Indirect Hot Water Heater

Indirect Hot Water Heaters

What are the most important stats when comparing indirect hot water heaters? Standby loss, peak flow etc are a few examples.

Any suggestions on the ideal materials? Stainless steel, cupronickel, ceramic etc. One manufacturer suggested if I am using minicipal water, which I am versus well water he recommends cupronickel. There have been instances of the municipal water "eating" the stainless welds and causing leaks.

THANKS!

Good man, Robert...

I totally agree about the Viessmann; that thing can make hot water like nobody's business.

Regarding the original question, I personally have been paying attention to: materials, square feet of heat exchanger surface area, diameter of heat exchanger, tapping size, insulation and weight.

I LOVE the Viessmanns!

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IWH

HX surface area can be a deceiving number, because finned tube surface area does not all have boiler water directly behind it. The fins are hanging out there in the domestic water. The fins are beneficial, but a direct comparison of heat exchanger surface area between a finned coil and a smooth coil is not quite accurate.

Coil designs are not all the same. Some are compact and not conducive to convection and therefore have reduced output (Amtrol). I would be hesitant to use one on a low mass boiler.

Tank-in-tank designs (Triangle Tube & Weil Mclain) have excellent surface area and are very resistant to fouling. Cleaning could be an issue, though it likely will not be needed. The main drawback I can see is that the hotter boiler fluid is in the outside tank and this increases the heat loss.

Cupronickel finned coil water tanks (Superstor and Slant/Fin) work well, and the tanks are stainless steel. These tanks have good cleanout ports. Cupronickel has nearly 2-1/2 times the thermal conductivity of 316Ti stainless steel. Some of the older versions had small diameter coils and will not work well with low mass boilers because the boiler bounces off high limit. The new versions have bigger coils.

Stainless coil water tanks (Viessmann 300 and Burnham Alliance/Crown Megastor) have good sized cleanout ports and good coil designs. I see Bradford White has a stainless tank now too that looks very similar to the Crown/Burnham. Viessmann drawback is primarily cost. Burnham/Crown drawback is moderate coil head loss in some models, though that is only an issue if you are trying to use the internal pump in the Viessmann Vitodens boilers.

Steel tanks are fine too. Bradford White has a large diameter coil, as do Viessmann and Burnham/Crown. The price is not as much lower than a stainless tank as you might think it would be. Viessmann being the exception; their Vitocell 100 tanks are much less expensive than their stainless tanks.

Indirect water heaters seem simple on the surface, but everyone has their angle. Viessmann and Crown also offer horizontal models that may have the boiler stacked on top.

Excellent points all around...

I just wanted to give my thanks to Andrew for his thoughtful post. He's made a long list of good things to consider.

One last thing you can look into is how much water each tank is supposed to deliver, and at what supply temperature. Manufacturers that aim their indirects at the non-condensing boiler market typically only publish recovery numbers at 180°F supply, which is higher than you'd want to supply it with using a mod-con.

Indirects aimed for use with mod-cons will have published recovery rates using water in the 120-160°F range. Those are the numbers I'd hunt for. The tables developed by manufacturers vary, but the last time I looked into this the Triangle-Tube Smart III (or the private-labled Weil-Mc Ultra equivalent) indirect had the fastest recovery at lower temperatures, followed by the Vitocell 300 series.

The faster recovery is indicative of the efficiency with which a indirect can transfer the heat from the boiler water to the water inside the tank. The better the efficiency, the lower the potential supply temperature for a given desired tank recovery rate.

Another thing to look into is the insulation of the tank and the thickness thereof. The thicker that layer, the lower the standby losses. Ideally, look for published standby losses numbers as they capture all losses.

If you have any degree of water hardness in your area and your home does not feature a water softener, then I'd probably stick to an indirect where the HX is not finned and which features a clean-out. That way, liming should be minimized and invasive cleaning is possible.

Hi Jack,

Thanks for the kind words and I hope all is well in ME...

DVD production is happening as we speak... Does 10 copies sound like a good start? Or would you like more?

All the best to you. Constantin