DHW preheat

[free75degrees]

I have an oil fired boiler with an indirect hot water heater that works just like any other zone. I am adding a Tarm 40 with a storage tank this summer and i noticed that some people use a domestic hot water pre-heat coil in their storage. What is the purpose of this? Do people think it is a good idea?

 

[Eric Johnson]

If you don't have an indirect, one way to heat your DHW is with a coil either in the boiler itself or in your storage tank. Typically, water circulates between the water heater and the coil.

Since you have an indirect already, you can just connect a zone from your Tarm to the indirect and accomplish the same thing. You already have the coil, in other words.

 

[free75degrees]

Yes, that makes sense, but I noticed that nofossil's heating diagram (and I think i have seen others) show an indirect DHW tank AND a preheat coil which mixes with the DHW output.

 

[EricV]

I have the same setup you have. I put a 42 gallons steel tank inside my storage tank to pre-heat water so the indirect doesn't ever fall behind. Works great.

 

[BrownianHeatingTech]

By having the preheat coil, even when your tank temperature is too low (ie, below what you want your domestic hot water to be), you can still pre-heat your water, which reduces the amount of energy necessary to "top it off" to the temperature you want.

Joe

 

[free75degrees]

OK, so I currently have two ways to control the temperature of the DHW that comes out of the faucet... I have a thermostat on top of my DHW tank and a mixing valve that mixes the DHW tank output with fresh cold water. I figured it would make the most sense to set the mixing valve so that I was using almost all hot water mixed with almost no cold water. This allows me to set the thermostat to the lowest possible setting and still get nice hot water, which in turn should reduce my heating losses from the tank. The problem with this strategy is that it might make me run out of hot water faster. However I have a 60 gallon tank and have never run out so far - if I did I would just up the thermostat and mix in more cold water.

Back to the point though... The thermostat sets the temperature inside the tank, so the staright hot water temp is fixed. If I vary the temperature of the water being mixed in (by pre-heating) won't it just cause large fluctuations in the water temp at the faucet?

 

[EricV]

No, the mixing valve takes care of that. I have my temperature cranked up on my tank and let the mix valve take care of the final temp.

 

[Vtgent49]

The tempering valve will take care of that, unless your preheated water is above the target temp. I'm going to change my set-up, so the valve gets pure cold water, and the preheated water goes into the tank. My preheated water can be 150, and my tank can be 160, and I now have the preheated going into the c side of the tempering valve.

No fossil have a set-up using two Temper valve in series, the first of which gets pure cold water, and the second gets DHW from tank as a last resort, and only if the preheated water is too cold.

 

[free75degrees]

Oh I see. I didn't realize tempering valves had thermostats built in to adjust the hot/cold mix. Makes sense now, thanks!

 

[Eric Johnson]

The cheap ones don't.

 

[Nofossil]

Tempering valves come in many different flavors. I've used Watts (terrible experience), Taco (worked, but needs large temperature drop) and Honeywell. The Honeywell needs only 3 degrees to operate, so it's perfect for my needs. The part number is on my site as part of my writeup - link in my signature below.

My goal is to preheat the incoming water as much as I can with a coil in my storage tank. I mix that with 55 degree well water if it's too hot, to get water as close to 115 degrees as possible. I then mix that with water from the DHW tank to get my water at my target temp of 118 degrees. the 'makeup' water entering the DHW tank is also 100 degrees, which is an improvement over the normal 55 degrees.

If my preheated water is 100 degrees, then the output of the first mixing valve will be only 100 degrees, and I'll need more hot water from the DHW tank at the second mixing valve. On the other hand, if my preheated water is 140 degrees, then I'll get a first stage mix of 115 degrees, and I'll need almost nothing at the second stage. The makeup water entering the tank is also 140 degrees, so I could be raising the temperature of my DHW tank by using hot water!

 

[SteveJ]

nofossil,

Thanks for the details on your two tempering valves.

I noticed on your diagram on your web site that your wood recirculation is controlled with a zone valve. Why did you use a zone valve instead of a tempering valve? What is the contolling logic for the wood recirculator zone valve?

Thanks,
Steve

 

[free75degrees]

Tempering valves come in many different flavors. I've used Watts (terrible experience), Taco (worked, but needs large temperature drop) and Honeywell. The Honeywell needs only 3 degrees to operate, so it's perfect for my needs. The part number is on my site as part of my writeup - link in my signature below.

Cool, thanks. I'll definitely look into getting a honeywell or two since my current mixer does not have a thermostat.

 

[Nofossil]

nofossil,

Thanks for the details on your two tempering valves.

I noticed on your diagram on your web site that your wood recirculation is controlled with a zone valve. Why did you use a zone valve instead of a tempering valve? What is the contolling logic for the wood recirculator zone valve?

Thanks,
Steve

I used a zone valve because I couldn't find a 1 1/4" tempering valve for anything close to a reasonable price. The zone valve is controlled based on inlet temperature - trying to stay above 135 degrees. I also open it as needed when the fire is dying to help raise the outlet temperature to usable levels.