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Unpressurized storage with 4 way mixing valve

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by yellowhead, Nov 6, 2012.

  1. yellowhead

    yellowhead Member

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    Having finally got my boiler (Biomass 60) installed and attached to a chimney I'm trying to figure out how to do the hard part (plumbing :)

    I'm planning to build a 800g or so unpressurized tank with a copper coil heat exchanger for storage. Pressurized might be simpler but lack of a suitable tank and physical constraints with getting it next to the boiler make it difficult.

    What I'm finding hard is coming up with a simple way to manage the boiler and storage. The attached diagram (with apologies for the literal cutting and pasting) shows what I have come up with so far. This is really no more than a primary/secondary system with a 4 way mixing valve. Here is how I think/hope it might work.

    When the boiler is cold the secondary loop will be off and the 4 way valve will be held in the 'neutral' position allowing the (short) primary loop to circulate directly as the boiler warms up. Once the boiler inlet has reached a good temperature, the 4 way valve will be moved towards the position shown to force circulation into the HX and add heat to the tank. How far it goes can be controlled based on boiler inlet temperature to make sure this doesn't fall below its set point. If the secondary loop calls for heat that is less than the currently available output from the boiler the 4 way valve will move back towards neutral to accommodate. If the secondary loop calls for heat that exceeds the currently available output from the boiler the 4 way value will return to neutral and the mixing valve on the secondary loop will reduce the heat taken from the primary loop to hold up boiler inlet temperature.

    As the boiler starts to go out, the 4 way valve will move towards the opposite direction to that shown reversing the flow in the HX and providing heat to the primary loop from storage. Initially this can be used to top up whatever heat is still being produced by the boiler based on the load on the secondary loop. When the boiler finally goes out the 4 way valve will move fully to the opposite direction and all heat to the primary loop will come from storage. Although its not shown on the diagram I would add a 3 way diversion valve to bypass the boiler from the primary loop once the boiler is out.

    Well that's the theory. I'm a total newbie at this though. Think it will work in practice? Any and all comments gratefully received.

    Cheers,

    Simon. Scanned Document.jpg

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  2. Bob Rohr

    Bob Rohr Minister of Fire

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    The intent of that schematic is to reverse the flow thru the HX coils. So a single coil can be used to add or remove heat from the tank. The valve motors from one direction all the way to the other, it does not modulate.

    The boiler return protection is provided by the sensor at the return which simply switches the pump on and off at the boiler temperature rises and falls at the return.

    A cleaner method would be a thermostatic valve/ pump assembly. That would give you both the primary pump and return protection in one device. Figure 6-17a from Idronics #10

    I have found a way to build that valve with off the shelf, or off the internet components :)

    Attached Files:

  3. yellowhead

    yellowhead Member

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    Thanks for the feedback. Quite correct, this is adapted from Idronics #10 figure 6-17a and as described there the 4 way mixing valve is driven 'bang-bang' to just control flow direction in the HX. This requires a 3 way diverter around the valve to allow the HX to be taken out of the primary loop on a cold start and a separate thermostatic valve/pump to keep the boiler return temperature up.

    I guess I'm trying to understand whether that 4 way valve could be used to take on those functions if appropriately controlled. Could it take the HX out of the primary loop if held in the 'neutral' position? Could it modulate the boiler return temperature if moved from that position towards the one show on the diagram (by diverting more or less of the primary loop flow via the HX)?

    I'm fine with implementing this as shown but reducing the part count on the plumbing side seemed like it was worth a try. This stuff is expensive :)

    Care to share how you built a ThermoBloc/Laddomat from off the shelf parts?

    Cheers,

    Simon.
  4. Bob Rohr

    Bob Rohr Minister of Fire

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    I think you will find the motor and control for that 4 way valve to be fairly $$. Are you trying to have the output go directly to the loads, and bypass the tank at times? If so a 3 way zone valve would do that for a lot less $$.

    Yes you can build your own return protection/ pump devise. Look for a fixed temperature, high Cv thermostatic valve, then add the correct pump for your system. I'm partial to the Caleffi 280 ThermoMix, Danfoss Esbe, Paxton, Laddomat are other brands.

    What is the output of that boiler?

    The 4 way reverser is a Tekmar 4 way valve with a Belimo spring return accuator. I homemade a coupling but I believe Belimo offers a spline adapter in their catalog. I think you can see the Bellimo numbers.

    Attached Files:

  5. yellowhead

    yellowhead Member

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    Some of those 4 way valves are spendy in the bigger sizes but I found a cast iron 1 1/4 from Honeywell that looks promising. See http://www.pexsupply.com/Honeywell-V5442N1031-1-1-4-4-way-mixing-valve. Less than $100. The actuator is a bit over $100 but still reasonable.

    I think bypassing the HX entirely makes sense when starting from cold. That way whatever heat the boiler is producing can go to satisfying loads first rather than storage. A 3 way zone valve would definitely do the trick but if I can hold the 4 way in the mid position won't that have the same effect?

    The Boiler is a Biomass 40 (not 60 as in the original post - dunno what I was thinking) so ~135K btu/hr peak. The specs for the 4 way valve suggest a Cv of 18.7gpm and it has 1 1/2 inlets. I'm thinking that 1 1/2 iron and/or 1 1/4 copper and a suitably sized circulator will take care of the boiler loop. The HX will be 4 parallel coils of 1/2 copper exact length TBD; long enough to transfer most of the heat from the boiler but not so long as to create too much resistance when taking full flow. Am I totally out to lunch?

    Thanks for the suggestion on boiler protection.

    Cheers,

    Simon.
  6. Attached Files:

  7. Here is a similar diagram with a backup oil boiler

    Attached Files:

  8. yellowhead

    yellowhead Member

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    Perfect. I had a nasty feeling I'd be re-inventing the wheel (and probably a square one at that). That's still pretty simple and better yet its proven to work without fancy controls. Still not sure I 100% understand how it works though ...

    Running on storage I get. C-3 is off and TV-1 will block return flow at 2. C-1 is running and pulls heat from storage as required via TV-2. ZV-1 must be open.

    Running with the boiler leaves my wondering. I assume C-1 and C-3 run together and heat from the boiler sort of gets 'injected' into the C-1 loop as required with any excess going to storage based on the flow through TV-2. Am I right? If so why is ZV-1 in there? It needs to be open in this case as well ...

    Good find another BMW owner :) Is it treating you well? Do you have storage configured ion this way as well?

    Cheers,

    Simon.
  9. I am in the process of adding pressurized storage to my boiler using a similar design. Been very happy with the bmw. Only problem is that all the zones can be calling for heat along with twin 120 gallon indirects and the boiler still goes into idle mode. And thats when I only load it half full.

    I believe zv1 is closed when there is no call for heat from the house circuits. C-3 will be running if the boiler is up to temp to charge storage. The zone valve prevents any water pushing through the circulators for the heating circuits. If you are using zone valves on your heating circuits than I don't think zv-1 is necessary.
  10. yellowhead

    yellowhead Member

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    OK thanks, that makes sense. I was planning on going with zone valves. Would you still need a separate pump (P-1 as shown in example D) or could C-1 take care of that? All my zones will be in floor so I want to keep the temperature of the water in the loop driven by C-1 below 120. At first I thought the diverter (TV-2) would handle this but now I'm not so sure. Its going to see the temperature of the return flow not the supply. Depending on the delta T across the open zone(s) the supply could be quite a bit higher couldn't it?
  11. Tv2 diverts water above 160 back to the house circuits. Below 160 goes back to storage. This helps keep the storage stratified.

    I don't know if you'd get enougH flow through the zones without p1. You could use an alpha at p1 to keep from over pumping and save electricity.
  12. yellowhead

    yellowhead Member

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    I think I get it. When running on storage C-1and TV-2 create a loop that pulls heat from storage as required. The flow through storage is the minimum required to keep the return side of the heating circuits at 160. That prevents running cool water through storage continuously which would mess with the stratification. Am I right?

    So for in floor zones couldn't I just replace everything to the right of ZV-1 with the usual 120F mixing valve, zone circulator, and manifold with zone valves? The return manifold flow would split to feed both the mixing valve and cold return to storage. The mixing valve would ensure that return flow through storage was only what was required to keep the manifold supply at 120F. Of course this assumes I can find a pump that will push water through the HX as well as radiant floor zones but I'm assuming that the HX will be pretty low head ...

    I guess I'm trying to find a way to avoid having 2 pumps going when supplying heat to a zone from storage rather than just 1. Think that's possible? I live off grid to I'm keen to keep the power consumption as low as possible. Hope that makes sense.
  13. That's how I understand it to work. You might find using two alpha pumps that vary there speed to match demand will use less electricity than a single larger pump sized to meet the max demand.
  14. yellowhead

    yellowhead Member

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    Thanks. I like that idea. The alpha looks to be pretty thrifty power use wise which is great. They are a bit more expensive up front but I'm sure they'll work out to be worth the $$ long run (off grid or on grid for that matter).

    Looking at that diagram again though I'm wondering if C-1 and C-3 don't need to be running at similar flow rates. If C-1 is pumping less couldn't C-3 short circuit the heating loop by pulling flow through TV-2 the 'wrong way' (i.e. from port 1 to port 3) when TV-2 is partially open? I don't have a Termovar diverter but looking at the Danfoss VTC511 load valve I have (which claims to be able to work as a diverter as well) it looks like it would behave this way; maybe the Termovar is different.

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