Keeping the BTUs Inside my Indirect

Larry_10

My system has been running for a while, and I've noticed that on the indirect DHW tank that a couple of the pipes running from the tank are always warm. I equate this to BTU leakage, and it bugs me. In particular, the pipes for the DHW supply to the house and the boiler supply to the indirect are the warm ones. Is there a good way to stop this? From what I can tell, they didn't install any check valves, thermal traps or expansion tank on the domestic side of the boiler (it's a Vitodens 8-32).

What can be done to solve this problem?

michael_15

insulate

You could insulate the first 10 feet or so of piping. This is generally recommended even if you have a DHW tank.

-Michael

hr

You could have

ghost flow to and from the tank depending on the piping arrangement and location of checks or zone valves.

It is possible for pipes to feelo warm merely fromn conduction transfer from a warm indirect.

How often does the indirect call for heat when no DHW is used all day? Most manufactures claim less than 1 defree per hour loss through the jacket.

hot rod

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Larry_10

The Vitodens 8-32 has a dedicated set of supply and return lines to and from the indirect. An Vitodens internal solenoid valve directs the boiler flow to the indirect when the aquastat calls for DHW production (all other boiler control settings also allowing this). After DHW production, the solenoids flip back to the space heating circuit.

In my case, the boiler is directly plumbed to the indirect without any additional flow checks, expansion tanks or other things between the them (except for the temp/pressure safety relief).

Attached is an older picture showing the connections. The current piping is the same as the picture with the addition of piping from the temp/pressure relief to a floor dry well.

Steve_35

You can easily help

the loss on the DHW side by rotating that T&P so the branch is pointing down. Put a short piece of pipe in a male adapter, 90 over and 90 up. Easy, quiet thermal trap.

For that matter you could do essentially the same thing on the boiler/indirect piping.

BTW, where's your drip tube?

Larry_10

I assume the drip tube is the outlet from the t&p relief valve. The drip tube wasn't in place when this older photo was shot. It's now plumbed towards the back wall, and into a nearby dry well. I had all the waste water (boiler condensate, humidifier flow through, ...) plumbed so it flows down into the dry well without using pumps. Thought gravity was more reliable than a pump.

I do like the simple notion of a thermal trap. Is 18" down about the right amount?

I've also heard that an expansion tank would be appropriate as well. Aren't these usually placed on the cold water supply side? In this situation, does it make sense to consider such a tank on the DHW outlet side?

Larry_10

hot rod,

Forgot to mention that I really don't have a feeling for how often the DHW tank fires. The current piping has a flow switch on the return side from the indirect to the boiler. I've had that switch installed in order to properly control my hydro-air handlers when the system goes into DHW priority mode. A side benefit is I'll also be able to monitor this switch and data log when it goes off.

As an electronics project I plan on placing a bunch of thermal sensors and state sensors all around my heating system probably using Dallas/Maxim Semiconductor 1-wire devices on a network. Initially I'll collect the data on a PC, but might move the code to a cheap little embedded Java processor acting as a web server. In several months I'll be able to report back with some real data on this question.

Steve_35

We just run a very short piece of pipe down. 3" is enough.

If you're on a well with a pressure tank and no check valves between the cold water inlet of the WH and the pressure tank you usually don't need an expansion tank for the WH. If you're on city water you really should have one.

Larry_10

At this point the contractor has installed thermal traps on the domestic supply to the house and on the boiler supply to the indirect. The thermal traps seem very effective in stopping the BTU migration out of the tank.

Since these two heat sinks have been plugged, the BTU leakage has migrated now to the return line from the indirect back to the boiler. Previously this line was ice cold (unless DHW was being produced). I guess the laws of thermodynamics are coming into play here. I assume once this is plugged, it will then migrate to the cold supply. Is this correct?