Tankless boiler with indirect hot water heater

[Marcel R. Cyr]

Originally Posted by Brian A. MacNeish

The first hour draw has the tank 40 gallons standing in reserve at 140 degrees. The unit then produces another 148 gallons at 140 degrees.
I am still trying to comprhend this, This means that the recovery of this unit is at least 2.5 gals. per minute according to your numbers. So that means that for the first hour, including the 40 gals. of storage and recovery rate, it could or would put out 190 gals.

To heat the well or municipal supply water volume of 148 gal from 60 degrees to 140 dgrees at 80% heat transfer efficiency, the boiler will burn 148,000 btu's of gas ( 1+1/2 therms or 148 cu ft of gas approximately)

The above was all calculated assuming there is no call for house heating and all the heating capacity of the boiler was used for domestic hot water purposes.

In the second hour, we don't have the already heated 40 gall in reserve as we are heating cold water from the start of the hour to the end. The unit may be able to heat 130-150 gal in the second hour if it has a large enough burner.
In the second hour according to the recovery rate as noted above, and without the 40 gals. reserve, the most you would get in an hour is 150 gals.
And this is assumming maximum effiency.

In cold weather, some of the heat from the burner is needed to heat water for the house heating zones and it may only be able to produce 50-60-70 gal of domesstic hot water.
Ah, now we are down to .83 gals. per minute to 1.1.
Well, I think my wife would be having a fit while trying to take a bath.

Interesting.

Kenton, do you get it now?

I think, I have constituted my confusion even more. ha. ha.

Marcel


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[Brian A. MacNeish]

Michael:

Here's another example:

40 gallons is the most common size electric hot water tank in Canada. Most of them have a first hour draw of only 46 gal or so - the 40 in the tank already heated and a big 6 gal extra. Read on- To reheat the whole 40 gal again will take about 3 hours!!

Why? The 40 gal tank has 2-3500 watt (10,239 btu/hour) elements but only one is "on" at a time. They work in what some call a "flip-flop" arrangement. The top has priority since hot water is drawn from the top of the tank; when the top is satisfied, the bottom element completes heating the cold water at the tank bottom. So to heat 40 gal from 60 to 140 degrees will require about 32,000 btu's. Divide that by 10, 239 btu's /hour and you get......3.1 hours!!!!

Your wife will divorce you if you put one of these babies in!!!

[Marcel R. Cyr]

She almost did until I got one of these.

[sturetsky]

I've installed somewhat similar systems as this, but am not quite sure about this one. But let me take a stab at it.
As far as 188 gals @ first hour, I think that is the maximun output. 188 gals per hour... isn't that about the same as 3 gals a minute? So in a "on demand" system, with domestic water being fed directly from the boiler, that might be how much hot water it can supply.
Most on demand systems have two temperature zones. One for the heating loops and one for domestic... which would require a lower temperature so no one gets burned.


By using the tankless system for domestic, the unit would have to operate every moment hot water is being called for. By using a storage tank, with high and low settings, the boiler has to work less often. It only has to heat the storage tank when necessary.
Another thing is, that if you want "instant" hot water, you can hook up your loop to the tank and it can circulate when needed.

If this is a single temp unit, or even if it's not, the storage tank could aslo be used, along with a mixing valve to regulate the temp of the domestic water delivered. This is another way to reduce the frequency that the boiler has to operate.