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Hot water buffer tank hack advice

user-1147460 | Posted in General Questions on

Over covid I have pulled out the old hot water system from my hotel. I’m replacing it with a heat-pumped, mains pressure, circulated etc etc system. As you’re aware, hospitality hasn’t been a dream run lately and I’m not exactly rich. 

The new system will be 2400l of hot water and requires a buffer tank to ….well…..buffer. 

The thing is I have a number of, now redundant, 180l mains pressure cylinders. Can I use one (or more?) as my buffer tank? Water connection at the bottom and air pressure in the top. 

To maintain the cushion I’ll need to isolate and drain regularly and then repressurise the air. 
My questions are:
Is this a dumb idea?
How often should I drain, reload the system?
Can I pour some kind of oil into the tank to act as a buffer/separator between the water and the air to slow the dissolving of the air into the water?

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  1. charlie_sullivan | | #1

    So you are looking for a way to buffer pressure only, not as an additional reservoir of hot water?

    If you had a way to measure the air volume remaining, you could simply add pressure with a tire pump without draining it, if you had an additional port for that. I'm not sure how to measure the air volume though.

  2. Expert Member
    NICK KEENAN | | #2

    I usually use the words "expansion tank" to describe an air-filled tank that is used to limit the increase in pressure when water is heated and expands. I use "buffer tank" for a tank that increases the volume of a system to limit swings in temperature.

    I'm a little unclear on your description but it sounds like what you are talking about is an expansion tank. It used to be common to implement expansion tanks as just a tank full of air attached to the rest of the system. There are two main drawbacks. The first is that over time the air dissolves and the tank has to be periodically refilled. The second is that the oxygen that dissolves from the air attacks the metal parts of the heating system, in particular the cast iron of a boiler. Modern systems tend to be closed, in that no air or water is introduced in normal operation. Once all of the oxygen in the water in the initial filling is reacted no more corrosion takes place.

    You mention that this is a "mains-pressure" system. Does this mean it will be open to domestic water? If that's the case you have to make sure all of your components are non-corroding. You also have to make sure that everything is safe for potable water. Fittings that have ever been used on non-potable or closed systems cannot be used on potable water.

    1. Expert Member
      NICK KEENAN | | #3

      Let me add that I'm not quite sure what you're trying to accomplish. If it's to save the cost of an expansion tank, I bought one today and it was $50, I just don't see a big potential for savings.

  3. BirchwoodBill | | #4

    It sounds like you want to run a heat pump into a 50 gallon buffer tank. I would look for Caleffi iDronics 17 which describes thermal storage. IDronics volume 12 covers the fundamentals. These are free downloads from Caleffi.

  4. user-1147460 | | #5

    Yes it’s an expansion tank I’m after. The pipe work is all PEX and/or copper. It’s the domestic hot water for taps & showers. (I’m also doing a separate central heating system, which will be glycol of course, but that’s not what we are talking about here)
    So, yes, I envisioned a valve in the top and pressurising it. I’d guess monthly topping up the pressure (and maybe draining out the water?)
    The expansion tank is off to the side of the water flow, of course, so I doubt much water would pass to/from it, but it would tend to slowly fill up over time as the air dissolved.
    I realise that proper expansion tanks have a diaphragm in them to physically separate the air from the water. But is there potential to pour a small amount of something (oil?) in there to float on the water and do a similar job?

    1. charlie_sullivan | | #8

      Technically, some kind of oil might work. But it's not that easy. An oil layer on top of the water in a drain trap just has to stop water from evaporating. Stopping air from dissolving in the water underneath is probably a more difficult challenge and I'm not enough of a chemist to know whether that would work.

      But even if it did, you are introducing something into a potable water system that is not approved for that use. That's not OK, even if in theory it's non-toxic and stays in the expansion tank.

  5. Jon_R | | #6

    +1 on just paying $50 for the small diaphragm expansion tank that you need. Otherwise you create a system that could be dangerous if passed on to anyone else.

  6. user-6184358 | | #7

    You need to put in plumbing parts that are made for plumbing and NSF certified. Her is a Bladder tank mfg.

    Oil - yea dumb idea - sick people will make you poor

  7. Expert Member
    BILL WICHERS | | #9

    This isn't a "buffer" tank, it's an expansion tank. Just buy one, they're cheap, and they last a long time. The usual "separator" is a butyl rubber diaphram. This is the proper way to build an expansion tank into a system like this.

    A buffer tank would be for bulk storage of heated water, and such tanks need to be insulated. You typically either feed and draw from opposite ends of the tank. You don't typically drain such a system. Note that you would have to have a recirculating pump for this to work, and you'll greatly increase your standby losses with all the additional surface area from the extra tanks.

    No, you can't put oil in the tank. You really need to use the proper components here, and as others have mentioned, everything used on a water system like this -- including on the hot water side -- has to be rated for use with potable water. That means NSF certification on the tanks (and everything else), all lead-free brass fittings where needed/used, etc.

    *** DO NOT COMPROMISE ON SAFETY. *** This is not the place to save money.


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