Decoupled buffer tank (four port)

Why are you connecting a 200k unit to about 50k of panel rads?

A right sized modcon with either heat pump water heater, power vent tank or indirect would cost less, be WAY simpler to install, take up about the same space and be more efficient.

P.S. Panel rads with a mix of TRVs/zone valves/ always on zones, piped direct to load on a right sized modcon is a great combination.

7 zones, what is the total btu load?

That is a 185,00 output boiler, quite large for most residential applications

True, you can lock it down probably a 10-1 turndown?

The boiler needs to size to the largest load. In some cases the DHW is the largest load.

Once you define how much or how quickly you want hot water. I doubt most people think about that, until they don’t have enough hot water.

The 199 could give you 4-5 gallons per minute based on 50-55 well water at a 77 rise. So itwould keep up with a standard 1/2 tub filler, or two showers running.

You use hot water differently with a combi or tankless.

With any tank type you fill a tub or run hw until it runs too cold, wait for it to recover. With a combi it just keeps providing as long as you want to feed it gas or lp.

DHW production tends to taper off as the HX gets scaled, add the flush valves, knowing that you will be descaling based on water hardness and how many gallons pass through

As far as the heat side sizing and buffer option.

Look at the smallest zone load. How does that match the lowest turn down? That is one way to determine if a buffer adds value.

Regardless, combis do cycle a lot. Anytime any hot water faucet is opened it will fire

My mother in law would hand wash and rinse dishes. She was old school, didn’t trust an automatic dishwasher. My father in law did sort of trust the DW, but would hand wash the dish before putting it in!
With every dish they would turn the faucet on and off. So the Laars Mascot combi probably cycled 1000’s

, maybe 10’s of thousands of times. It ran without any problems for 16 years, on LP

the buffer tank for a modulating boiler will be much smaller than a non mod boiler.

Here is some sizing math.

You will want a tank with 1-1/2 or 2” taps. Boiler Buddy, Lochinvar, Laars.

Zilmet has some small wall hung versions.

There are several goals a buffer can accomplish.

On a conventional boiler, say a wood or pellet fired we would run the tank to 180F or higher. A long hot, efficient burn.

Pull radiant loads off via a mixer on outdoor reset. So now you leverage maybe a 100° ∆.

On a condensing boiler it is more about lessening the cycling than a long drawdown. The ODR limits the tank temperature.

Really all you are doing is turning a low water content boiler into a higher water content boiler, with two components. AKA a condensing tank water heater :)

That was the attraction to the Polaris, Hydropulse, HTP voyager and Phoenix and others. The buffer was built in. They work great with even only 5-1 turn down.

If you run ODR, the boiler just needs a call for heat and it will modulate. Just jumper TT connection on the boiler.

I see the system sensor connection on the board, #6 & 13. Perhaps this is the sensor in the boiler. Or is it intended for an additional sensor?

As @Kaos mentioned, you could wire the boiler pump to run 24/7. Or tie it in to a relay to go off when ODR turns the boiler off, around 68° typically.

Yet another option wire any setpoint control to the boiler, run the tank to a specific temperature. Skip the ODR just run the tank to a specific temperature.

My first choice would be to let the boiler control run the tank temperature if the board allows that?

I don't see Laars offering a separate sensor in their option kits, however.

Or buy a boiler like the Lochinvar Knight that give you a row of additional sensor connections. #23 & 24 is what I have in my buffer. You have connections for flow switches, LWC, three different heat calls, power for louvers, variable speed input. Most any option you can think of.

Pretty much all the higher priced mod cons have a more robust control. More adjustments, more input options.

My Lochinvar did come with a spare sensor. It is a universal sensor. The sensor can be placed anywhere, in a tank, on a pipe, it is just a thermistor. The only unique sensor is the ODR, it is in a weatherproof enclosure

I have done a number of small buffers with electric tanks, here is a 6 gallon and maybe a 12 or 20 gallon.

I leave an element in for backup should the boiler go down I still have some heat.

The small Lochinvar electric tanks have a bunch of ports you can use,

3rd pic shows fittings to increase the 3/4 ports to 1-1/4 or 1-1/2"

Check what models your local distributors stock spares for. You want to get something you can easily get replacement parts for in a pinch.

You have to be careful with add on bits, it doesn't take much to add a couple of grand of BOM cost to an install.

The continuous circulation setup feels wrong at first glance but it works very well, it is what I run with my AWHP. Simple to set up and control, all the bits for setting it up are built right into the boiler. An ECM pump is a good idea for this type of setup. With decent outdoor reset, proper fire limit and delta T limits on the boiler you can get nice long burner runs at low fire.

Sounds like you are also micro zoning. There is usually no need for this, lot of the zones can be plumbed always on which increases the effective min zone size. Find areas that you want extra toasty, I run my bath floor floor heat, towel warmers and rad by the entrance always on. This also saves cost as there is no need for controls there.

For the rest of the house think about places where you actually want programmable thermostat control and where it can be TRVs. For example a spare bed is a good place where a TRV is good enough. You can also group zones with similar solar gain together under a single thermostat.