Emergency Shower Tempering systems

[tbonebanjo]

Has anyone any experience using gas fired instantaneous water heater with ASSE1071 mixing valve serving combination Emergency Shower/Eye-face wash? I need tepid water between 60 and 90 degrees F final temperature at 23 gpm. It seems most of the tempering valves I have seen require fairly high (120-140 degree F) hot-side temperatures to operate properly. It seems rediculous to heat water that much just to temper it back to 80 degrees, however I do not have room for a 120 gallon storage tank. All I seem to come across for this application is electric (I cannot do 180 kw) and steam (not available this project). Any info would be helpful.
Thanks,
Carl in Maryland

 

[IRstuff]

Why do you care how the final temperature is achieved? Is fuel efficiency such a big issue for something that, hopefully, never, or seldomly, occurs? In the end, the net BTUs input is about the same. And, with the higher temperature high-side, achieving the desired temperature is essentially instantaneous. Trying to do direct control of the output temperature using the heater is a blunt tool and is almost guaranteed to be slow in response.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[PEDARRIN2]

The temperature for emergency fixtures is set by an ANSI standard, but there is a range. Set it a bit on the cooler side and you can downsize your water heater.

Use of instantaneous water heaters (electric or gas) would seem to be ideal for this type of situation, i.e. seldom used, but with a large dump load.

Heating water to a higher temperature and then cooling it is typical.

Concerns with use of instantaneous heaters:

1. If you have any significant hardness, remove it. The flow path of these heaters is small and it will scale (fail) before you realize it.
2. Depending on the BTU requirements, you will need fairly large gas pipe to provide the heat to comply with the flow requirements. So if the applicable heating requirement is 100,000 BTUH, you will need 400,000 BTU because you have to provide it in 15 minutes to comply with the ANSI standard.
3. The instantaneous water heaters typically are lower flows so you will likely need more than one. You can minimize this if you have higher water pressures.
4. You have to get the flue gases out. Even though these heaters are typically high efficiency, you should not use PVC for the exhaust.
5. Make sure you have adequate combustion air because of #2 above.

You have not mentioned whether there are other hot water requirements in your building. I commonly size a storage water heater to cover the emergency fixtures with a combination of storage and generation and use the heater to also generate the hot water for the rest of the building. The heater is a bit over sized unless you have showers in the facility, at least you would be getting use out of the stored hot water.

If some of your concern about using a storage water heater is due to standby losses, they are not as significant as you would think. Running the calculations from the ASHRAE standard indicates the payback on an instantaneous water heater is longer than the water heater would last under normal operation.

 

[tbonebanjo]

I do have other hot water requirements and the first thing I did was size the storage for the additional load of the emergency equipment. However, the architect is not giving me any floor space for water heating even though this is a huge warehouse/office/processing application. She wants me to put water heaters on a platform above the janitor's closet. The combined weight of the equpment using stored hot water is over 5000 lbs. I can go that route, but that's a hefty platform. I need to weigh the cost of the structure and storage heater against the instantaneous multiple heaters, beefed up gas supply, etc. I have never designed emergency equpment with instantaneous but it makes sense to me if I can get it to work at reasonable cost. It is weird that all of the hits I get when I google this are electric heaters with about 180 KW (no, we do no thave that extra capacity in the building) or steam converters.

The mixing valves I have looked at so far are Leonard TM850 with minimum recommended inlet temperature of 120 deg. F., Guardian G3700 with minimum hot water supply temperture of 140 deg. F., and Powers ETV200 with MAX hot water supply of 180 deg. F (no minimum listed, I have contacted them for more info).

I have considered lowering the final tepid temperature to 65 degrees F, however even though it meets the requiremts it's pretty cold to stand in for 15 minutes. My understanding is you want the injoured person to stay in the flushing fluid long after they think they need it which is why they went with the tepid requirement in the first place.

I am trying to find an ASSE 1071 valve with a delta T of about 20 degrees or less so I don't have to waste so much expense and energy to heat the water way above the set-point of the valve discharge. It seems these valves are adjustable to a wide range of temperatures, making the delta T of 20 degrees above the highest adjustment of the valve. In other words, if the valve is adjustable to 60-100 degrees F output, the minimum inlet temperature would need to be 120 degrees to make a Delta T of 20 above the highest set point of 100. I will call some of these manufacturers today to find out if that's the case, or if I can realize a delta T of 20 above MY set point of 65 or 70.

 

[LittleInch]

I can't work out why you're so concerned about the water temperature. To heat the mass of water that is going into your shower requires the same amount of energy to heat from your incoming cold supply to 65F regardless of whether you mix 85F water or 140 F water. You just heat less of the 140F supply than the 85 F supply, that's all. You are mixing this, not cooling it via a heat exchanger. The capital cost and size of a smaller flow but higher temperature boiler should favour this option and reduce your hot water pipe size etc.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[PEDARRIN2]

I have used the AO Smith ATIO-910 instantaneous gas water heaters.

Since this contains a warehouse, it is hard for me to believe the architect will not give you enough space to put a storage water heater. Sometimes architects do not live in the same world as the rest of us. But I would not put a storage heater of the size you would need on a platform. That would be a large amount of steel.

If you were using storage, you could go with a lower temperature because you would not be limited on flow rate to mix the waters. But since you are wanting to go instantaneous, you need to heat it up as much as possible so you minimize the flow rate (and pressure drop). From what I have been told, the greater the approach temperature, the better the control on the mixing valve.

If you go with storage, I would recommend the Hubbell EMV which is a package system for emergency equipment.

 

[Ziggypump]

Am I understanding this correctly, the instantaneous water heater is for both emergency and building use? In this case, why wouldn't you want to supply 140 degree F water? I'm assuming mop sink, which would like to see 140, and tempering valves for building water can be supplied. If it's one water heater for each application, I can see varying the water temperatures, otherwise you're looking at a controls nightmare.

Emergency mixing valves do not work the same as regular mixing valves. Tempering water to lower temperatures is a must, so they should be able to do it. And they work like this; if you get 80F water from the water heater to the mixing valve, the cold water side of the valve should remain closed, allowing 80F water to reach the emergency fixture. Don't look at this as heating water to a higher temperature just to bring it back down, look at is as a safety thing in the event that the water heater brings the water temperature up too high.

I don't have a lot of experience in instantaneous water heaters in this type of application either, but with water heaters, isn't legionella an issue with the system or just the inspector?

 

[PEDARRIN2]

You never (never) want the valve to close the cold water side. Typically there is a feature for cold water bypass, in case the valve fails so you never are without cold water. You want it to close the hot water side to prevent potential scalding in case pressures drop and all you get is hot water.