How much pex for inslab heat.
 

!,200 sq ft house, hope to put in the floor next season. Will be 2 inch concrete, planning 2 0r three loops,
1 living room,
2 2nd bdroom, bathrm, & utility rm
3 master bed.
May combine 2 & 3 since they are adjacent and take up about 1/2 the house.
House is pretty well insulated & about 1/2 bermed on 3 sides in Zone 4.5
Plan on using 1/2 pex. May use outside wood boiler, or nat gas hot water heater for heat.
1000 feet of pex is not that pricey so am wondering what experience there is here to indicate total requirement for 1.200 sf.
Any links to sites that will calc it, would be appreciated.

nib

I used 1 ft centres for 5/8 ID tube as that was the tech available. 1/2 pex. will be perfect on 1 ft. centres. So for your calculations 1200 Lin ft. and really its quite in-expensive. Are you planning a slab build or a gyp-crete overpower (self level) on subfloor.

Don't over zone, 1 thermostat controlling the slab temp. and 200-250 ft loops.

I've built mine on slab and heated floors are awesome. Some here have eluded to in-floor being expensive but I've ran the numbers for other requests and they are less than duct-work especially if your doing the grunt work.

If I were to do it again I would run two pex systems 1/ for your outside wood or gas fired boiler and the 2/ for solar hot water. This saves the cost of a heat-exchanger and efficiency losses. We have solar hot-water heating the floor and some yrs it supplies 1/2 our heat load. The solar hot water would heat your floor directly AWESOME!!!

Randen

nibs

Where are you located? The total heat load, slab thickness, etc are all dependent on how cold you get!


Steve

Your local temperatures will have a lot to do with your results.

2" slab is pretty thin. Spacing your PEX at 12" centers will not give you very even heating of your slab.

If your location is such that heating efficiency is not critical, it won't matter much. If heating is a serious issue, you will want to be much smarter about your project.

-AC

"House is pretty well insulated" indicates an existing house, so probably an over pour on a wood subfloor?

"Zone 3.5" gets pretty cold a times.

I would recommend R20 under subfloor. Bond-breaker plastic on top.

1/2" O2 barrier PEX-A on 8" centers (to minimize hot/cold bands), so 1,800' in 6 loops (300' max per loop) with Sioux Chief copper manifolds with 3/4" pex feeds, one manifold of each pair needs to have the 1/2" pex ball valves to allow air to be pushed out by water, 1 loop at a time).

Clamp pex down with 1 screw clamps as it will try to float up.

3x thermostats controlling 3x 3/4" zone valves, 1 Grundfos Alpha pump moving all the water.

The floor will be slab on grade,
from the bottom, waterproof membrane, 2 or 3 inches of epscrete, 2 or 3 inches foam insulation, pex piping, and 2 possibly 3 inches of fiber entrained high strength concrete.
Am building in zone 4 and much of the house is bermed, we have been
making tilt up ferro cement walls for much of the exterior and most of the interior walls. All the walls are on their own footings on 2 inches of foam.
AC I have no problem making a slab 2 inches thick with a compression strength of 6000 lbs, using high cement content fly ash, fiber and low water content. & have worked in construction for about 40 years starting as a laborer, then carpenter, then foreman/superintendant then consultant.
My biggest problem is that in my mid 70's am not as spry as once was.
I hope to get the second half of the house closed in next season and if we are able may start laying the floor. You seem to be quite critical of some of my posts, please recognize that I have at least a year to finalize my floor design so don't worry.

I do not worry about your slab.

My comment relates to the significant thermal variation in a two inch slab that will result from 12" PEX spacing.

If you are interested in a very high efficiency floor, you should reduce the spacing.

At your age, efficiency will never pay for itself, unless you take into consideration benefit to subsequent owners.

-AC

P.S.: Don't whine about your age... I'm older than you.

"I'm older than you."
Then act your age and drop the insults.

The tiny amount of insulation used in the US building industry is amazing from a European perspective.
In Europe nobody would consider putting less than 150mm of PUR insulation beneath a heated slab and even unheated, most of that would be required just to meet building regulations.
Our own standards took a long time to get to this level as the construction industry is conservative and slow to change.
Why not learn from our mistakes and start insulating to a high standard?
Surely foam is cheap?

If you have to heat your house like we do (in the Netherlands 55°North), I would suggest to make the distance between the tubes much smaller. This causes a lower required water temperature in the tubes. In that case your floor is ready for future use of a heatpump. Or if you heat with gas, the efficiency of your boiler is increasing, because of more condensation from vapour in the flue gass. I have 6" spacing (built in 1987) but with the heatpump we have since 6 years, I would have liked 4"spacing. Every °C that the water in the floorheating system can be reduced, gives me around 2,5% extra efficiency of the heatpump.....

Thank you for the input. It is great to get the positive feedback. sounds like I will be putting in 2,000 lin ft.
Is it better to stay with 250 ft loops?
I was thinking that perhaps a good way to circulate the water is using an RV water pump for each room, (3 pumps) each on its own thermostat, less than $100 per room and inexpensive to swap if/when they fail, they are very long lived and can handle reasonably hot water.

Nibs -

I would not have thought of RV water pumps being especially reliable and long lived compared to the "bumble bee" and other highly efficient water pumps. I have found that most RV infrastructure equipment does not seem to be especially reliable and certainly NOT inexpensive.

Don't know the cost comparisons, but any water circulation pump is easy to replace.

Yes, the use of a common loop length means less balancing issues.

I would look into the costs/reliability of: one pump, one manifold and thermostat controlled values on each loop. Each of these valves has a small gate valve built in so you can fine tune water (heat) flow in each loop.

Equal lengths of pipe can can different resistances due to numbers of turns, etc. But this small effect can be handled easily with the gate valve.

Less large current wiring (one pump), higher operating efficiency with one pump and simplicity of using inexpensive and incredibly reliable zone values (See Uponor for some nice stuff).

Great project.


Steve

Have not researched other pumps, but am very impressed with the Shurflo style pumps, they operate on 12V, we will have a battery bank for solar so 12 v is no problem and if we use mercury switch thermostats can drive them directly with no 110 interface other than a backup 110v charger.

Nibs - I applaud the use of 12 volt solar and batteries . . . . but.

The use of a 12 V water pump will be far more energy costly compared to an equal flow 120 V AC pump. Ohm's law dictates that as power = current squared times resistance (i**2 x R).

Compared to a 120 V circuit, a 12 V pump must use ten times the current to get the same power (P=Ei). Assume the winding resistance is about the same for both. It is the square of the current that kills the efficiency. That is why higher voltage water pumps are more efficient than lower voltage ones.

These hydronic pumps are going to be on a lot of time so even small (and the above is not small) decreases in efficiency will cost you a lot over even a few years - a huge amount over ten years. Do your homework carefully.

I am a BIG advocate of rational use of PV and stored power when applicable. If you were off grid, then I might (and this is a might) suggest a 12 V pump - but it is a very inefficient way to pump water. Most off gridders all have a 12 to 120 V AC inverter for appliances/applications that work best on 120 VAC.

I might suggest the use of the 120 V AC mains for this hydronic application and the use of a PV system and inverter (highly efficient) to create the 120 V power you need for this application.

As a PV advocate, I appreciate the used of stored power, but the 12 V pump inefficiency, the cost of the 12 V battery, its upkeep/replacement and the associated costs just make this a 120 V application.

Hope I am not disappointing . . . .


Steve

European Advice
 

I think that this thread may have ignored a couple of very important contributions to this discussion.

Energy prices are currently low-ish, but as a very wise machinist said to me, "we are only a camel-fart away from skyrocketing energy costs."

Housing has a very long life, compared to cars, etc. so a house should be built not for today, but for 50 or 100 years from now.

Right now, Europeans are paying far higher prices for energy, in other words, regarding energy, they are living in the future, and we in North America would be well-advised to pay attention to their practices.

Fionn (Ireland):
6 inche PUR (rigid foam) insulation under slab

Zwerius (The Netherlands):
PEX spacings 6", preferably 4"

These seasoned EcoRenovators are using very thick foam (6") under the heated slab not just to support the foam industry, nor are they tending from 6" PEX spacings toward 4" spacings so that more PEX is created in the world.

They know that they will require less energy by doing this. This means less money required to heat and if you care about our impact on the earth, CO2 generation will be reduced.

We should not only thank them, we should act on their good advice.

-AC_Hacker

Steve, the main reason I thought about 12V pumps is familiarity, have been using them for years and know them well. Also they are inexpensive I have several, I buy them at yard sales, the weak link on them is the pressure sender, not a prob with heat loops; to be able to switch them directly by thermostat is handy as well.
Will research pumps over the winter probably.
Envy your southern insolation, we hardly get enough solar to pay for itself this far north. With grid tie inverters becoming so inexpensive will likely not build a large battery bank.
Since I have virtually no experience with in floor heat, many of my ideas will be changed as the build time approaches, as ideas flow from this and other sources, my thinking becomes more flexible, and the design evolves.
Am carving an elephant here, so we just keep knocking off the bits that don't look like the elephant.
Tony.

Tony - great analogy of carving the elephant! Had not heard of that one . . .

Spent time before you build. It has low cost, gets your mind "around" the problem and by the time you implement your ideas, they are well versed and worked out.


Steve