Adding an electric boiler to old cast iron oil boiler

asfd

I bought an electric boiler and would like to put it in parallel with my existing 50 year old oil boiler. I want to keep both to be able to take advantage of temperature based electric rate and in case of power failure. The electric boiler is thermostatically controlled (not sure this is the correct term but the elements turn on when the temperature drops) so I think I need a 3-way valve to be able to bypass it when I want to use oil. My current setup is illustrated on the first picture and the proposed modification is next. The boiler already has a flo-control valve so gravity flow should not occur.



Does this modification make sense to you guys and is it worth it moving the expansion tank so the circulator pumps away from it?



I would also like to add a radiant floor loop to the existing cast iron baseboards and would like to know if I could only put the radiant loop with its circulator (and possibly a zone valve) parallel to the baseboard loop. This would allow me to keep the existing circulator in its place but I am not sure it would be able to "freewheel" when the radiant floor calls for heat but not the cast iron baseboards.



Finally, how hard is it to make modification on 50 years old iron pipe (I plan to use copper for the pipes I will add)?

Brad White

First Things First

The first thing I would do is to relocate your expansion tank connection to upstream of the circulator (OR move the circulator to as closely downstream of the expansion tank connection point -the boiler- as you can; this is the same thing). Do this regardless of what else you do.



The second question is, do you know what the pressure drop of the electric boiler is? If it is modest, I see no reason for the 3-way valve and bypass around it. You will not save pressure drop because at some point the water will flow in series through both.



Unless I am missing something, I see no need to go around it.



For controls, I would stage them with a 2-stage aquastat giving the oil boiler priority or electric boiler priority depending on what your rate structure is. If your rate is that good and predictable, a digital time clock function might be the best enable/disable means. You could also set it up so that you pre-heat your return water to keep your boiler from condensing, a few options. But I presume your electric rates are much lower or your cost of oil is huge.



For a radiant zone, can you spare one of the monoflow circuits? An abandoned radiator perhaps? That will allow sufficient flow with a P/S circulator for a small space. Otherwise, yes, run in parallel. For working old pipe to new, the pipe thread standards have not changed on over a hundred years, so you should be fine. I have connected new IPS and copper/IPS fittings and valves to pipe installed between 1900 and 1922, no problems. For iron to copper interface, I would use a thread-pattern brass ball valve, nipple to iron and valve, male to copper adapter out of the valve.

Henry

Dual energy

Dual energy systems are very popular here. The electrical utility even gives you a break on the Kw/Hr rate in the off season.

We do a fair amount of these. Here is a proper way of doing it:

http://www.thermo2000.com/pdf/schemas/bth/valve_trois_voies.pdf

Your boiler manufacturer also has similar instructions in his manual.



Henry