Boiler replacement, piping configuration?

Luc

Hello everyone,



Just want to have some advise on a boiler replacement.



Presently we have two oil boilers of 37 BHP. The boilers are really oversized and one is enough to heat the whole building. We want to replace one of the oil boiler by an electric boiler. We will control the electric boiler so that we stay under the peak KW demand. We have a good price for the kWH so it is advantageous to have an electric boiler that can meet some of the heat demand when we have a low KW demand in the building.



The oil boilers are in parallel between the supply header and the return header. There are 4'' pipes entering and leaving each boiler. We have three pumps that are pumping away from the supply header for the zones:

- Pump #1 is for a baseboard zone and gives 60 USGPM @ 40 head feet.

- Pump #2 is for a baseboard zone and gives 35 USGPM @ 35 head feet.

- Pump #3 is for a heating coil for the ventilation and gives 40USGPM @ 10 head feet.



The electric boiler has a 2'' pipe entering and leaving the boiler. I was wondering of the best way to install it.

1) Reduce the 4'' to 2'' and install the electric boiler in parallel with the oil boiler.

2) Use a Primary/Secondary arrangement for the electric boiler. Use a pump with closely spaced tees on a 4'' primary pipe joining the supply header and return header. The other oil boiler would be in parallel with the primary pipe that the electric boiler supplies.

3) Any other suggestions?



Kind regards,

Luc

Tom Blackwell_2

Demand limit

If you are going to use a demand limit strategy with the electric boiler, it will need to be in series with the others ,so supply temperature setpoint can be met. Looks like the best bet would be to pipe it with close spaced tees in the common return before the other boiler take-offs and use an "injection circulator" to pump out of the electric boiler into the header. Size the pump for the boiler rated capacity. With this arrangement whatever heat output you get from the electric boiler will unload the other boilers downstream from it. Are you using outdoor reset?

Luc

Thanks.

Thanks Tom for replying.



Yes we will use an outdoor reset.



Do you think that leaving out the second parallel 4'' pipe will have some restriction on the pumps?



Luc

Tom Blackwell_2

Bypass

No, a bypass should not be necessary. The system will just continue to pull through the boilers; fired or not. By installing another boiler in an injection configuration, the existing hydraulics are not affected. Hope this helps...

Luc

Boiler breaking down

Thanks for replying Tom.



One problem that I would see with the electric boiler in an injection prior to the oil boiler is the case we have a leak in the oil boiler or some other problem and we have to shutdown the flow of water. This will shutdown both boilers.



Luc

Tom Blackwell_2

Maintenance Bypass

You could add a valved bypass to be used only in this situation...