both of those are storage tanks to hold your heated water.
You have a tankless boiler that will heat your domestic storage tank and heat your home.
the important thing about the tanks would be capacity size.
so you have plenty of water for showers ectt.
Not sure if one is better then the other.

I think turbotax is for 23 gallons and Heat Flow is for 60 gallons (although I was told turbomax heats up wayer faster because of coils inside). Since what matters is capacity in terms of gallons, should i go with the 60-gallons Heat Flow and not risk it with a smaller turbomax?

Thank you so much for responding to my question!

There are two basic measures of tank water heater performance, capacity and recover time.

It is difficult to make any judgement based on the available information. Hot water usage patterns and the temperature of the incoming water make a big difference.

In fact, rather than building a new model, it might be more productive to know what system you have been using and is it adequate for your needs.

I would not want a 23 gallon in my house. However if it has a super fast recovery rate then maybe. Most 2 bath house is 40 gallon and 4 people.
I will let plumber chime in on this area.

Since you have natural gas, your other option would be to get a direct tank hot water heater. They come in various sizes (40, 50, 60 gallon). The advantage is the direct tank costs less than half of the equivalent sized indirect tank and operates independently from the boiler. If your boiler ever has problem you still have hot water. The traditional type uses no electricity so you can have hot water during a power outage.

We have not really lived in the house, we just bought few months ago and are renovating before we move in. The old water heater and boiler were very old, so we decided to replace them at this time. So I cant really tell you about the performance or our water usage. We live in Northeast, will have 2.5 baths, and we are a family of 3 (soon to be a family of 4). Not sure if this helps. I would rather have a larger tank than worry about running out of hot water. We were thinking about 60 gallons, is that too much for a family of 4?

That's a good point about possible power outtages and not having hot water. Having survived through Sandy, I want to make sure we have access to hot water during storms, especially if we have no electricity. So if we go with the old fashioned water heater that works independly of the boiler, do we need then Navien 240 condensing boiler, or should we change the boiler too?

Thank you!

It makes sense that if you are using a boiler for space heating alone, removing the hot water requirement, you might get by with a smaller one. If you can, it will work better.

How much of a hot water heater do you have in the old place? Keep in mind that teens use a lot of hot water.

We live in a coop apartment now, so we never had to deal with hot water heaters, usage and capacity.

Do some direct water heaters sill require electricity and some don't?

Aso, are there specific venting requirements to vent a direct water heater? Our water heater will be in the basement, and i wonder if venting will present an issue.

Thank you!!!

Some direct hot water heaters have a powered flue damper. The damper closes when the heater is not operating in order to minimize heat loss.The damper is powered by electricity. You can get a tank that does not have this feature and use no electricity. The difference in operating costs is very small. It is not worth it in my opinion.

A 50 gallon hot water heater with a 50K BTU input will keep a family of four in plenty of hot water. Update your shower heads. New heads are required to be all low flow. The newer heads will save a lot of hot water.

And how does this gas combi boiler come into play if we decide to go with a separate direct 50 gal. tank water heater? Does this combination make sense?

Any opinion on Naven 240 condensing boiler in particular?

Thanks.

If you got a direct hot water heater then you would not need a combination boiler. You could get a traditional high efficiency boiler.

Go to Amazon and read the reviews on the Navien 240 tankless hot water heater. There are some very bad reviews. It seems Navien has quality problems with their products and the customer service is not good. There are no reviews about the Navien combination boiler, but you get a sense of some of the problems people have had with Navien products.

"Do some direct water heaters sill require electricity and some don't?"

You can find significant reading on the department of energy and Energy Star web sites about different types of water heaters. You might find some pics to help you understand. IIRC, it probably does not treat auxiliary water heating on boilers as completely as for stand-alone water heaters.

A conventional tank water heater has a flue right up the middle of the tank where combustion gasses go to heat the water. When the water is not heating, indoor air is drawn up through tube in the middle of the tank constantly. This is an air leak in your house and cools the water off more quickly. A more efficient water heater has a blower, mechanical damper, and the associated safety equipment to close this off in addition to a combustion system to wring more heat out of the combustion, like the Navien.

Your choice 'boils" down to higher efficiency and higher initial cost for a modern water heater vs. lower initial cost and lower efficiency for the old-fashion tank heater. Cost savings in equipment might not be that great, however, because you need a flue through the roof for the stone-hatchet tank heater.

If I were you, I'd just use the Navien with the appropriate equipment for making DHW. If you are worried about HW in a power failure, make provisions to hook a nat gas genset to your boiler. It won't take much of a genset, portable or automatic to run that. You don't need an automatic genset because you don't need hot water when you are not home, but you might decide to install automatic backup for other reasons.

Thank you everyone for your advice.

After speaking with my contractor, here is some additional information about or situation.

The conventional tank boiler is out since we took the chimney down and they must vent through the chimney.

So we are left with direct vent units. What also complicates things is that we are installing Runtal radiators which were calculated to run at 180 degrees. My research indicates that condensing units only make sense when they work at lower temperatures, below 140. Does it mean that condensing unit is not appropriate for us? We can set the Runtals to run at lower temperatures (such as 140) but that will cut out BTUs output by 50% which means the rooms will either be cold or we would need to double up on the number of Runtals (which is a no-no since we don't have any more space on the walls to go bigger)

Since Navien only makes combi units, and we are going with a separate stand alone hot water heater, Navien models are out.

I looked at some Weil McLain models but they all seem to be condensing units or old-fashioned boilers requiring a chimney. Can anyone suggest a high efficiency direct vent WM boiler that is not condensing?

I also found these 2 non-condensing boilers - Burnham ESC5 104,000 BTU Cast Iron Gas Boiler and Slant/Fin Victory VSPH 111,000 BTU Direct Vent Hot Water Boiler. Are these any good?

Any other suggestions/advice/ideas you can think of?

My head is spinning at this time from all the information I had to read about the boilers, and you guys have been very helpful!!! Thank you very much!