Preheat DHW with heat exchanger off hydronic baseboard zone

Jamie Hall

Chasing efficiency is a really interesting game. It's also likely to lead one down rabbit holes...

The first and perhaps primary thing to remember is that for any given heat output is going to take a certain amount of energy. Nothing you can do about that. The trick is really two fold, then. First, minimize the amount of energy input for that much output and, second, make sure that any losses in the process (there will always be some) are recovered and put to use -- either in that specific process or in some other process -- to the extent possible.

Sometimes recovery of unavoidable losses in one process in some other process can result in significant gains in efficiency. The best examples there are in power generation, where it is often possible to recover some of the "waste' heat, present at too low a temperature to generate power, to provide usable heat for something else. Examples abound -- district heating from low pressure exhaust steam in New York City and some other cities, for example, or even the heater in your internal combustion engine powered car.

Waste heat shows up in other ways, too -- someone up there mentioned heat recovery from drains in a home plumbing system, for instance. The improved efficiency of condensing boilers comes from arranging the whole heating system to operate at a low enough temperature to recover at least some of the latent heat of combustion present in the flue gas.

Another avenue is to minimize waste and direct heat to where it is needed -- the obvious example being insulating pipes, of course, but also any insulation of the structure.

What does not improve overall efficiency in many cases, however, is changing one power source for another. Indeed changing from direct fuel fired heat in a structure to electric heat will, in most parts of the world, actually reduce efficiency, due to the relative inefficiency of thermal power generation.

I guess what I'm trying to say is that one needs to consider the problem as a process and as a whole, and not in individual bits and pieces.

One also needs to remember that energy efficiency relates very poorly to cost, and not to confuse the two.

hot_rod

wholesalers in rural areas, and pump suppliers still have plain galvanized tanks in stock. It would transfer ambient heat into your pre-heat.
It may sweat in some climates on hot humid days, however.

apsinnott

Something like this would work, they are basically using what I’ve been describing but with an air to water heat pump. I’d love to do that, as I am familiar with and have the tools for heat pump installs. But air to water is very rare. This company seems legit, I think they’ve been around a while but everything is low stock and pricey. 

If you replace the heat pump with my rarely used hydronic zone I described it’s what I started the thread with to a degree. 

They describe the preheat process here
https://www.arcticheatpumps.com/heat-pump-domestic-hot-water.html

hot_rod

I like the 3 pipe a bit better for the "direct" to heating load ability, with HPs.
With a 4 pipe buffer, you always cross and involve the top of the tank to get heat out to the zones. With 3 the tank may not need to be involved if the heat load and HP output are matched.

But you still get the advantage of the cold return across the bottom for some buffering and the coolest return to boost HP efficiency.