply the grid. I'd go with 1/2" on 12 and do a good reverse return on the properly sized mains.

 

[Firebrick43]

There is a BIG difference between a water heater and a boiler !

Thanks Mr obvious, first time caller long time listener.

But they all still have high powered electric heating elements.

 

[rburke65]

rlitman.....Aside from steps, with their tight loops, and perhaps really small rooms, 1/2" is not generally recommended for heating slabs.
Do you have any reference for this statement?.......no......I didn't think so........

 

[rotortuner]

Appreciate the replies, you guys are confirming everything I have read in the past. 1/2" is really the standard and on the 1ft centers, a little closer on the outer edge where you loose the heat. Im starting to research better insulation. any recommendations on a "reasonable" good electric heat source for the water?

 

[79rallysport]

For electric heat I would go with a Steibel HydroShark tankless boiler or a Seisco SH series tankless boiler.

I have a Seisco SH09 for two years and it has done its job.

BTW, this is used in my 850sqft shop with 1/2" pex on 12" centers, 6" centers on the perimeter. Works good. Check out LoopCAD for a computer generated manual J and pex layout.

 

[rlitman]

rlitman.....Aside from steps, with their tight loops, and perhaps really small rooms, 1/2" is not generally recommended for heating slabs.
Do you have any reference for this statement?.......no......I didn't think so........

The OP is building a shop 68x60. Do the math on the number of loops required for 1/2". Then, when you realize the number of pumps and how ridiculous the manifold will look, look up a more reasonable size tubing for the job.

In a residential job, 1/2" is fine if you want 1-2 loops per room. Past a 2-car garage, it's not so fine any more.

 

[rburke65]

Well you said 1/2" is not recommended for heating slabs.

 

[forAK]

I've got 8 loops heating a 40x40 using 1/2" on 10" centers. One pump drives the whole thing. R-21 walls and roughly R-38 lid, slab is 5". I've only seen 1/2" used up here.

 

[finn]

His floor area is 2.55 times your 1600 square feet.

He's going to need several pumps.

My 48x75 has three pumps and 3/4" tubing. 16x60 isn't heated and one of the pumps is for the office/bathroom.

 

[bzinsky]

The OP is building a shop 68x60. Do the math on the number of loops required for 1/2". Then, when you realize the number of pumps and how ridiculous the manifold will look, look up a more reasonable size tubing for the job.

In a residential job, 1/2" is fine if you want 1-2 loops per room. Past a 2-car garage, it's not so fine any more.

He's probably need 15 loops if he uses 1/2 with 12" spacing

But OP said the guy only wants to use 300ft loops with the 7/8th's at 16" spacing, which is probably around 11 loops. Still a lot.

Probably go with longer loops with 7/8th's. I guess the ability for the system to radiate heat is not all that important with electric.

I'd definitely go with a heat pump (assuming he has to do electric) with that much sq footage.

 

[bowhuntr311]

My 40x40 is 4 loops of 1/2. I put 2inch rigid foam underneath. I would say most of my loops were 12in OC. Beginning of the runs went immediately to the outside edges so the most heat was delivered to the edges.

 

[rotortuner]

4000ft with 1/2" on 12" would be about 16 loops. Could break that into two 8 manifolds and probably wouldn't be so bad. Any recommendations for other designers out there?

 

[Radix2]

I have about 4000 sq ft. 1/2 pex, 3 zones (2000,1300,700 sq ft), 3 pumps. With zoning, you will break the huge manifold up anyway. I would definitely consider some zoning in that size of area.


2" foam under floor, 4" under outer 4'. R28 walls, R60 ceiling.

 

[yeldogt]

There is so much mis-information -- it's hard to know where to start. The tubes don't care what made the heat (you will from a cost basis). Give the tubes enough hot water -- and they will heat almost any building regardless of the insulation or lack of in most cases.

I know many who did not insulate slabs and spaced the tubes far apart and never turn the systems off -- they think radiant is great. They also don't know how much more they pay to run the system. Radiant can be comfortable in drafty buildings -- all you have to do is raise the slab temp. Many systems have oversized heat sources, so that is easy to do.

The key is to build it correctly from the start -- the more insulation you install, the lower the heat load for the building (forever). Lower heat load = less Btu's needed per foot = lower temp water = less $$ to operate. If you want quick response you have to space the tubing closer -- especially with a thick slab. Will larger tubing and or wider spacing work -- sure .. with higher temp water.

My SIP building is like being in a cooler -- almost any heat will keep it comfortable. I still use constant circulation in the system.

A few extra dollars spent upfront makes for a better system.

 

[johnnyradiant]

His floor area is 2.55 times your 1600 square feet.

He's going to need several pumps.

My 48x75 has three pumps and 3/4" tubing. 16x60 isn't heated and one of the pumps is for the office/bathroom.

Why several pumps I don't understand?
I've got an 82 unit apartment building halls are about 325' long 3 floors high run into and out of units is about 22' and there is about 20' of fin tube in most units, about 10 have 5'-10' more of fin tube. I also have about 220' worth of fin tube in common areas. All that water leaves and comes back to the boiler room on ONE pump.
I've got 1 boiler serving 2 houses with an assortment of different fin tube length zones, a 40 gal and 60 gal DHW makers, and 2 out buildings with about 2000 sqft of radiant. One pump gets me water everywhere and I use a little pump in one of my zones because I modified a zone from fin tube to a little bit of fin tube and some radiant - I needed a two working temps in that zone.

If you have more than one zone, and you plumb your system as primary secondary then a pump for your heat source (primary) and one pump per zone (secondary(s)). If the shop is on one thermostat (1 zone) there's not much need for primary secondary, depending on your target boiler temp range vrs your zone temp range, therefore 1 pump. If you start adding pumps to a single zone/boiler run your just asking the pumps to play off each other. Lots of places have multiple heating zones where the system is not set-up as primary secondary and they work without multiple pumps.

 

[finn]

Must be a really large pump.

I'll bet you're not flowing all that water through 1/2" tubing, either.

The issue is flow restriction with small diameter tubing in large systems. The head is greatly reduced if he stays with the 5/8" or 3/4" tubing.

 

[foleymech]

We do them all the time. 1/2" 300 ft loops grundfos Alpha pumps. Put the most insulation under the slab as you can afford. Tekmar house control link right into boiler 10vdc input.