Tankless boiler with indirect hot water heater

A real common set up in newer, energy-efficient homes around here. The tankless boiler supplies hot water for radiant or baseboard home heat and they also connect an indirect water heater to it.

The “first hour” rating of 188 gallons means that’s how many gallons of hot water it’ll supply during the first hour? Will that amount change after the first hour?

**Will that amount change after the first hour? :shock: :shock: :shock: **

The average family of 4 (not water wasters nor severe conservers) will use 60-80 gal US hot water per day. 188 gal/hr!!! Why would anyone need such a system for domestic hot water.

7-8 years ago, I was seeing the same systems in new houses but it seems to have stopped here. The heating salesmen were overselling what the customer really needed just to earn more commission. When the customer does not know what the average or norm is, it’s easy to sell them on something they don’t need.

My background isn’t in plumbing, but maybe it’s got something to do with heating a three-story home with hot water radiant in-floor heating when it’s 20 below outside.

So, the “first hour” rating of 188 gallons means that’s how many gallons of hot water it’ll supply during the first hour? Will that amount change after the first hour?

Kenton, my specialty isn’t plumbing either, so maybe we can both learn on this one.

I have never seen a heat plant like your picture. Not much gas fired units around here.

In trying to answer your question, would it make logical sense that between the storage capacity of heated water and rate of recovery of the unit, it would produce 188 gallons the first hour , but once the storage capacity is gone, would you not be down to the recovery rate of the unit?

Does that make sense? ha. ha.

Marcel :slight_smile: :slight_smile:

I used to regulate the energy efficiency of equipment and appliances in my province. “first hour recovery rate” is a term used only for domestic hot water heaters so that when a usage need is known for a facility, the proper unit can be specified. The indirect DHW is rated by adding the volume of stored water available at any time and then its ability to heat extra water to temp. in the first hour.

The fact that the the indirect unit is supplied water that is heated by the boiler immersion coil is redundant. An indirect tank has its own heating coil installed and is treated as a separate zone from the boiler using boiler house heating water as its heat source.

Using the immersion coil in this setup probably makes the whole system more inefficient.

he whole system has to be sized according to the design heat loss of the dwelling + the possible coincident btu need for hot water + a bit of a safety factor.

I think this explains it well…

Other info, such as its source, from the DOE site is quite instructional:

Source: Gas Appliance Manufacturer’s Association

And this little tidbit:

*The above worksheet assumes no water conservation measures.

This is where everyone should be directed to!!!

*]Reduce Your Hot Water Use for Energy Savings

How much hot water would a tankless boiler have to produce per hour to heat a 4000 sq. ft. home if the home were being heated using raidiant in-floor heating? Isn’t tht a large part of why this boiler appears to have such a high output?

tankless boiler

Do you mean a “low mass” or “cold start” heating boiler that only holds 3-4 gal of water. For energy savings, these units go back to ambient room temp between calls for heat. They act similarly to wall hung tankless/ instantaneous DHW heaters.

No matter what the size of the home, a design heat loss should be done to determine the heat needed in the worst conditions. 4000 sq ft is not 4000 sq ft when one house is 50+ years old with no energy retrofitting carried out versus a new high efficiency house with double low E/argon windows oriented for solar gain. The old house may need 3-4X the size boiler!! If any one gives an uncalculated answer to this, they are shooting at the stars!!

It’s the one in the picture in my first post, Brian. I don’t think there’s any water in it except for what’s in the coil around the heat exchanger.

Good point. In this case it’s a new strawbale home, so walls are around R-30 and ceiling around R-60. Good windows, but the whole thing is not super tight because they are pulling makeup air through the building envelope.

Here’s the project. The girls are cutting and tying flakes to fit odd size holes. The guy in the pink shirt is stuffing loose straw into every place he can. That’s why straw bales make 2-hour walls. Practically no oxygen in the wall to burn. Almost all straw bales that burn are lost during construction to careless subs.

It’s the one in the picture in my first post,

Sorry, Kenton, I only looked at the tank and never got back to that post. The second picture could’ve saved me and you at least the thousand words.

That would be so low mass as to be instantaneous. There is no immersion coil in that wall hung boiler as there is in the older, heavier steel or cast iron boilers sitting on the floor.

For the given test conditions, that unit will produce only 188 first hr gal of domestic hot water (not heating water) at say 140 degrees F.

Will that amount change after the first hour?

It most likely will not be able to supply 188 gal in the second and following hours as it has no standing hot reserve of say 30-40 gal. It definitely won’t be able to supply that amount of second hr water during a cold weather period unless it is oversized.

For example: for 180 gal at 140 deg in the second hour, with 60 deg supply water heated at 80% domestic hot water efficiency, the unit wll need to produce and dedicate 180,000 btu’s to the hot water function! and not to house heating!! But luckily we don’t need that much hot water.

Kenton; Isn’t somewhat like I was saying? :???: :-k


I don’t know Marcel, I think it is but I’m confused…

The indirect water heater tank is 40 gallons. That doesn’t count?

…and I don’t understand this explanation. Sorry Brian, I appraciate your effort but you lost me.

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.

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 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.

Interesting. :slight_smile:

Kenton, do you get it now?:wink:

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

Marcel :slight_smile: :stuck_out_tongue:



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!!!

She almost did until I got one of these.


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.