Retrofit single-zone hydronic baseboard to 3 bedroom zones with 3 way diverter valves.

Yes, It is a viable approach . I would try to fit the Diverter Valve and the new bypass pipe (to the next baseboard unit in the next bedroom) within the existing baseboard end caps. The bypass pipe can run below the finned element in each baseboard unit. Providing power to the diverter valves is another issue I'm not sure how to address but it's low voltage so it's easier than 120 VAC and its code requirements. The only other approach I can think of now would be to have each baseboard supplied directly by a home run from the boiler with a 2 way zone valve and a common return pipe. Don't know if you have easy access to run PEX tubing in place of copper ( a cost savings). You would need to build a small manifold at the boiler with 3 zone valves and 3 returns. All wiring would be located at the boiler and you could use smart thermostats like Ecobbe with a wireless remote temp sensor in each bedroom. This eliminates the need to run thermostat wires in each bedroom. Can you do this type of work yourself or do you need a heating guy? There are two costs: 1) for the piping and valves and 2) the electrical for the valves. Doing either one yourself is a big cost savings. My experience is that it's cheaper to just get a good plumber to do the work as you designed it rather than a Heating Contractor with his overhead costs. PEX is easy to work with and much cheaper than copper. I assume you need to have big differences in bedroom temperatures to justify the cost of this work. Do you have un-occupied bedrooms that can have a low temperature? A rough ball park cost would be about $1000 in material ( valves, transformers, thermostats, pipe and fittings) and maybe $2000 in labor( based on your location). There is one negative factor in that you may have a lot of boiler short cycling because of the small size of each zone. In the ideal world, you would install buffer tank to reduce the short cycling. Also, how is your insulation in your 1980's home. If you are looking to save on heating costs with this zone design, would the money be better spent on improved insulation and windows. Just a thought. Good Luck.

You either need a buffer tank or a modulating boiler because of the microzoning.

TRVs might make a whole lot more sense than zone valves and thermostats.

Doing a heat loss calculation and sizing the radiation to be balanced might be a much simpler solution. I bet you have a series loop and they just put the baseboard wherever they could instead of sizing for the decreasing water temp in each room further in to the loop.

I would think all the 3 way valves would add up to a large Cv and be restrictive. I agree with 2 way valves but with 3 bedrooms on 1 floor do you really need this?

Are the temps that uneven?

EBEBRATT-Ed,

Can you explain how a two way valve would work in this setup. I think you would need to convert this setup to a single pipe system with mono flow tee's and have the 2 way valves on the side of the tee feeding the baseboard but then would you have the same concern about a large Cv through the mono flow tees when 2 of the 3 valves are closed? Would this setup work with regular tee's ? Thanks

It depends on how it is pipes, as far as zoning options.

Most common in small homes is a series loop. Figure 3-1

With fin tube, you can regulate heat output in various rooms by adjusting the damper opening. Or put aluminum foil over some of the fins to limit output. A cheap way to djust room by room without zoning.

A monoFlo or diverter system is another method found in older homes Figure 3-6. If you have this system TRV thermostatic radiator valves can be added for individual room zoning.

With a series you cansplit off a zone by addingdiverter tees. Then a thermostatic valve.

Nice about TRVs is they do not require wiring.

Most zone valves are the dimension to fit inside standard fin tube enclosures if you want to zone with a 2 way.

A buffer tank is a band aid for a oversized boiler system. Ideally the boiler gets replaced with a correct size unit, it may pencil out the same $$ as adding a buffer. If you are paying someone to do the work.

But with a fixed output boiler, even correctly size, when you zone the system you are building in short cycling issues. Mild days, one zone calling will cause those short cycles.

Older cast iron boilers could hold 10, 15 or more gallons, so they were a bit of a buffer by design. Modern cast boilers are down to 3-5 gallons ijn some cases.

Here is one method I have used to add a common 40or 50 gallon electric HW tank as a buffer.

Replace the two elements with 1" pipe nipples connect boiler and zones in at the tank nipples. A setpoint control monitors the tank temperature and calls on the boiler. About as simple as it gets.

Util we know exactly what piping you have for the fin tube, tough to answer your question about options. Rarely do you need a 3 way ZV.

EBEBRATT-ed, Thanks. Yes, I see how that would work with a pressure balancing valve at the end of the circuit.

pretty much all the new systems use ECM circulators with any type of zone valve or trv

The pump senses that a valve has opened and starts flowing automatically

So if you use zone valves, the thermostat turns open the valve the pump revs up

Same with trv.

Since the buffer is hot, there is no need to call the boiler on from a thermostat

Any 24v zone valve

1 is a zone valve

2&3 are arrows showing flow direction

4 is an air purger