Why are mixed cast iron and aluminum rads a problem? Are they not connected with dielectric junctions?

Why do mini split heat pumps not work? They are very versatile.

Basement ducts are OK since they are inside the house envelope. I'd stay out of the attic if it is vented. Duct leaks turn forced air HVAC into a power vent.

How well insulated and sealed is your home?

If you have access to natural gas then you should consider switching fuels ASAP. You will cut your heating bill at least in half and possibly to one third what you are currently paying. I would also get a stand alone gas hot water heater.

I am not an expert in this area, but I think the mix of cast iron radiators and aluminum baseboards is one of the problems. If you like the old radiators then you could replace the baseboards with refurbished units. There are companies who specialize in this. You could probably find radiators which resemble you current radiators.

Having two zones is going to help heat the house uniformly. The layout of the plumbing will determine the level of difficulty.

The other option is to install heat pumps in the attic and basement and keep the oil heat as back up. This would provide the AC in the summer and help lower the current heating bills.

What is the size of the house, and what are the gas and electric rates?

Switching out your oil boiler for a gas Combi boiler, such as a Navien or some other brand might be a good idea as you will have one compact very efficient unit heating the house and your domestic hot water as well.

Cast iron and aluminum-finned radiators in a single zone configuration usually wonâÂÂt work, as they require different water temperatures. You can however have a single boiler with multiple zones requiring different temperatures by incorporating the appropriate mixing assemblies into the system and having only one type of rad in any given zone based on required water temperature. The other option for single zone would be to replace the aluminum finned rads for appropriately sized cast iron ones.

As for A/C you could have either an attic system or mini splits. I would probably favor mini split heat pumps in your case.

You need to find a competent contractor in your area that is thoroughly knowledgeable in modern hydronic systems, techniques and controls as well as older systems too.

SR

Thanks for your help! We looked into mini splits but concluded that we would need too many units because of the way the house is laid out. We'd basically need one for each room. Too bad, because I liked them on principle. The good news is that we can at least do the attic ac system without having to open up walls.

I wasn't sure what you meant, Ionized, about duct leaks turning HVAC into a power vent?

Leaks in supply ducts depressurize the building. Leaks in returns make the pressure positive. That sucks outdoor air in, and blows it out, respectively, through any air leak in the house. Note this is for "outdoor" ducts only. If you create fur-downs inside the house envelope, you can tolerate duct leakage. Sucking air in randomly causes a lot of problems in my climate where condensation can readily occur.

Central ducted AC systems also tend to cause pressure differentials between rooms unless there is a return in each room. Keep in mind that you will have to undercut each door halfway to your knees unless you have a return in each room or install jump ducts of some sort. Ducts in conventional attics pick up a lot of heat in the summer too.

These are all reasons that I installed 7, high wall units in my home in the hot, humid South. Keep in mind that there are other types of mini-splits that do involve shorter duct lengths that might be more easily accommodated by fur-downs. There are also ceiling cassettes..

FSQ4CW, do Al and Fe radiators "require" different temps or are they just sub optimal on the same loop? If the latter, how bad is it really?

I'd suggest the natural gas conversion asap, as all ready noted. Your AL baseboard is, I assume, regular old hot water fin tube baseboard, correct? Manuf of baseboard usually state outputs/ft at 180 and 140. Outputs drop dramatically for the 140*. I think the 140 would be good for the radiators and you could change out the baseboard for panel rads. Panel rads run well at 140. Initially your oil boiler could be converted to NG by simply installing a power gas burner.

Adding duct work to a home is extremely difficult to do in an existing structure. You basically have to tear the house down in order to get the access you need for the ductwork. I went the other way and pulled my entire h&C ducted system and went mini-splits and Rinnai Energysavers. All high efficiency and "net to the space" with no transmission losses. Google "doeductleakage" and read a bit.

I would recommend that you take a deeper look into the mini-splits. For an add on system they are very versatile.

Re Ionized:

Cast iron rads are usually rated at 140F water temperature whereas copper tube/aluminum fin rads are usually rated at180F. A 40F âÂÂT is very significant. Aluminum fin would be sub optimal at 140F due to the attenuated Btu output of the rad at that temperature.

As always, you have to pay attention to the technical specs provided by the manufacturer for Btu output per linear foot or per emitter device at specified âÂÂEntering Water TemperatureâÂÂ, flow rate in GPM, as well as âÂÂEntering Air Temperatureâ flowing over the emitter. In some cases, such as geothermal heating and cooling, the same emitter is used for both hydronic heating and cooling further complicating matters of design.

SR

We converted from oil to gas in late 2012 and used a Buderus GC124 conversion boiler. It is designed to replace old oil boilers, so the connections were just moved from old to new. We already had a gas line for the range and water heater, so adding the boiler was relatively easy. Gas bills have been very reasonable.

We also have all cast iron radiators, but I want to add heat to my basement and I've looked at all the issues of using a combination of radiators and some other type of hydronic heat. Every manufacturer recommends separate zones because a radiator needs different temp than lighter aluminum components, and the mass of cast iron allows it to radiate heat much longer after the pump shuts off.

A typical baseboard heater only gives off heat when the water is pumping since it has no mass to retain the heat. In-floor heat needs water lower temps because 140+ degrees feels too hot on the feet.

Bruce

This is a little off topic. Are cast iron rads not run at higher temps like Al because the aluminum is enclosed in a structure and the cast iron is not so it is a burn hazard?