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Anti freeze in gas hot water heater??

Discussion in 'Home Power and Microgeneration' started by Jaggy Taggy, Jan 27, 2006.

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  1. Jaggy Taggy

    Jaggy Taggy Guest

    I have pipes in the floor of a new building and I am thinking of heating the
    water (rather water-antifreeze) mix with a regular gas fired hot water tank.
    Of course I could never use that old water heater for potable water again,
    but that doesn't matter, I have an entirely different system for that

    The question I have is if the anti freeze would somehow break down during
    this usage, being exposed to very hot temperatures at the heated surface of
    the tank (but probably not much different from the temps inside a car

    Has anyone around here experience with this, is there any reason not to do

  2. SQLit

    SQLit Guest

    If the area is above freezing what do you need the anti freeze for?

    Water treatment for minerals would be what I would consider.
  3. Bughunter

    Bughunter Guest

    Yes, it breaks down over time and you will need to replace it periodically.

    I have anti-freeze in my boiler that uses hydronic baseboard type radiators.
    Building codes in my area dictate that this must be a non-toxic antifreeze,
    and not toxic automotive type anti-freeze. Also, a system is present to
    remove air bubbles ( a scoop, with air escape valve), an expansion tank, and
    a special one way mixing valve to allow water to automatically be added to
    the system as some of the fluid gets lost over time. That has a tendency to
    dilute the anti-freeze over time. Periodic checks for anti-freeze
    effectiveness is warranted. The reason that non-toxic is required by code is
    to keep the system from contaminating potable water in the unlikely event of
    a failure in the one way mixing valve from the potable water supply. In a
    completely closed system, not connected in any way to a potable system, it
    may not be necessary. But, you may want to ask a code enforcement official
    to be on the safe side.

    This is all pretty standard stuff on heating system boilers, especially in
    the north east US where a power outage could cause you loose heat and risk
    the possibility of freezing part of your hydronic heating system.

    If you are thinking of using a hot water tank, I can't think of any reason
    it would not work. The downside is that you will have lots of excess storage
    in the tank, and the antifreeze is expensive. Also, a most tanks designed
    for domestic hot water do not have sufficient recovery rate. A better
    solution would be a higher capacity tankless type heater. They make some
    that are sized to perform whole house hydronic heating.
  4. It's been done - usually using Propylene Glycol. You need an air
    bladder, like a "wel-trol" tank to maintain pressure and keep air out
    of the system.And you need the normal circulating pump.

    However, most domestic boilers are higher efficiency than the water
  5. Jaggy Taggy

    Jaggy Taggy Guest

    Well Bughunter, I am in the woods of Maine, the house to be heated is well
    insulated and I thought of this gas hot water heater is because of its low
    output (35K BTU). I need less than that to heat the place.
    I don't have to worry about building code.

    I use tankless water heaters for other buildings etc, but what do I do with
    125K BTU??
    Also, I use hot water solar panels on the roof and tying it all together is
    the challenge. Heat exchangers are difficult and I would need to run a bunch
    of pumps for the various loops, so an exchange-less system where the solar
    panels heat some water in the boiler and if it is not heated enough the gas
    kicks in seemed like a good idea, but I wasn't sure about the antifreeze as
    I mentioned in my earlier post.

    But input from anyone who has actually built such a thing is appreciated.

  6. I have a 2-car garage that is heated in exactly this fashion.
    DHW heater coupled to hydronic floor heating. There IS an expansion
    tank and a circulating pump. There is no valve to admit water from the
    water system.

    The garage was built in '92, and I bought the place in '98. AFAIK, the
    system has been trouble-free over that period. Honestly I don't know
    what flavor of glycol is in there, propylene or ethylene. There's
    never been any drippage from the relief valve.

    Apparently, this installation met code at the time it was installed,
    but code now prohibits this use of a DHW heater; around here (Alberta)
    at least.

    It certainly does work, but it may be less efficient than a
    pupose-built heating boiler.

    Gordon Richmond
  7. Bughunter

    Bughunter Guest

    Even if you are not subject to building codes, it is still not a bad idea to
    understand them because they are often enacted for reasons of safety for you
    and your family. I'm in "Live Free or Die" state of NH, in a town without
    building codes, but there are still state codes.

    If it were me, I would use a non-toxic anti-freeze such as those for home
    boilers. It is probably less expensive than toxic automotive anti-freeze and
    I would assume just as effective. If you are fortunate enough to lie in an
    area without codes, then you can do whatever you please.

    Too many btus are better than too few. You can always put a thermostatic
    control in place to shut off the heater whenever the room gets to the
    temperature that you desire. If you have a 125k btu unit, you will get there
    faster than if you have a 35k unit. Undoubtedly you will need a differential
    thermostat between the tank and the solar panel. A similar control could be
    used to determine when the gas powered assistance is required and turn
    on/off that unit.

    I have a place in northern NH. 35k btu would hardly put a small dent in my
    heating needs. I am not yet through insulating and it is 2200 sq ft. If 35k
    btu is enough for your place, then a simple hot water tank would be easy to
    acquire and adapt for your purposes. It picked up a used propane tank for
    free from a place that was converting to oil. How big is your place?

    In the case where you have solar heating panels on the roof, a tank in the
    systems makes a whole lot more sense. You did not mention panels in your
    original post. I had a friend who had a solar heated house. It had a
    standard 40 gallon electric water heating tank that acted as a backup/assist
    to the solar heated loop. They also had a huge (500 gallon) main storage
    tank. That was several years ago, and I don't recall specifically how the
    system was plumbed. It was a drain back system that did not use

    For a very small cabin, a single hot water heater tank might be enough. I
    have never constructed a solar heating system, but I do some reading on the
    subject because I am interested in ways to heat my own place. For a seasonal
    place, in a northern climate, anti-freeze is pretty much a requirement.
    Drain back systems are subject to catastrophic failure from freezing.

    Mythbusters had a show on last night where they tried to set a ship afire
    using mirrors. Then were successful! That inspired me to begin thinking
    about some type of a concentrating parabolic collector to grab a little heat
    for my place. I am doubtful that I'll be able to do much more than
    supplement a more traditional propane heating systems, and an existing coal
    and wood stove. I'm no expert, but I might be inclined to tinker to collect
    a few free btus.

  8. Bughunter

    Bughunter Guest

    Why? I would think that a higher differential between supply and return
    would indicate a more efficient operation of your radiators. Or, are you
    talking about a different supply and return?
  9. Bughunter

    Bughunter Guest

    It makes sense to me now, since the goal is to get a nice even heat
    distribution. I guess if all the heat does not get into the rooms on the
    first pass, it gets there on later passes through the loop.
  10. Guest

    So use more radiators per square foot of floorspace towards the end
    of the string, with less pump power and more boiler efficiency.

  11. daestrom

    daestrom Guest

    Or c) Pipe more radiators in parallel?? Seems like if you pipe a couple of
    branches in parallel, then each branch has the same inlet temperature (don't
    need more/larger radiators), and you can run the water slow enough to drop
    the return temperature.

    But slow moving water doesn't transfer heat through the radiator wall as
    well as faster moving water.

    Boiler efficiency only improves with cold return *if* the bulk water
    temperature drops enough to affect the vent temperature. Most boiler
    thermostats are set for a narrow temperature range and will cycle just as
    much if you drop the return temperature twice as far with half the flow
    (same heat demand). So other than saving pumping power (and needing more
    radiators), what's the *real* issue?

  12. Cosmopolite

    Cosmopolite Guest

    Add a surfactant.
  13. daestrom

    daestrom Guest

    Not the same thing. Surfactants will reduce the surface tension. Often
    this is referred to the 'wettability' of the liquid. But the thickness of
    the boundary layer against a pipe surface is determined by the viscosity of
    the fluid, and surfactants do little to change this.

    50/50 glycol is denser, carries less heat per kg, conducts less heat through
    the boundary layer, has a thicker boundary layer, and takes more energy to
    pump than plain water.

    Bottom line, unless you really have to worry about freezing down to -40 or
    something, use less glycol. With the OP's setup, unless the room is going
    to lose heat and freeze, he would probably be better off with plain water
    and a corrosion inhibitor instead of glycol.

  14. daestrom

    daestrom Guest

    Doesn't really matter. If the vent temperature is the same, then boiler
    efficiency is unchanged.

    Remember, you're were suggesting using lower flow through the system to get
    that lower return temperature. Even in a counter-flow heat exchanger, lower
    flow along with lower temperature is not a 'slam dunk'. The lower flow
    would also increase the boundary layer size and that would hurt efficiency,
    not help it.

  15. Well, if you live in an area that is subject to hard frosts, and there
    is ANY possibility that an interruption in fuel supply or electricity
    supply could cause your boiler to go down for a day or more, you had
    better have anitifreeze in your hydronic heating system to avoid the
    possibility of VERY expensive damage to your structure.

    Can you imagine the cost of replacing frozen and split heating pipes
    in a concrete floor?

    I have floor heat in my garage, and baseboard heat in the house, and
    both systems have antifreeze in them.

    Gordon Richmond
  16. PCK

    PCK Guest

    would piping all rads to a 3 way valve for bypass help to balance such a
  17. daestrom

    daestrom Guest

    We get 'hard frosts' around here starting in October and ending in April.
    Even when the house loses power for three days, it doesn't freeze. The
    magic of insulation. Concrete floors, if insulated around the foundation,
    will take a long time to freeze (several days). Not that you want to go
    more than a day or two, but it isn't the 'lose power and the slab freezes in
    one day' sort of scenario. If you have an idea how long it will be out,
    you've got a couple of days to do something about it, not just one night.

    More danger of the water piping to the second floor bathroom freezing than
    the floor. Folks don't put anti-freeze in that piping ;-)

    Interesting question, does PEX tubing fail when water in it freezes? I know
    some plastics are 'freeze-proof' because the plastic has enough 'give' and
    doesn't split wide open like copper or iron piping.

  18. daestrom

    daestrom Guest

    Yep. Now that you say that, ISTR seeing just such a set up in an old HVAC
    manual. One loop of piping all the way around the first floor and a second
    around the second floor. Two 'T' fittings connected each rad/baseboard in
    parallel with a portion of the loop, and a 'tuning' valve in the main loop
    piping between the 'T's. Can't completely stop all flow from going through
    the radiator with the valve full open, but can tune the system by starting
    with all the valves full open and then gradually shutting the ones in the
    room that is too hot, until the thermostat has warmed up the cold rooms.

  19. Bughunter

    Bughunter Guest

    Interesting question, does PEX tubing fail when water in it freezes? I
    PEX is "freeze resistant", their term not mine. It will expand some and is
    less likely to crack than copper, but the brand I used (Quest PEX) in my
    house explicitly said not to let it freeze.
  20. Daestrom,

    I might be away from home for 3 weeks at a stretch for work reasons.
    Or a family could be away for winter vacation.

    Being as you CAN use antifreeze in a hydronic heating plant, it seems
    to me to be prudent; if freezing is indeed a risk. I know the plumbing
    can freeze, too, and it's hard to anti-freeze protect that in a
    regular home. But at least if the worst happens, you can get the heat
    plant up and running again and work on the plumbing in relative
    comfort. :>)

    Gordon Richmond
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