Combi Boiler Design Suggestions

Does the combi have the same output as the Turbomax? Do you need the output of a Turbomax? Rain heads? A lot of people? Jacuzzi? Laundromat?

a combi might keep up with 4 showers at once in some place that is warm where the incoming water is say 60f but where i'm at the municipal water is like 35f at some times in the winter and a single residential size boiler isn't going to be able to heat that instantaneously.

if you are buying a stainless indirect, look at the chloride content of your water and the specs of the tank, places with a lot of road salt tend to have municipal water with a lot of chlorides.

with a reverse Indirects you will need to maintain that tank around 180 to get the rated performance. So the mon con efficiency will drop when operated at those temperatures.

Got enough fin tube to run at lower than 180 SWT temperatures. probably some of the heating season the outdoor reset will lower the operating temperature

Plus look at the btu/ hr input required with reverse Indirects. There are some advantages if you need a buffer for the radiant. Most mod cons turn down 10-1. So unless you have some micro zones used constantly, you may not need a buffer? With a two zone system and a mod con, I doubt you will need a boiler buffer.

Another common approach is use an indirect with a lot of coil surface area. This allows you to generate DHW to 120 with maybe boiler running 140 SWT, so staying within condensing range.

Heat-Flo has these hi performance tanks with large dual coils, Viessmann also.

It takes X amount of btu to heat water regardless if it is in a tank, coil, or tankless.

Tankless and combis are usually rated at a gpm flow at 77 degree temperature rise. Some are at 70 degree rise, so read the spec carefully.

Most DHW systems run 120 degrees, so a 77 degree rise takes 43 incoming to 120.

people show with water in the 100-106 degree range, so you are adding some cold to 120 water to shower.

If your incoming water temperature drops below 43, obviously the gpm performance of a tankless or combi drops.

Really the main difference between a tankless or combi is that you are not storing hot water, and you have a near endless supply when sized properly. It is a family preference question, both are commonly used with fine results.

In northern climates with above ground storage towers water can drop into the mid to high 30’s in winter,

While Bluetooth is a clever feature on a circ, how often do you plan on adjusting the pump? Do you have design numbers to adjust to?

My fav is a single delta p style pump with zone valves. The auto adapt function allows the pump to adjust to your system’s needs. That may be a better pump operation mode than you and your app can select🤔

Here are their high output models and specs.

The show perfomance from 200 boiler supply all the way down to 120 supply.

It really comes down to what you want or need, there is no one size fits all.

If the previous tank was adequate with a GV-5 boiler, use that recovery number to size a new one.

Personally a Lochinvar Knight 105,000 with one of the high performance Heat Flo tanks would be my choice. a similar size to the boiler you have and plenty of btu for a big DHW supply.

A 60 gallon high output with 80,000 boiler input give you plenty of Dhw. Pipe it with a dedicated 14 gpm pump for full performance