Condensing boiler with storage tanks

LeakyPete

I’m set to do a job at work with a couple condensing boilers and storage tanks for a condo building. What’s the best temp to set the boiler to turn on to maximize efficiency while still providing a minimum 140f in the tanks? I will be piping the cold feed into the inlet side of the boilers. Will this be enough? Would have a 140 turn on for the boiler and 160 off be sufficient? 

Please note we are installing a digital mixing valve to be set at 120 to supply. 

Jamie Hall

At 140 F you're not going to get much, if any, condensing -- so your efficiency will be no better than a conventional boiler. Is this recirculating? Are you using the boilers as though they were domestic water heaters (open loop)?

A little more info would help...

Hot_water_fan

Are these boilers or tankless water heaters? You could also use a condensing tank type unit.  

LeakyPete

Hot_water_fan said:

Are these boilers or tankless water heaters? You could also use a condensing tank type unit.  

They are not tankless but more volume water heaters. 2 Lochinvar armor 650,000 BTU with 4- 120 gallon storage tanks for a condo 

Jamie Hall

OK -- progress. The next question is... are the storage tanks heated by heat exchangers (often coils in the tanks -- referred to as indirect water heaters) or is this thing piped so that the storage tanks are on the same loop as the boilers, so that the boilers take feed water in and heat it -- and, presumably, recirculate -- and it is stored in the tanks for later use?

I sincerely hope you are talking about indirect water heaters when you are mentioning storage tanks here, with the boilers on a completely separate loop.

Would you clarify, please?

GGross

@Jamie Hall

If I am not mistaken the Lochinvar armor is a volume water heater. So it doesn't use a separate boiler loop and heat exchanger, it's rated for DHW. They add storage tanks (not indirect water heaters) just to have the storage volume. These are not used for space heating

basically just big commercial water heaters that look and function more closely to a boiler

Jamie Hall

Thank you. Now. What manner of condensing boiler or water heater are you replacing them with? If the system is once through the water heater -- cold domestic water coming in, 140 to 160 domestic water going out -- you will be able to get condensing efficiencies.

However, this is not the same kind of application as a more normal condensing boiler, which is designed to handle closed loop heating systems. You will have to consider different pressures and different heat exchanger designs, to cope with potential scaling problems.