Lowering return water temp on a combo boiler/ preheat dhw

njtommy

Currently have a Navien combi NCB-240 boiler (yes i know all jokes aside) my supply water temp is 140 degrees return is 120 running off a delta t pump set at 20 degrees feeding hydroair handler and a fan coil unit for my garage. I'm wondering if its worth preheating my domestic hot to lower my return temps down to another 20 degrees to gain efficiency. I have an 50 gallon electric hot water heater I can use for storage. Would just need a heat exchanger either plate style or a side arm. With the side arm I would not need to run a circulation pump, but not sure which would work better. I could buy a Indrect tank, but I'm going to leave the electric hot water heater wired for back up.
Dhw usage is 2-3 showers a day and along with the normal dishes and laundry.

Thanks for the help guys.

Snowmelt

im wondering why he has an electric tank and a Navien that can supply domestic water

njtommy

Lol thanks snowmelt I just converted to gas last year about this time and left it hooked up just incase.

njtommy

Yeah 5% is really any thing and even that could be a high figure. Do to once the tank gets warm enough It would no longer be lowering my return temp.

My highest gas bill was $220 in February.
I have SJ gas and its $1.30 a therm delivered. Not including the other $10-20 in fees. I think my usage was 160 therms or so.

So far it looks like I'm saving about $1000-1200 a year from running electric dwh, stove and heat pump.

I do still run my heat pump down To 35-40 dgrees do to NG prices.

Rich_49

You'll gain at the combustion assembly of the boiler but don't forget that you will use consume less electricity . NG is about 1.00 where we live and I pay .14 KWh . Figure out how much hot water you use . An electric heater takes not much energy to raise water from 110* to 120* .

kcopp

You will do better running the return water through a cast iron radiator in the area near the boiler....or a room you want extra heat in.

njtommy

@Kcopp I thought about that too. I could dump the heat into the basement. Was going to add Baseboard down there at some point any way. I also just pulled out a bunch of baseboard out a of a house so I have plenty to use.