Indirect water heater stretch suggestions

[DIYorBust]

Trying to get a little more peak capacity out of my DHW system, and would appreciate suggestions. I have an SSU-45 indirect water heater heated by a Navien NHB-150 which also provides space heat. It's ok most of the time, but when the cold water supply gets really cold in Jan/Feb, it can a little short on hot water when I fill the tub, or have 2 or 3 showers going. Here are some ideas I've had, but they all have drawbacks. Do any of these ideas sound practical, or what approach would you folks take? Much thanks in advance for your responses.

1. Add a 30 gallon storage tank in series with an aquastat to call for hot water from the indirect.
-Whichever is closer to the fixtures would be on priority.

2. Install a second indirect, in series with the first, and put one of them on DHW priority over the other.

3. Install a larger indirect.
-The SSU-45 requires about 150K BTU input, so if I upsize the indirect, the performance and recovery time might suffer.

4. Install a tempering tank to feed the indirect at room temp.

5. Install an electric water heater, perhaps in parellel.

6. A combi plus an indirect or storage unit might be possible, but is this practical? Plus I'd be replacing a working boiler, and combi's don't have a great reputation.

Other considerations:
-NHB's cannot be common vented with water heaters, and a tankless would probably require a gas service upgrade which is currently not possible.
-The hot water is stored at 180 degrees and downmixed at the outlet.
-Space heating load is 60-100K BTU.
-I cannot easily install a stand alone gas water heater because I do not have a flue or chase available, and cannot wall vent due to the lot configuration. The roof is about 50 feet up from the boiler room with lots of interesting stuff along the way.

DIYOB

 

[Reach4]

I presume you made the water heater a zone priority. Does the boiler switch to high when the hot water cools?

https://terrylove.com/forums/index....heater-stretch-suggestions.83686/#post-603037
https://terrylove.com/forums/index.php?threads/boiler-and-indirect-questions.83669/#post-602941
https://terrylove.com/forums/index.php?threads/running-out-of-hot-water-dilemma.83596/#post-602438

 

[Sponsor]

 

[Dana]

>"The SSU-45 requires about 150K BTU input..."

NO, it doesn't "require" any such number. That number is only relevant to the rest of the performance specs. There are plenty of SSU-45s out there doing just fine on 50K boilers. And yes, it makes a difference on recovery time, but them's the breaks. The NHB 150 puts out about 4x the heat of a typical 40-50 gallon standalone- does anybody really worry about the recovery time of a 50 gallon standard water heater?

If it's showering capacity your're after, if you have at least 5' of vertical drain downstream of the showers, for about a grand in hardware cost a 4" x 54" (or taller) EcoDrain V1000 or 4" x 54" PowerPipe (or taller) can double the showering capacity, by pre-heating the water entering the indirect (and the "cold" side of the shower mixer) to ABOVE room temperature. With the indirect zoned priority the NHB-150 could easily keep up with 3 simultaneous showers with drainwater heat recovery unit behind it.

In an open basement this is pretty DIY-able, dead easy if it's plastic drain, a bit riskier with cast iron, but still not a high risk project for the handy homeowner.

Mind you, the NHB-150 is crazy oversized as space heating boiler for most normal sized homes on Long Island, and might even be oversized for your radiation. Run this napkin-math on last winter's gas bills. Then run this math on your zone radiation.

At least the NHB-150 throttles back to a nice low 10K-in, unlike most combi-boilers with burner sizes that large. At 10K-in/9.5K out in condensing mode it only takes about 45-50' of fin-tube baseboard (per zone) to not run into cycling issues, and even the cycling on a 35' zone won't be so bad that it can't be tamed by setting up the boiler parameters correctly.

 

[DIYorBust]

Thanks Reach and Dana, these are some great ideas. I'm going to analyze the space and see if I can make one of these suggesetions work. Do you think upgrading to a larger indirect would increase the shower capacity significantly? Recovery time isn't a problem if the water last long enough for the peak demand. I think running 3 showers for 20 minutes would likely have us covered. There are 5 showers and 3 units in the building, but 3 of them are in one unit, and rarely do I think more than three would run.

I'm also interested in the shower drain recovery idea, but will this work if the drains are 40 feet up from the water heater, probably 70 feet total distance. I could use it to heat the cold side of the shower input but then i need 5 units, and I don't think I have the space for them. Are there any horizontal units? My sewer line is only 4 feet from the ceiling or so in the cellar, so it could be difficult to fit this in. All plumbing is no hub CI, but I think I could DIY this if I had the space.

Regarding water temp, the indirect hot water is on priority and the outdoor rest is not connected, so the boiler runs at max temp. When the project is complete, the hot water will call through the DHW channell for 180 degree water, and the space heat will run on outdoor reset. It sounds like I have some options though, which is great.

Thanks!
DIY

 

[Dana]

Thanks Reach and Dana, these are some great ideas. I'm going to analyze the space and see if I can make one of these suggesetions work. Do you think upgrading to a larger indirect would increase the shower capacity significantly?

A 100 gallon indirect will have about twice the showering capacity of a 50 gallon indirect- it's pretty simple math.

I'm also interested in the shower drain recovery idea, but will this work if the drains are 40 feet up from the water heater, probably 70 feet total distance.

The water temperature doesn't drop much flowing through 70' of drain. The amount of surface area contact with the water is minimum on horizontal sections of drain, but is quite high (nearly 100% of the pipes internal surface) on vertical sections. The thermal mass and thermal conductivity of plastic drain is pretty tiny too, a bit more if it's all cast iron, but still not a huge problem.

Are you saying the bottom of the showers are 40 vertical feet above the top of the water heater?

The heat exchanger doesn't have to be located next to the water heater. Even 50' of 3/4" plumbing is about a gallon of water, or about 30 seconds-worth of showering time with a low-flow shower, so there isn't going to be a huge loss there either. While in-situ monitoring studies of these things in college dormitories shows a hit in performance relative to typical residential applications, the plumbing lengths are in hundreds of feet, and the diameters are also larger.

I could use it to heat the cold side of the shower input but then i need 5 units, and I don't think I have the space for them. Are there any horizontal units?

Thanks!
DIY

The common way to deal with it is to feed the entire cold water distribution system to the house with the output of the heat exchanger. That way the cold side of the shower mixer is getting output from the heat exchanger, as well as the cold feed to the water heater. The only down side to that approach is that the cold water for other uses at other taps doesn't get very cold when people are actively showering, starting out at about room temperature or slightly above. A 60% efficiency unit at the NRCan test flow of 2.5 gpm is only about 40-45% return at 5-6 gpm (three low-flow showers). So with incoming water at 40F and 106F at the shower head the output of the heat exchanger is about 78-80F when just one person is showering, 70F when three showers are running.

In practice it's never been even close to an issue at my house with a 2 gpm (bucket verified) shower and a 4" x 48" PowerPipe that tests a bit under 50% return efficiency.

There are horizontal units, but the horizontal orientation doesn't deliver the 90% coverage gravity film of water on the interior of the drain than happens with vertical orientation, so it take a much bigger unit to achieve the same return efficiency. EcoDrain's B1000 is probably the best bet, but in your application it's not necessarily a good bet.

 

[DIYorBust]

Thanks Dana, is it worth feeding the cold water to the fixtures as well as the the water heater inlet?

Also, I'm looking upgrading to another HTP indirect, I see they have a Superstor pro, which has a smaller heat exchanger than the SSU for the same tank size, but a higher first hour rating, and a faster continuous rating. How is this possible?

Thanks,

DIY

 

[Dana]

Thanks Dana, is it worth feeding the cold water to the fixtures as well as the the water heater inlet?

Absolutely yes- that is the configuration under which they are tested for efficiency, the tepid water out of the heat exchanger feeds both the water heater and the cold-side of the mixer. If it's feeding ONLY the water heater it'll take more than 10% off the efficiency/capacity numbers (instead of 58% heat recovery it drops to 45%, etc.)

Also, I'm looking upgrading to another HTP indirect, I see they have a Superstor pro, which has a smaller heat exchanger than the SSU for the same tank size, but a higher first hour rating, and a faster continuous rating. How is this possible?

Thanks,

DIY

I'd need some model numbers to be able to ponder the full specs. (A bigger BTU boiler would be my guess.)

 

[DIYorBust]

Absolutely yes- that is the configuration under which they are tested for efficiency, the tepid water out of the heat exchanger feeds both the water heater and the cold-side of the mixer. If it's feeding ONLY the water heater it'll take more than 10% off the efficiency/capacity numbers (instead of 58% heat recovery it drops to 45%, etc.)



I'd need some model numbers to be able to ponder the full specs. (A bigger BTU boiler would be my guess.)

SSU-60 vs SSP-60. BTU is actually lower on the SSP with a higher hot water output. Perhaps the exchanger design?

So my concern is that with the bigger tank, the 1st hour rating would be higher but...

HTP says to calculate the continuous rating as First hour - .75*tank size. So the larger tank would have a lower continuous rating, which could reduce shower time once the tank temp approaches 106. Is it possible I would be more likely to run out of hot water with the bigger tank?

 

[Dana]

The 140F first hour rating of the SSP-60 are predicated on having at least 155 KBTU/hr of boiler output (see p.3). The 140F first hour ratings of the SSU-60 are predicated on a 174K of boiler output at 180F (see p.6)

The most you're going to get out of an NHB-150 with 180F output would be about 130-135 KBTU/hr, so no matter which indirect you're hanging on it, it will underperform the published first hour specifications. The tank size will matter.

The cooler the water in the tank (as it approaches 106F) the more effective the heat exchange, but you're still burner limited on the continuous flow numbers. At a 66F rise (40F in, 106F at the showerhead with 135K of burner output it's adding at best 135,000/66F= 2045 lbs/hr, or (/8.34lbs/gal.=) 245 gallons/hr potential first-hour gallons beyond what's stored in the tank. Figure on ~280-300 first hour gallons (which is still quite a lot) with that setup. That's 4.5-5 gpm, enough to support 2+ simultaneous 2gpm showers for a full hour. Does anybody in the household take showers even half that long? The US average is about 8 minutes, and the amount of heat drawn in that 8 minutes will be replaced in about 4 minutes.

With a decent drainwater heat exchanger 130K of boiler output will support three 2 gpm showers forever without losing ground.

 

[DIYorBust]

I like it. Thanks Dana. I checked with HTP and the SSP-60 does indeed have a different exchanger design that apparently can trasnfer heat a little faster than the SSU version. I think with thedrain water heat exchanger that would likely get me where I need to be for showers. No one takes an hour long shower, but it's a 3-unit building, so 3 simultaneous showers could happen at the same time. The key is whether I can find a place to fit this. Ecodrain says the horizontal unit is out of production at the moment.

 

[DIYorBust]

Well I looked into this, and ultimately it seems that it's not really that practical to use since most of the bathrooms in this project are only a short vertical distance above the building drain, and those that are not do not share a drain so I'd probably need multiple units, a lot of copper, and even then it would only recover heat from half the bathrooms. I think I may upgrade the heater to an SSP-60 or SSP-80. 2 showers for an hour sounds good, but the reality is probably not as good. There's some loss in the long pipe runs, plus sinks often are running. If a tub gets filled during a shower the drawdown is much faster. The units are expensive too, and since I have natural gas, it's not a big money saver on fuel, for now.

Another concern I had is how it might interact with a potential hot water circulation system. It seems like it might have the effect of heating up drain water under certain circumstances unless a return line is installed.