By Roy Collver
Hydronic space heating systems are ideal for doing double duty by generating Domestic Hot Water (DHW) through an internal heat exchanger inside a storage tank.
These tanks are called indirect water heaters due to the two stages of heat transfer that take place. Stage one occurs in the boiler’s own heat exchanger (HX) where hot gases from the combustion process are transferred into the hydronic system water. Stage two occurs inside of the tank where the hot boiler water transfers heat through the tank heat exchanger to the colder domestic water.
These have been around for years. But the continuing trend towards lowering system water temperatures to take advantage of condensing boilers and heat pumps has created issues. Older designs were based on high-temperature boiler water being available at all times of the year. This was always inefficient in the warmer seasons, but when fuel was cheap and climate change was just a whisper – system designers skimped on heat transfer surface in order to lower the cost of equipment. Heat emitters, including indirect tank heat exchangers, were small in high temperature systems.
It was never a great idea. When you look at the nuts and bolts of heat transfer in an indirect tank, the way to build a better heater by using a generous amount of heat transfer surface becomes obvious. As a bonus, when an indirect HX does not have to deal with high water temperatures, repeated thermal stresses from hot/cold cycling are reduced dramatically, resulting in longer service life. Mineral-scale build-up is reduced or eliminated completely when the HX surfaces on the domestic water-side are not subject to high temperatures as they are in direct-fired or high temperature indirect tanks.
Four design factors
Fig. 1 shows the way heat moves about in a hydronic heating system. The four design factors shown are locked in a dance of physics where the movement of one partner will affect the movements of the other three. Some partners are restricted to a small area on the dance floor and the others can’t drag them away from it.
Factor No. 1 deals with the tank’s internal heat exchanger – most tanks use one or more coils of metallic tubing to carry the hotter boiler water through the tank of cooler domestic water. Designers can increase surface area with longer tubing, or with larger diameter tubing, but they need to be careful to consider that factor No. 4 – flow rates – can only reasonably dance within a small space. Longer tubes resist high flow, while large diameters suppress turbulence and heat transfer at reduced flows.
Factor No. 2 deals with how much heat can move (transfer) through the heat exchanger walls from hot to cold. When using Imperial measures, heat output is commonly described in Btu/h transferred per square inch of HX surface per Fahrenheit degree temperature difference (Delta T).
A bigger difference results in more heat transfer, but for any given Delta T, the other three factors can conspire to hold back the amount of heat that gets through. There are also external design factors that could restrict the maximum temperature available, such as restraining the boiler temperature to promote condensation.
Factor No. 3 only helps if the other dancers are able to boogie to the same beat. A big boiler with high heat output is not an asset unless the indirect tank has a big HX coil, operating at a high Delta T, combined with high flow rates to be able to absorb all that heat. In other words, the four partners need to be fit, and dancing to the same tune, or they will start tripping over each other
Sizing to load
In most systems, the HX is sized to a DHW load which is less than the heating load. In these cases, the HX will not be able to transfer the full heat output from the boiler and the boiler will modulate or short-cycle to match the capacity of the HX.
An important class in Hydronics 101 should require students to observe a modulating boiler’s operation through a complete DHW heat-up cycle while they monitor system inlet and outlet temperatures. If the tank is cold (high Delta T between the inside and outside of the HX), the boiler will modulate up to a high firing rate because Factor no. 2 is taking the lead. As the tank warms up (decreasing the Delta T), Factor 2 slows down the whole dance troupe even though Factor 3 still wants to boogie hard. What about kicking it up a notch by increasing the flow rate in the coil to increase the average HX temperature?
Factor 4 runs hard up against Factor 1 – both facing down the high cost of hydronic and domestic water compatible metals. To increase flow rates, the tubing diameter needs to be increased as the tube gets longer – basic fluid dynamics. More HX surface area requires more tubing and/or a larger diameter. Both solutions mean more metal – more money.
Fluid chemistry in these systems requires high-grade materials for longevity and robust heat transfer. The best materials have proven to be 316L or 316Ti stainless steel, and copper. Copper is expensive, but a very good conductor of heat. Stainless is not so expensive, but not as good for conducting heat, so you need more of it.
This all presents a conundrum for tank designers: Add more metal and drive up the cost of your tank – or reduce the amount of metal to keep the tank cost down, but end up with poorer performance.
Choosing the right tank
Many indirect tanks will do a great job in traditional high temperature systems, but the requirements of the “new” hydronics can make life difficult.
The first thing to study when choosing an indirect tank is the manufacturer’s published capacity charts (See Fig. 2). They will show you DHW outputs at various boiler flow rates, outputs and temperatures, and various domestic water inlet and outlet temperatures.
The DHW outputs are often rated using an IBR standard U.S. gallons per hour continuous flow, or for the first hour draw (which accounts for the tank’s storage capacity). Be prepared to wade through pages of charts. DHW tank manufactures have had to ex
pand their specifications to account for a wider range of conditions.
New hydronic systems may only be designed to supply 130°F water to the heat exchanger, rather than the 180 to 200°F in days of yore. DHW temperatures of 115°F are now more common than the traditional 140°F. The example charts included give you an idea how much variation there can be in a single tank size with different HX coil options.
The three coil photographs come from a manufacturer who can build you a tank with your choice of three different HX coils – standard – high capacity – and extra high capacity. Expect to see more manufacturers offering expanded choices like these.
There is no need for explanation, you can easily see the design changes made in order to gain more HX surface area and allow for more flow as the coils become too long. The tank connections also have to be up-sized, as both the boiler and domestic flow rates increase with higher capacities.
There are many other options not covered in this article like dual coil tanks for solar and micro-load service – reverse HX where the DHW is drawn through the coil and the boiler water is in the tank – tank-in-tank heaters and more. They will have to wait for another day
Yes, a properly designed tank can get expensive, but don’t forget that a well-built tank made of premium materials will likely outlast two or three boilers and save the owner money in reduced energy costs as well. To the best of my knowledge, no one has ever complained that their DHW tank lasted too long.