A2W heat pump as primer for 4 bank hydrothermo ( 864,000 btu )

With a return of 127.2F the water coming out of the heat pumps is going to have to be 137F or so. That's awfully hot for a heat pump, the COP is going to be disappointing. Which means you probably won't save any money.

At 46F it can work. An EVI unit should be able to provide 130F water at about COP of 3. I'm mostly familiar with residential units which max out at around 5tons but there are larger commercial units out there. Quick google:

https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/equipment/chillers/modular-chillers/APP-APG021A-EN_08222022.pdf

Roughly how many therms of fuel are you using in a heating season to get a feel for actual heat load.

I recommend the article outlined in this article to get an estimate of your actual heating load:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

Since — according to the article — heating equipment is routinely oversized (typically by a factor of 2 to 4) you need to get a handle on your actual heating load before doing anything.

This is a sound strategy.  The control system does not need to be that complicated.  Whenever water is returning to the boiler the heat pump operates.  I don't have any experience with air to water HP with the exception of two customers that used geothermal for their HP which is W2A.  (I wonder how installing them backwards might work? LOL). I’m sure there is something out there that will work for you.

I'd be pressed to install high efficiency boilers in your situation. You might save $2,000 in fuel costs, but that savings will get eaten up by repair costs. And you'll be replacing those boilers every 15-20 years as that's how long they last. There are exceptions; Polaris comes to mind.

Love those Hydrotherms.

I haven’t seen a new HydroTherm boiler in years. My local dealer lost the line ages ago.
I would look to replace it with a Burnham cast iron boiler or even a Peerless. Quality American products.

40f outdoor (so 30F delta) is somewhere around zone 4 say 3000 heating degree days.

26000therms, say 20% for DHW and 75% boiler is 16000 therms of heat.

16000therms / 3000hdd*24h=0.22 therms/degree hour

30F delta *0.22therms=660k heat load. Those boilers are not that far off actually.

Heat pump makes sense if your electricity is cheap. Lets say you size to supply 1/2 your heat cost (so most of the shoulder season plus part of heating) from heat pump, you need 8000 therms of heat out. COP3 that is

8000term*29.3=234MWh of heat/3=78MWh of electricity.

As for piping, @hot_rod I think this is the better option here

This puts the two heat sources in series and controls is pretty simple. Heat pump runs all the time, boiler runs only if the supply to building drops.

I think in case of the OP, the better option is to figure out a way to stage the boilers so only one needs to fire in the shoulder season (thus higher efficiency and less wear) and can pick up a bit of extra efficiency with a flue damper kit to reduce stack losses.

The above diagram would also work with a modcon pre-heater. You could install a big residential modcon which are cheaper and have it run all the time. This would provide a decent fuel savings without a big cost plus gives you backup in case a hydrotherm or two fails.

Another good piping option.

That was showing a HP addition to an existing system that had injection piping used on a cast boiler/ radiant system. That was a common method in the early non condensing boiler/ low temperature radiant installations.

If starting from scratch a sep or small buffer could make more sense.

@DCContrarian With a series setup, this is not an issue, you need slightly different reset curves on each unit. One on the heat pump (or modcon) that is say 5F higher than the one for the boiler.

If the AWHP is targeting 130F and the boiler is 125F, as long as the heat pump is SWT is 130F, the boiler won't run. When the AWHP starts loosing steam and say output drops to 120F, the boiler will fire. This will bring the loop temps up to 125F and return will rise a bit. As long as the return doesn't go above the AWHP setpoint, the unit will continue to run. My AWHP runs at max power with a 10F delta, so as long as the return stays bellow 120F in this case, it will continue to supply full power even with the boiler running.

Most AWHP units can be set to shut off bellow certain outdoor temps. You can set this at the cost crossover point.

@hot_rod The issue with a standard hydraulic separator is that it will make controls difficult as now both units operate in parallel and there is a high risk of mixing flows. AWHP efficiency tanks with very hot return water, something you avoid completely with the series closed spaced Ts. Even though not floor heat, I also think with cast iron boilers, the injection loop is a good idea as it lets you run a much more aggressive outdoor reset on the heating loop without condensation risk.

@walnut This is all fun to think about, but I don't think the cost pencils out. About the only way to make this work is if you are looking to renovate units and switch each to their own water source heat pump. This is a common setup for high rise construction as each tenant is responsible for their own heating cost plus they also get AC. The building still needs to supply a heat/cooling loop but it is now at much lower temperature (50f-60f) which is easy for commercial chillers.