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Community and Q&A

Hydronic in-slab heat for basement

beaubristow | Posted in Mechanicals on

I’m remodeling a 1700 sq ft basement under a single story 1959 ranch house in Nashville, TN (zone 4). The slab was so poorly done that we decided to cut it up and re-pour. This will give us 4-5 extra inches of basement headroom, around 7’11”.

I’m planning to install hydronic in-slab heat for the entire basement. There is already an HVAC system for the main floor and the duct work is in the basement. I think a few registers will handle cooling in the basement in the summer. I assume we can always close off the vents in the basement in the summer if it gets too cool.

I assume the hydronic in-slab heat will handle basement heating in the winter and we won’t need a split system. I like the idea of having in-slab heat. I know it won’t feel warm to the touch, but I assume the floors will be much more comfortable than they would have been otherwise.

I have two questions:

1. Am I overlooking anything with this approach?
2. If we had to choose between 1″ EPS under the slab or 2″ EPS but with 1″ less headroom in the basement, would it be okay to use 1″ EPS? The basement is walk-out for about 30 ft on one end of the house. The rest is below grade (grade ranges from 6-8′ above the slab around the rest of the house).

Much thanks.

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  1. user-2890856 | | #1

    Beau ,

    1" will work but 2" would be better . Headroom is huge though , the heat loss would be minimally impacted by the additional 1" .

    A heat loss done for the basement should definitely be performed to determine supply water temps , surface temps and what , if anything you would need for a source . I use and often recommend Barrier X5 slab insulation for slabs because it will have less seems , has vapor barrier and adhesive in one product . Material only is R5 and within an assembly that has 6" stone below the R value is 10.3 .
    What type watwer heater do you presently use ? There is the opportunity to use the water ehater as the source for your space heating while still keeping it separated from the domestic hot water . This makes these jobs doable many times as opposed to purchasing and installing a totally independent system .

    Is the basement to be one large space or will there be walls and individual spaces ?

  2. beaubristow | | #2

    We are planning on 4 zones. (See attached plan. We decided to combine the living area and laundry/stair zones.) We have a gas tank water heater currently in the mechanical room that will supply 3 full bathrooms when the remodel is complete (2 on the main floor, 1 in the basement). We are planning on using that water heater in an open direct system for now, but I'm using Wirsbo HePEX which has an oxygen barrier and is rated for potable systems so we can potentially switch to a different heat source in the future. Thanks for your advice!

  3. user-2890856 | | #3

    Beau ,

    Did someone tell you Uponor HePex is rated for potable systems ? If that is the case you have been misled . HePex is not approved for domestic or potable applications . Printed right on the tubing is , "Not for potable use ".

    An open direct system is quite possibly the worst application that can be installed . Open direct sends cold domestic water through the entire heated panel which robs stored heat from same . Hardly energy efficient .

    You'd be much better off installing something like a Taco X pump block or 2 circs and a flat plate and doing it right .
    As far as switching to a different heat source later , I ask why ? You can get a modulating / condensing water heater and change it out with the heater you have now or later without any headaches .
    Using a water heater for a radiant system makes sense , that mass let's one avoid lots of problems also .

    What room temps for what zones are you wanting to achieve ? How will the basement be zoned , what rooms on what zones ?

    Please disregard the thought of an open direct system , it is a bad idea and I have worked on many of them to the extent possible where the poor unsuspecting folks who bought and installed packages were not happy . Beware internet companies with orange looking home pages from a New England State , you'd not be the first nor the last unwitting consumer .

  4. GBA Editor
    Martin Holladay | | #4

    There are several issues here.

    1. You live in a relatively mild climate (compared to the northern sections of the U.S.), and your basement shouldn't need much heat. If you already have a furnace or air-source heat pump, with ducts in the basement, that system is the logical one to use for heating.

    2. If you want to improve the comfort of your slab, the most important thing you can do is to install an adequately thick horizontal layer of rigid foam insulation under your slab. Two inches of foam with no hydronic tubing will do more for comfort than 1 inch of foam and hydonic tubing.

  5. beaubristow | | #5

    Richard - Here is what I've read regarding HePEX and potable water:
    Thanks for bringing the Taco X pump to my attention. I'll definitely be including it in my system now. There is a pdf of my zones and floor plan attached to post #2 above. Let me know if you can't access it and I'll repost. I'd like to have winter temps in the 70-75 degree range for the basement.

    Martin - My concern with using the existing heat pump to heat the basement is that it would be based on the thermostat on the main floor and might not heat the basement adequately in the winter. Am I overlooking something?

  6. GBA Editor
    Martin Holladay | | #6

    Is there any insulation on your basement walls? There should be.

    Assuming that your basement is insulated, and that your basement includes forced-air ductwork connected to a heating system, it would be highly unusual for your basement to be cold. In most homes -- even homes in colder climates than yours -- this type of basement stays warm without any supply registers at all.

    If you install some supply registers in the main trunk duct, you can make the basement as hot as you want.

  7. user-2890856 | | #7

    Beau ,

    Installing some registers in the main trunk is bad advice . If the ductwork is balanced and right sized now (unlikely) it will in all probability get worse , or it could get perfect . zoning will not be possible without spending much more money than installing the tubing . You are on the right track .
    Martin is not thinking of the ability for later change , system upgrades . Maybe someday you'd think of getting rid of the heat pump and doing something else . Maybe that something else requires water , you'll have that ability . He may also have overlooked the apparent possibility that there will be a handicapped person/s occupying the space and that their systems may be compromised and sense the "feeling of heat " differently than others .

  8. GBA Editor
    Martin Holladay | | #8

    Richard is correct that adding registers to the trunk line requires airflow balancing work (usually accomplished by adjusting existing dampers, assuming that the original forced-air system was designed properly). All kinds of other problems are, of course, possible.

    Who knows? If the existing forced-air system is poorly designed, or lacking dampers, or already responsible for comfort problems, all bets are off. When in doubt, consult an experienced HVAC contractor.

    All of that said, most basements that include forced-air ductwork will be warm during the winter, even without registers, as long as the basement is relatively airtight and well insulated (as it should be).

  9. Dana1 | | #9

    Whether the slab is used for a radiator or not, put at LEAST 1" of EPS under the slab, along with an EPDM or polyethylene vapor barrier. Without the sub-slab foam the temperature of the slab in summer will be below the outdoor air's dew point, resulting in a higher moisture content and the "musty basement" smell.

    As Martin suggests, if both the basement foundation walls & exterior wall are insulated, and the slab is insulated, if there are ducts & air handler also in the basement the parasitic losses to the basement may be enough to heat it to a reasonable temperature without additional air flow or thermostatic control. The difference (if any) to be picked up by the radiant floor may be vanishingly small. This will be especially true if any supply ducts routed in the basement are not insulated. The losses will be primarily on the walk-out side, and you should at least run an I=B=R type load calculation of those losses to understand just how small or large that heat loss really is.

  10. dinnerbellmel | | #10

    We live in Upstate NY and have radiant heat for our basement slab. If you are going to be spending much time in the basement and use it as everyday living space then there is nothing better than radiant heat. Being in a much colder climate than yours, we insulated both the slab and walls. We used Crete Radiant Panels with 2" of insulation under our slab. Unheated, our basement stays in the high 50's to around 60 degrees even on the coldest days so the delta isn't that great to get it heated to around 68-70 and it seems that the large mass maintains that temperature fairly efficiently. Again, comfort was the main goal for us. It looks like you have a couple bedrooms and maybe an in-law suite in your basement so that may be your concern as well?

    I do agree that being in TN, if your space is well insulated then you may find cheaper or more easier ways to heat your basement than radiant, especially with the up-front costs.

    Good luck...your design looks great BTW!

  11. user-2890856 | | #11

    Steve , See my first post . Load calcs are always the first step .

  12. user-2310254 | | #12


  13. Expert Member
    Dana Dorsett | | #13

    +1 on load calculations being the first step.

    Without the load numbers it's difficult to determine out the most appropriate solutions for the load. Radiant floors are nice, but only "worth it" if they're sufficiently above the average room temperature to matter. In a location with a +16F outside design temp the peak heating load isn't going to be very big, even for a walk-out (assuming you keep the doors & windows closed when it's +16F or cooler outdoors. :-) )

    Your deep subsoil temp in Nashville is ~60F, so even with 1" of foam the slab is going to be reasonably comfortable even without the radiant, but without the foam the slab would be cool enough to be a mold issue in summer when the outdoor dew points are averaging in the high 60s.

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