Hot Water Heat Exchanger

[ssn61]

I am working on design-replacement of two older heating water boilers with two new condensing boilers- 3,300 MBH each. One of my colleagues mentioned that during summer we can use the heating water out of these boilers and preheat domestic water via a heat exchanger. Does this have any energy conservation benefit?

 

[MintJulep]

How much domestic hot water do you need?

 

[EnergyProfessional]

I doubt it is more efficient than a dedicated condensing-modulating water heater. Especially since you need higher water temp due to heat exchange and you also need to run a pump (or two). Plus all the piping has more heat losses.

what you save is space and possible some upfront cost, but you need more controls, possibly a larger boiler...so you may not really save much.

One problem also is you need a double-wall heat exchanger because your boiler system has anti-corrosion and biocide chemicals in them and if your HX has a leak, you don't want to drink that.

In the end there is a reason why domestic and heating hot water often is separated. I know some residential and "hack" installations use waterheaters as boilers with just a single-wall HX. but they all have in common that a code-official never had a look at that.

 

[PEDARRIN2]

All you are doing is using energy to heat it in another location. No savings there.

 

[LittleInch]

During the summer, why do you need the heating on?

Only if your heating demand is so low as to make your burners inefficient would it seem to make an sort of sense.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[urgross]

For preheat, absolutely none. If preheat only, you still need an additional sanitary HX and pump set.

 

[dbill74]

Done right with the right type of domestic water heaters, there is the potential for energy savings.

The only way I can imagine there being energy savings is if the domestic water heaters are 'standard' efficiency gas fired heaters. Even so, you will need a complex sequence of operations to get the energy savings. And what saving you do get is minimal.

You could see utility cost savings if the domestic water heaters are electric and the cost of gas per BTUH is significantly lower than electricity.

A couple years ago, I worked on a new building project where we did something similar to what you are suggesting in an effort to get a couple extra LEED points. Royal pain in the a$$ to get setup right.

In my opinion, it's not worth the effort.

 

[semago]

Depending on the cost of gas (assuming your boilers will be gas fired) and the cost of electricity in your region, and how you produced your domestic hot water before the project, there may be some savings.

If you have an inefficient gas fired (or oil fired) hot water heater heater, you may save energy by using your condensing boilers to preheat domestic hot water, because your new boilers may have a better efficiency. That's if you don't have to keep an entire network of pipes hot during the summer just to preheat domestic water because you will use energy to keep those pipes on temperature and also the heat losses from the pipes may have to be cooled by your A/C if you have A/C in the building.

One efficient thing we do here is to install a high temperature heat pump to do simultaneaous cooling (for A/C and heat recovery) and heating. If you have a low temperature HWR (I'm assuming you have one because of the condensing boilers you are installing), you can reject the energy from the condenser of the high temperature heat pump to your hot water network. In winter and mid-season, that energy will heat your building and in summer, you can preheat domestic hot water and other things (reheat after deshumidification, preheat steam makeup, etc). Basically, we recover energy that otherwise would be lost in cooling towers, air evacuations and drain pipes to heat the building and domestic hot water . This is probabely out of reach for your boilers replacement project though.

 

[collosus]

well it all depends on the pricing of gas to electric in your area. Electric heaters could prove to be 98% efficient as opposed to gas being 85%. Also the way it is set up plays a big portion in the efficiency department as well.