Episode Transcript
[00:00:00] Speaker A: Hello and welcome to the Consulting Specifying Engineer podcast. I'm your host Anna Steingruber and today we'll be joined by Greg Ochtenhagen to discuss low temperature water systems. By lowering the temperature of water systems, building owners and engineers can meet sustainability goals in a cost effective and comfortable way. These systems can be put in new buildings or in retrofit projects. To talk to us more about this topic and dive in depth, Greg Ochtenhagen is here.
Greg is a professional engineer and lead accredited professional and a senior manager with Cleaver Brooks. He has over 45 years of experience with H Vac Design for commercial healthcare and institutional clients. Welcome Greg. Thank you for joining us.
[00:00:43] Speaker B: Thank you.
[00:00:44] Speaker A: So, to get started, can you just tell us what is a low temperature hot water system?
[00:00:51] Speaker B: Well, traditional hot water systems were typically 160 degrees Fahrenheit, return water to 180 degrees Fahrenheit, sometimes the supply temperature would go as high as 240.
Low temperature hot water systems typically are going to have a supply temperature of less than 140 degrees Fahrenheit and sometimes even as low as 120 degrees Fahrenheit. So that would be considered a low temperature system.
[00:01:24] Speaker A: Great. And so why design a low temperature hot water system? And can you give me some advantages or disadvantages to these systems in particular?
[00:01:35] Speaker B: Sure. It's really all about energy. And of course, if you're burning less fuel, you're also producing less carbon dioxide. So it helps the environment as well.
The big driver is typically condensing boilers. In order for a boiler to condense and get its high efficiency, you have to have a return water temperature of less than 127 degrees. And the colder the water, the more efficient it is.
So we want to design ideally a system with maybe 100 to 110 degree water coming back.
Now, to get the terminal units to operate right, you're typically working at a 20 to 40 degree Delta T. So that puts you in that 140 degree temperature going out.
The other reason you do it is there's a big push for heat pumps nowadays. And heat pumps also work better and are more efficient when they're not making as hot a water.
So that's really the push for low temperature hot water systems to get the boiler or the heat pump to operate at its most efficient point.
[00:02:43] Speaker A: What design changes are necessary for low temperature systems?
[00:02:49] Speaker B: Well, there's a number.
What you first have to look at the device you're using to make the hot water. So again, if you're using A boiler system, it has to be a condensing boiler because the whole point of the low water temperature is to get the flue gases to condense, which are very corrosive and would destroy a conventional steel boiler, a fire tube or a flexible water tube. But a condensing boiler's made with stainless steel and will not condense.
So that's the first change. And generally you're using a condensing boiler. So the push to a low temperature water system is to maximize the efficiency of that boiler.
The second part is your terminal units or your, your heat emitters.
So things like perimeter radiation reheat coils, radiant floor systems or air handling unit coils.
The, the drawback to a low temperature system, if you take perimeter radiation for instance, if a system or they're typically rated at 180 degrees Fahrenheit and when you cut the water temperature down to say 120, you lose about half the heat output per foot. So you need twice as much radiation.
Now this sounds like a problem, but in many cases it's really not. Because if you look at how perimeter radiation is designed, we're generally have a long wall and we're running the cabinet from one end to the other end of the wall to hide the piping.
But in that cabinet, say it's a 10 foot long wall, we may only have a 4 foot piece of thin pipe that the part that's actually emitting the heat. So even if we lose 50%, we just have to double that to 8ft. But if it's a 10 foot wall, it's not a problem.
So radiation, you need more fin pipe, but it doesn't change the cost much.
Reheat coils are a different matter.
Generally we have to go to a two row or a larger coil.
And sometimes if the you're using the coil that comes with the VAV box, you have to go to a larger VAV box.
Some manufacturers are now allowing you to have alternate coil selections to work with low temperature water. But obviously the two row coil or the larger coils going to result in some increased cost.
Air handling unit coils surprisingly don't change at all or change very little.
And the reason is the size of the coil is really dictated by the cooling coil that's in the air handling unit. And the heating coil is, is many times grossly oversized.
So having the lower temperature doesn't change the design at all. And then of course radiant floor systems or radiant ceiling or radiant walls are generally designed with 120 degree water or less. So they really don't have any change at all.
[00:06:06] Speaker A: So you mentioned that those floor systems don't have much change. What else wouldn't change when designing these kinds of systems?
[00:06:15] Speaker B: Okay, so basically, if you think about it, whether you're using 120 degree water or 180 degree water going out, you still need the same BTUs.
And BTUs are a function of the delta T, not the temperature of the water.
So for instance, If I need 10,000 BTUs and I have a 20 degree Delta T, you need one gallon of water.
Okay.
And whether it's at 120 or 180, it doesn't, it doesn't matter. I still need 20 gallons or one gallon of water with a 20 degree Delta T to get that 10,000 BTUs. So what that means is your piping doesn't change your pumps, your air control system. Now sometimes with the reheat coils, you may find where you used to use a 40 degree Delta T, you may have to go to a 30 or 20. So in some cases the flow rates may have to go up. But in the systems I designed over the years, I typically found that the flow rates and the piping didn't really change dramatically. So, so that's nice because it does.
[00:07:23] Speaker A: Mean.
[00:07:25] Speaker B: You'Re not adding any extra costs there or extra energy pumping.
[00:07:31] Speaker A: Great. Thank you, Greg. I don't have any other questions planned. Do you have anything else that you wanted to say about these systems?
[00:07:40] Speaker B: You know, I think the biggest thing with these systems is people, especially in the cold climates.
My practice when I was a consulting engineer was in the upper Midwest. So we had consistently designed temperatures of minus 10, minus 20 in parts of the state.
And, and we had systems that were designed around 120 degrees going back to the 80s.
You, you can design the system. People tend to think they need hot water to get the heat, and that's not always the case.
It does require a little bit more thought on your terminal selections.
It should be getting easier nowadays because with the advancement in energy codes and window types and insulation, the amount of heat needed in buildings is being reduced from when it was when I started practicing.
So that actually helps with low temperature water systems, but it can be done in very cold climates and has been done in the past.
[00:08:48] Speaker A: Well, great. Thank you so much. This has been so informative and we really appreciate having you on here today, Greg.
[00:08:55] Speaker B: You're welcome. I enjoyed it. Thank you.
[00:08:59] Speaker A: So once again, this was Greg Achtenhagen on low temperature hot water systems. For more information on hot water systems or any other H Vac topics, visit consulting specifying engineer online at www.csemag.com. thank you so much for listening and we'll talk next time.
