I just built a new home and installed radiant heat in my basement floor.  I have prodex under concrete insulation with 1/2" pex about 1 1/2" up off the insulation.  This is in a 4" slab. I have a 40 gallon hot water tank with a taco 007-f5 pump on a closed loop.  I have 6 runs at 300' this is fed by 3/4" line.   The water going in the slab is around 140 degrees.  I don't know the exit temp is, but it not hot at all.  I can not get the space above 70,  so do I need to buy a bigger pump or split my zones up on multiple pumps.  Like I said this is my first go at this.  Any ideas would be great because I have my garage done the same way but not plumbed in yet.

Every good radiant floor heating systems start with a room-by-room ACCA Manual 'J' heat load performed by a qualified radiant floor designer or experienced engineer using dedicated software purpose-built for the task. If we new where you were, size of the space and the heat load-determined by climate, building features and use--we could better help.

If I understand you correctly you describe "Prodex" as insulation. On this matter you are mistaken and it may have been your second and most serious mistake.

All those professionally experienced with radiant slabs know that "reflective" insulation such as the "Prodex" you refer to, does not reflect anything when buried under a slab. It works as a conductor and offers precious little of any resistance to downward heat flux.

http://www.healthyheating.com/Page%2055/Page_55_o6_bldg_sys.htm

Hopefully you have some real insulation, not bubble-foil, at the perimeter and the system may heat the ground to equalibrium at which point the heat delivered to the slab and thence to the ground below will start upward to the living space.

The temperature you are delivering to the slab is more than I use on my upper floor designs with 3 or 4 times the load of the typical basement, but your water heaters' output may not be sufficient to maintain the requisite btu's to match the load presented. L

More likely you are experiencing the same delay we encounter when starting up a large commercial radiant project without insulation in the field of the slab i.e. charging the ground for the first time in the fall. The same is true of heating a pool as the initial load is substantial and the maintenance load significantly lower.

Thank goodness you raised the PEX in the slab, or all would surely be lost!

I have 4- runs @ 300' of 1/2" pex-al-pex (though I have 2" XPS under slab) fed with 3/4" copper. My return water temps basically mirror the room temp. Supply temp is 100*F.

When I first started my floor heat I had the 007 pump. Water is moving pretty slow so the cement is sucking all the BTU's out of the water. So that's why the water comes back so cool.

I switched to a Grundfos Alpha pump and it helped a little.

I agree with Badger hopefully the slab will eventually catch up and begin sending heat upwards.

How long does the supply temp stay at 140*F? I'm guessing not very long.

Chris

Let it run continuously for a few days to heat everything up. Then see where your are at in terms of capacity.

You didn’t indicate your location (or any other specifics that would be helpful), but you likely have inadequate under-slab insulation. This is likely resulting in much more than the 5-10% maximum recommended downward heat loss.

Furthermore, 300’ is about the maximum recommended circuit length for ½” PEX. This is likely resulting in high PEX hydraulic friction which requires increased pump head to achieve the required flow rate to achieve the recommended circuit temp drop (typically 15-20F). It sounds like your circuit temp drop could be as large as 60F…very bad…

To overcome this poor design, you may need a higher capacity pump or a pumping scheme that will provide BOTH more flow rate and more head to overcome the resulting increased PEX hydraulic friction. A parallel pump arrangement adds twice the flow rate of the single pump at the given head. A serial pump arrangement adds twice the head of the single pump at the given flow rate. Alternatively, you could significantly increase your heating cost budget and also plan on waiting several days every time you want to change your indoor temp like others have suggested. Who did your HR system design?

Did anyone do a proper heat loss analysis? If not, how do you know if your water heater has the BTU heat capacity to heat this space?

It's bordering on criminal that they advertize Prodex as R16. ( http://www.insulation4less.com/TechnicaldataPopUp.aspx?ItemId=1 ). In the fine-print by the aterisk it states:

"Parameters of test: 24-inch on center 2" x 6" wood assembly. Roof application. Test method ASTM 1116. Airspace of 2.64 inch on each side of product. Heat-flow direction down. Interior side of product exposed."

That's exactly how you built your slab, right?

The slab rests on 2 x 6 joists, with the Prodex installed 2.64" below the slab, and 2.64" above the dirt/gravel. doesn't it?

If not, it won't come anywhere near R16.

If it's resting on dirt or gravel with the concrete contacting the top you'll be lucky to get even R2 performance out of it- probably more like R0.5 if the weight of the concrete compresses it at all, maybe R1 if it doesn't get compressed, given that it has 13/64" of closed cell polyurethane.

Well, maybe the design is intended to heat the neighbors floor too… What is criminal here is that this is a new home that needs higher than 85F supply temp.

More questions… Do you know the total heated floor area and the installed PEX spacing? What is your location or do you know your outdoor design temp? Do you know the required BTU heat gain that you require this 6 circuit system to generate at the design outdoor temp? Are you using circuit balancing values and what are the approximate valve settings (hopefully not nearly closed)? Do you have a manifold that shows the circuit flow rates and, if so, what are the current flow rates? It would also be helpful if you could better determine your current circuit supply/return temps. With this info, we should be able to make better recommendations.

Your loop lengths are well within specs. The pump is not undersized. If enough heat is generated and not coming back to the heat source resulting in the heat source cycling off on temperature, then you have enough pump, i.e. enough. In residential work it does not pay to get hung up on Delta T. Many of my systems are designed for much higher Delta T's, which will not effect comfort unless your radiant floor PEX patterns are poorly laid out--the reason we provide CAD drawing with our designs.

It would appear the OP froze to death and we will never learn who designed this brilliant system...

NOT "well within spec". 300' is right at max spec for a properly insulated slab...which this is NOT.

Yes, you can design to greater delta T than 15F and you can also design to a greater PEX spacing than 12"...and if you never walk barefoot on the floor or you have old, dead feet, you will never sense the temp gradient.

Most every store that sells PEX will provide a CAD drawing free of charge. And for an inexperienced installer, a good CAD drawing is helpful. However, an experienced installer doesn't need one and good CAD drawings that actually adhere to proper PEX placement principles (see John Siegenthaler PE, Modern Hydronic Heating) are not very common either.  The DIYers that use our free HR floor heating system design software just get their free CAD drawing too when they purchase their PEX.

As BBMN suggests, a Taco 007 and 6 3/4" 300' loops in parallel will deliver far more btu than a 40 gallon water heater will produce. Do not just add more pumping.

The OP will likely need BOTH more flow rate and more heat source capacity to overcome the massive heat loss created by this inadequately insulated slab. This will be another expensive HR system to operate and very challenging to control. Really not all that different from this other recently reported situation:

Radiant Heat Insufficient in Den

When you have to accept delta Ts higher than 20F and supply temps higher than 100F in new construction, you know you have an inefficient HR floor heating system design. When the designer or installer tells you this is okay, you know you chose the wrong designer/installer.

The real concern I have with this design, is that the OP may be have been painted into the corner of the design envelope... Near maximum circuit lengths (OP stated 1/2" PEX and NOT 3/4" PEX Jonr...) will limit how high the flow rate can be increased before the required pumping head becomes excessive. A higher supply temp can compensate for this, but the delta T may still be excessive (which is not good for barefoot comfort...for those who care about such things) and high supply temps are also inefficient and can be very costly.  Without knowing the required HR upward heat flux neccessary to meet the living space heating requirements, we can't provide better recommendations.