Need help with DHW design

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sgschwend

New Member
Jul 13, 2009
312
PNW
www.sawmillservice.com
I have been putting off turning on the DHW part of my system, but there really isn't a reason not to connect it up. I have forgotten to add a mixing valve, but beyond that I have taken this beast as far as I can go. Please help by finding ways to simplify or make it better.

Background:
There is a backup hydronic heating system connected to the DHW system. Those two loops with a propane fireplace can keep the house warm.

New stuff:
There is a second circulator pump that is controlled by the boiler that will move DHW in a loop through the boiler Domestic loop through the two storage tanks and then up and though the DHW tank returning to the pump.

The fresh water is brought up to the boiler loop, since this loop is inside the boiler tank it can handle cold water input, a check valve is needed to stop reverse flow to the HW heater.

Note the connections to the circulator pump (shown as an angle line), this concerns me.

The domestic supply selector is just manual valves to support non boiler months.


I have updated the schematic to help clarify
 

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I am guessing that the upper right label "cold water supply selector" is really hot water to the house?

You might consider running the wood boiler setup as a pre-heat to the existing water heater. Leave the existing water heater exactly as it is now, just change it's cold water supply point to come from the wood boiler setup (which includes the wood boiler and dual DW storage tanks). Do I have this correct? Cold supply is "DW in"?

You might also want to consider what might happen to the pair of tanks labeled DW storage in the off season. I get a little concerned about what might grow in these tanks if they are not kept hot, or at least have a large supply of fresh water moving through them. If the water in them stagnates and goes cool for months, things could get ugly in there. For safety sake, you might wan to keep the cold water inlet path moving through them year round and just bypass the boiler coil in the summer.

Letting cold water through the boiler coil in the summer could cool the boiler and possibly cause condensation on it. I think you would need to run a ton of hot water to cause a problem, but worth keeping in mind. Your idea to bypass at least this coil in the summer would prevent any problems.
 
Sorry Scott, that is another mistake I made not showing the house DHW load, it is actually the other side, the selector allows the for the bypass of the HW storage tanks so they can be drained in the off season. Updated schematic

Yes I want to use the boiler as a pre-heater for the hot water input, the circulator is also there to charge up the DHW with hot water as the boiler is run, something like 5 hours a day run time. Yes the cold supply is the DW in.


One of my concerns is this design circulates the DHW by removing it from the hot side of the hot water heater and returns the heated water to the cold input side. That seems backwards but is the way I think it is done to preheat the water going to the hot water heater. Does anybody have a better idea for this loop?
 

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Don't know why, but I can't seem to download the larger version of your sketch - I just get a blank page - tried several times, and nothing seems to work on it - which is unusual since I don't have trouble w/ other diagrams...

Gooserider
 
Gremlins,

I am using Firefox, and both links work for me.

or

Have you paid up your forum dues for 2010?
 
Steve,
You have the circulator pump correct in the diagram. One thing to note is that the circulator needs to draw water out of the DHW tank before the tempering valve (and you will definitely need one). You need a bronze / stainless circulator for this since there is O2 in the water which will corrode an iron pump. I bought one from this guy on Ebay which is a Taco 006 with 1/2" sweat fittings and includes a check valve and control board. The control board can easily be removed and use this as a regular circulator.
http://cgi.ebay.com/Taco-Bronze-Car...cmdZViewItemQQptZBI_Pumps?hash=item45f075188d

You might want to control the circulator so that the DHW tank does not get too hot. The tank surely has an ASME T&P valve on it. The wood boiler may get too close to the rated temp. See what you have, some will pop in the 175 range.

Also note that you will be causing large temperature swings in the DHW tank which means expansion. Normally this expansion pressure is minimal and relieved by the faucets. In my case, I have a well, so the expansion can push back into the well tank. If you have public water, there will be a check valve at the entry to your home. This gives the expansion no where to go. You may need to add in a small expansion tank depending on your situation. Your local codes may also require a vacuum breaker at the hot water heater.

Regards,
Scott B.
 
sgschwend said:
Gremlins,

I am using Firefox, and both links work for me.

or

Have you paid up your forum dues for 2010?

I agree - gremlins, as it started working again shortly after I made the above post... Not sure what to suggest, but the drawing doesn't quite make sense to me. For some reason I'm not following the pattern on it - maybe if you color coded it, and / or drew in some flow direction arrows? (Not that you need to make me happy...) Seems like you have two cold water sources going in, with one going straight to the hot water out... :-S

Are those two coils off to the left above the boiler heat sources or heat emitters?

I would consider trying to get it set up so that you had one loop between the wood boiler and the two lower tanks, with all the cold makeup water entering at that loop, then passing up to the upper DHW tank, with possibly a second circ loop between that tank and the lower ones.

If it is winter time, the boiler can pre-heat the lower tanks and feed hot water to the upper tank as it's used, with the circulator to allow additional pre-heating if need be (i.e. storage losses cause the tank temp to go below usable temps before you use the water...)

In the summer when you aren't getting pre-heat, you would still flow the incoming water through the boiler and lower tanks, just to keep that water fresh, with normal heating taking place in the upper tank...

Gooserider
 
Here is the new schemo. G.

I have made a change to suggest a different method. It is shown in the second schemo. The pre-heated water loop to but does not loop with the DHW heater. With the objective of keeping the long supply line warm too. It would require some sort of dual circulator control, one from the boiler the other as a timed circuit.

I have concerns about:
1) distance from storage to DHW (about 60')
2) what is the proper working temp for the DHW storage
 

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Yes, that is much more clear about what is going on, and makes more sense...

Two big changes that I would make -

1. Move the feed for the boiler pre-heat loop from the house HW loop to the cold out of the DHW tank, same place as the floor heat loop returns. That way you won't be pulling hot water out of the top of the tank to circulate it back to the boiler for more heating...

2. Lose the cold feed option at the input to the DHW tank - ALWAYS pull via the boiler preheat loop (maybe bypass just the loop) and DHW storage tanks - you don't want to have them sitting around stagnating, by always pulling through them you keep the water in them fresh... (Same concern as was mentioned by someone else earlier...)

Gooserider
 
Ok, dropped the extra valving, and did not loop through the DHW heater, but pre-heated the cold water line.

Any strong opinions on this method, versus looping through the heater? The backup heater would not benefit from the boiler since there would be no new water demand. I don't like that.
 

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Maybe this is better, just need to make sure not to overheat the hot water heater. Changes per your....
 

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sgschwend said:
Maybe this is better, just need to make sure not to overheat the hot water heater. Changes per your....

That looks really nice to me - consistent flow through the DHW storage tanks (Someone did mention concern about possible summer time condensation on the boiler coil, it might not hurt to have bypass on that)

Hotwater makeup comes in through the DHW storage tanks which would be hot in the winter time.

The backup heater coils would actually pretty much bypass the DHW tank if the wood boiler is running, as when the storage loop kicks in it will grab the return from the coils, run it through the boiler and storage, then do a little loop across the top of the DHW tank... As long as you have a reasonably good aquastat on the storage loop pump I don't see much chance of overheating anything...

Gooserider
 
Steve,
The pump I pointed you to has a thermistor strapped to it's output. If you leave the controls intact it will occasionally turn itself on and run until the pump gets warm then shut off. It is designed to be used to keep hot water at a remote faucet ready to go with no delay. They claim to save thousands of gallons a year by doing this. Whatever... In any case, you could use this, but it would be undesirable to run the pump any time the boiler is cold. If the boiler is cold, you will just be sacrificing your hot water to warm the boiler water if the pump runs.

DHW tanks are generally kept at > 120 degrees to keep things from growing.
Deliverd DHW is usable down as low as 105-110 degrees. Typical shower water is around 100.
The tank T&P valves limit the temperature you can run your tanks up to. These tanks will easily handle temps in the 145 range. I doubt you would have a problem into the 160's and I am sure you will end up with a mess if you go for 180.

What is the size and type of the DHW heater? You can measure it's standby loss and determine how hot the entering water needs to be to keep it above your desired temperature setting. A really good tank will lose 1/4 - 1/2 degree per hour.

Also note that the DHW tank cold input is a dip tube to the bottom of the tank. Your take-off is also on the bottom of the tank. As you pump water through the tank it will slowly de-stratify due to the turbulence, but it won't be as direct as if you draw from the hot output. Most of the water you are pumping will take the short-cut along the bottom of the tank. What you have drawn is best if you expect to use hot water while the boiler is heating. If you wanted to heat the tank as fast as possible, I would use the hot output to pump from.

Your drawing has 2 circulators in parallel. If both are expected to run at the same time, this aspect may need a little more thought.
 
Thanks Scott, I will take a look at my tanks and those pumps, and the schemo is getting better with everyones help.

I had wanted to trick my single pump into doing both jobs but gave up on that idea.
 
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