Actual oil usage vs heat loss for boiler sizing

DGA

I have an oil fired boiler currently heating my house that is approximately 50 years old (old Crane 70-290).  We have recently made significant upgrades in insulation and windows and now that unit is quite a bit oversized (and old).  I have installed an hour meter on the burner and kept a log of the overnight burn times and consumption during the coldest nights of this year, which I believe to be much below what anyone would typically use for a design temperature here (central Virginia).  My question is this.  If the current boiler is using a 1.5 gph nozzle and burning overnight no more than 50% of the time, would you expect a much newer unit operating wiith a .85 gph nozzle to be an acceptable replacement.  Specifically I have an Axeman Anderson WL140 that was given to me and waiting in the shop to put in.  My estimations tell me that I believe it would work well and be much better sized to the current heat load requirements of the house. Efficiencies should be better and longer burn times utilized.  There are no domestic hot water coils or indirect hot water loads associated with the boiler.  I have a separate hot water heater taking care of that. 

Thanks for your thoughts and expertise, this forum is very helpful.

David

Jamie Hall

For starters...

you don't say whether this system is steam or hot water.  If it is steam, the boiler must be sized to the radiation, not the heat loss.  So you will need to add up the installed radiation and size your boiler to that.



If it is hot water, you still need to take into account the installed radiation, but you can size the boiler more closely to the actual heat loss with the proper controls for flow and delta T.



As to whether a bigger boiler running half the time is more or less efficient than a smaller boiler running all the time... well, the condition and type of the boiler and the nature of the controls are going to be much more important than the duty cycle.

DGA

Thank you

... for responding.  Sorry, I should have given more details on the configuration.  It is a hot water system with 3 zones.  Each zone has a pump and thermostat controling it.  Radiation is reasonably equal on all 3 zones.  I may also consider adding an outdoor reset to it as well. 

My thought is this, and let me know if my logic seems off base.  This winter has been a good test, with several nights under 5*F.  That is exceptionally cold here.  My overnight usage at that temp has been very consistent from night to night at about .75 gph (worst case).  Replacing the boiler with a .85 gph unit that should be operating at a somewhat higher efficiency seems acceptable.  I already have the boiler and it cost me nothing.  If it can do the job more efficiently, I feel foolish not putting it in.  I understand that my ability to increase temperature in the space after a setback may take longer.

Thanks again for your thoughts.