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Alpha pump and 'backwards' mixing valve to control loop?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by 700renegade, Dec 5, 2012.

  1. 700renegade

    700renegade Member

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    Wondering out loud if I could accomplish a few objectives by deploying an Alpha pump pumping INTO the mix side of a typical Honeywell or simmilar mix valve.

    In my system my goal would be to minimize return volume and thus mixing into the bottom of my storage tank ( maximize stratification ). See my sketch. Would the mix valve set at say 120* cause a downstream head condition that the Alpha would 'see' as a varying number of zone valves? If there was little heat demand in my house the 180* supply water would come back to the shop at say 170*, the mix valve would clamp down on the hot side, and the Alpha would trim back to low flow. If the sidearm water heater, furnace fan ( coil in plenum ) and in-floor system were running in the house the return water cools below 120*, mix valve opens up, Alpha spools up, and it should stabilize at 120* ?

    I know the sensor on a mix valve is on the mix port, but will it modulate correctly with 'backwards' flow ???

    Also, since my in-floor system in the shop has to mix down to a low temp, instead of returning that 120* house water to the bottom of the tank, I'd like to route it as supply to the shop manifold. If the system were in perfect balance it could conceivably return only 85* water to the bottom of the storage tank.

    Somehow I know I'm missing a thing or two in this sketch to allow both the shop system and the house system to operate independantly without some weird repurcussions. If you see it, please chime in, as I've never seen a sketch like this before. There probably is a reason......

    One issue is the shop floor system has to pull it's full supply thru the house loop. I wondered if it would be 'starved' if the Alpha was clamped down - but even if it were at a meek 2gpm, with dT = 120*-85* = 35*, it would equate to 35,000BTU. I'd suspect if I needed more than that in my shop floor, that my house would also have a heat call somewhere.
    scheme .jpg

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  2. BoilerMan

    BoilerMan Minister of Fire

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    If the valve doesn't have anything cold to mix with the hot supply water (from house) it will just clamp off all the return water from the house and sit there dead heading the Alpha until it cools to 120F. At least thats how I see it.

    I think Maple1 has a plan to install a mixing valve on his return fed backwards, not sure how he was doing it though, he'll chime in.

    TS
  3. lotawood

    lotawood Member

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    Looks like you want a mix valve in a diverting setup. I think you still have to hook up the other side to the cold. I'm going to use a 3-way zone valve that diverts based on temperature.
  4. 2000 gallon lp tank, vertical? We need pics.
    Taylor Sutherland likes this.
  5. BoilerMan

    BoilerMan Minister of Fire

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    I think it's just hypothetical at this point, but yes if there is a 2K tank in the air that would be cool!

    TS
  6. 700renegade

    700renegade Member

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    Yes - per TS this is a very simplified version. The tanks are two side by side 1000's.

    I've omitted the 250gal expansion tank, the 30k btu electric backup boiler, the Maxim corn boiler and it's HX system, the multiple zones in the new workshop and a myriad of other little things that would just clog up the sketch and confuse things.

    Regarding the 120* mix valve, to prevent the Alpha from dead-heading I was thinking I could insert a 1/2" bypass line (and ball valve) parallel with the mix valve that would allow a base line flow like 0.5 gpm. With the natural convection in the furnace plenum I'm sure I'll get some drawdown in temp, but maybe not to 120. Looking at it's pump curve, I'd suspect it can go pretty low. How bad could 'dead-heading' be at a whopping 8 watts? If anyone has a mix valve lying around perhaps see if they close tight on the cold side when blowing in the mix port - I suspect some leakage may exist - maybe I don't even need the bleeder bypass.
  7. ewdudley

    ewdudley Minister of Fire

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    I'm not seeing the need to cap off the one leg of the diverter, it can just connect back to the house feed like Tarm does it.

    Shop floor circuit will pull as much as it needs whether Alpha is running or not, except if diverter gets hot then it will clamp off shop circuit.

    For mixed loads I've been real happy with a stratified hydraulic separator / buffer for the last couple seasons. It is simple, inexpensive, does a great job of supplying water at various temperatures, and excels at maintaining stratification of storage.

    Top zone of tank (which you may or may not need) is for DHW priority. If 'DHW T' sensor is below for example 140 degF, the 'XFR' pump runs until top section is hot enough. The setpoint is for the bottom of the tank zone, the top will be supplied with hot water from top of storage.

    Lower zone of tank is for high temperature loads like baseboard or WAHX. If "HOUSE T' sensor is below for example 120 degF the 'XFR' pump will run likewise. Again, setpoint is the return temperature to storage, hot water enters top of tank zone to be supplied to high temperature zones.

    'XFR' pump runs to satisfy temperature requirements of either tank zone. This means normally one zone or the other will be somewhat hotter than it needs to be depending on the season.

    Low temperature loads get leftover water from bottom of tank. Note that radiant floor pump pulls independently from the bottom of tank and all the way from top of storage. Normally there will be a reservoir of 'spent' water in bottom of tank, but in any event low temperature loads will get what they need by drawing through the tank from top of storage.

    Additional boilers are very easily integrated if they are adjacent to the separator. Also any boiler can be tied into the main lines to and from storage most anywhere and set up to maintain temperature at the top of storage, not all of storage.

    In your case you could put the separator in the house to make it convenient to feed the sidearm. In that case the shop circuits would draw from the house return line with a check valve.


    buffer_separator.jpg
  8. maple1

    maple1 Minister of Fire

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    ?. No mixing valves here.

    Well, I have one on my DHW-out in case my DHW tank gets too hot.
  9. BoilerMan

    BoilerMan Minister of Fire

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    Hmmm, Sorry I thought you thinking of doing some configuration of a valve to promote stratification, must have been someone else.

    TS
  10. I'm planning to use a danfoss 511 valve on the return from the house loads to help reduce tank mixing. By sending the water back through the loads until it has cooled below 160 should help keep the tanks stratified longer since less water will be going through the tanks.
  11. BoilerMan

    BoilerMan Minister of Fire

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    Mike, Maple........well you know they both start with M. LOL, I knew someone was going to do it. I think it's a great idea!!

    TS

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