[Greenbuilding] [BULK] Re: suspended slab

Lawrence Lile LLile at projsolco.com
Tue Sep 26 10:36:00 CDT 2006 

 We discuss the potential of radiant floors to breed bad bugs
occasionally here.  There are two schools of thought, both equally
valid:

1.  Isolate the radfiant floor from the domestic water supply. 

2. Integrate the radiant floor into the domestic supply, in a way that
there are no dead spots, the entire radiant floor system is refreshed
with cold water whenever the water heater demands water.  This is
accomplished with a system of one-way valves.  Think of it this way:
Does anyone worry about salmonella growing in the line feeding your
water heater in a standard non-radiant floor house?  No.  If that line
was ten feet long would you worry?  No.  How about 50 feet, or 100 feet?
No.  The radiant floor, in a non-heating season, becomes a 100 foot line
feeding your hot water heater.  The Radiant Company has some good
diagrams as to how this works.  

Either system can work safely, the second system needs to have some
careful planning to avoid dead spots that never get flushed out.  

I would recommend you dump your tank type water heater in any case, and
go with a tankless.  You'll spend several thousand on this whole system
by the time you are done, if you install a tankless you'll get a higher
efficiency water heater, and never run out of hot water.  It is likely
that your tank type is ready for it's ten year replacement schedule
anyway, just retire it early.  


Lawrence Lile


-----Original Message-----
From: greenbuilding-bounces at listserv.repp.org
[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of Shawna
Henderson
Sent: Tuesday, September 26, 2006 9:20 AM
To: rigaziodesigns
Cc: Greenbuilder list
Subject: [BULK] Re: [Greenbuilding] suspended slab
Importance: Low

Most definitely you want to isolate the DHW from the floor. The two
systems should never be fed from the same tank. The reasons are twofold:

1. If you run the floor off the DHW tank without a step-down or mixing
valve, you are going to be running the floor too hot, and you will end
up with extremely high bills. Also, the tank has a longer recovery time
than a boiler, so you will potentlally suffer cool showers if there is
too much demand on the tank at one time (think cold winter morning, high
heatling load).

2. And more important than #1 -- the cooler water running through the
floor and back into your DHW tank can drop the temperature of the water
to a point where salmonella and other greeblies can flourish. Then your
whole family's sick and you might not have enough hot water to clean up
after them...

There are DHW tanks out there that have an integral heat exchanger in
them, allowing you to store hot water iin the tank for DHW, but run the
floor (with its lower temperature demand) through the heat exchanger. No
cross-contamination. Loverly. Only one heat exchanger. Works divinely
with a direct-vent propane unit tied to a solar thermal system and a
second tank in parallel.

Also, with the slab, definitely insulate under it. And don't be swayed
by bubble pack insulation, 'radiant' or not. There's a Canadian
manufacturer (truefoam) that puts out a product that is specifically
made for hydronic floors. It's a 1" or so rigid foam that has grooves
cut into it at 6" centres, so you just fit the PEX into it. I wouldn't
recommend it for new slab construction, but for a retrofit situation,
its great. The reason I wouldn't recommend it for new constr. is that it
puts the heating tubes in the lower 1/3 of a 3"or better slab, reducing
the efficiency of the heat delivered and also increasing the lag time. 
With a retrofit 1.5" overpour (or suspended slab, a term that's new to
me), this is not so much of an issue.

Hope this helps

Shawna H.

rigaziodesigns wrote:

>George,
>
>When you say "The big question is do you need to isolate the radiant 
>floor from the domestic hot water, with a heat exchanger and 2 pumps." 
>do you mean he may be required by code or is this a functionality
question?
>
>Thanks all!
>Lisa
>
>On 9/25/06, George J. Nesbitt <geoedb at idiom.com> wrote:
>
>>   Uninsulated slabs can be a real comfort problem. A well designed 
>>forced air heating system can overcome this, although not as easily as

>>heating the floor directly.
>>   Ideally you would add insulation on top of the existing slab and 
>>then install the radiant floor. If you don't you will get more heat 
>>loss through the slab. If you use the existing water heater it needs 
>>to have enough capacity to heat both the domestic hot water and the 
>>floor. The Gas Research Institute has guidelines on sizing the water 
>>heater, I don't know if it is available on the Internet. You can 
>>possibly turn of the radiant floor when there is domestic hot water 
>>draw to prevent running out of hot water sooner. The big question is 
>>do you need to isolate the radiant floor from the domestic hot water, 
>>with a heat exchanger and 2 pumps. Taco makes an integrated exchanger
and pump system.
>>   Don't forget that you have to cut down any doors within the area 
>>being raised.
>>
>>
>>rigaziodesigns wrote:
>>
>>>A friend of mine recently moved into an existing house with slab on 
>>>grade floors in the main living space... and of course the floors are

>>>cold. He looked at the Radiant Floor website (see below) and thinks 
>>>the suspended slab installation is for him since if he uses the new 
>>>concrete as his finished floor he will only loose 1 1/2" head room. 
>>>He also assumes that
>>>
>>he
>>
>>>will be able to heat the space off of his domestic hot water heater. 
>>>BTW,
>>>
>>He
>>
>>>lives in the washington DC area. Does anyone have experience with 
>>>this
>>>
>>type
>>
>>>of system or know if they work well and are worth the investment?
>>>
>>>http://www.radiantcompany.com/details/suspendednotes.shtml
>>>
>>>Thanks,
>>>Lisa
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>>
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