[Greenbuilding] [BULK] Re: suspended slab

Tue Sep 26 11:55:07 CDT 2006

I think the health issue is not so easily shaken off. THe line feeding 
the water heater in a non-radiant floor house comes directly from the 
well or municipal line, at ±45*F. This is not salmonella-breeding 
condition in the line, nor in the water heater. Also, there is no 
recirculation from the end use back into the tank (ew, the thought of it!).

When the return from the floor comes into the tank, it is coming in at a 
lower temperature than the floor required. Say the floor requires 85*F. 
The return will be somewhere between 70 and 80*F. This dumps into the 
DHW tank when there is potentially no other demand on the tank. Anything 
under 120*F as a constant temperature is cause for concern in the tank. 
I simply don't see how anyone can guarantee that this condition won't 
ever occur. The careful planning of your second system is also not 
guaranteed, especially in a D-I-Y situation, where one cannot make any 
assumptions about the knowledge/skill level of the installer.

I can't bring myself to recommend the shared tank to anyone because I 
can't see the overall benefit -- the cost of careful planning and extra 
valves and such outweighs the cost of going to a different configuration 
all together, one that doesn't carry any risk of contamination of 
potable water. I usually spec an integrated heat exchanger in a storage 
tank or a small boiler system However most houses I design have such a 
small load that the boilers are embarassingly oversized. That's why I 
got 'hooked' on the integrated HX w/storage tank (which also lends 
itself very well to a now-or-future solar thermal installation).

Here's another bit of info from Natural Resources Canada re: Tankless 
waterheaters. Performance goes down dramatically when coupled w/tempered 
water from a solar thermal system or other upstream preheating source. 
THis is because the units are built to work most efficiently when there 
is a large temperature difference. Also, I have experienced challenges 
with propane-fired units w/piezo-electric ignition (Bosch specifically), 
where wellpump & expansion tank did not give adequate pressure to push 
the p-e ignition. THis resulted in no hot water except when faucets were 
cranked wide open. Problem mitigated *but not solved* by putting in 
larger expansion tank. Municipal feed at constant pressure, no problemo.

s.

Lawrence Lile wrote:

> 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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