Anyone have a passive hot water tank?

[jayjr1105]

I have a boiler (natural gas) that heats our house and provides domestic hot water. Our hot water is really hot but high flow really cools it down quick. Showers are fine but bath's for the kids sometimes get luke warm towards the end because the boiler can't keep up.

From what I understand somehow the passive tank keeps water hot even without heat being applied to it. Not sure how that would work but maybe someone has one and can explain it better. I would like to get something to supplement the hot water process. Not interested in a separate hot water heater unless it's absolutely necessary.

 

[14Accent]

A very well insulated storage tank will keep water hot for quite a while. Keyword: WELL. If you could find a used water heater and use it just for a storage tank, that would work quite well.

That being said, how do you get hot water without a heater or storage tank? Is it just on demand, where the boiler runs when hot water is requested?

 

[pair of Blazers]

I have an indirect w/oil heat.
It's set up as a zone to the tank and has its own thermostat for water temp I believe it also acts as the zone switch.

 

[walterjay]

From what I understand of your question you have a passive hot water heater off of the boiler. It is probably up high without a circulator between it and the boiler. Heat thermo-siphons upward through hot water coils inside the tank. To increase hot water output you will need a seperate zoned storage tank which will include a circulator and control system for the whole thing. This is also called a side arm heater. Pretty expensive usually costing over 2K for components plus installation. The cheapest thing to do is to install a 40 gallon electric water heater on the output side of the boiler domestic hot water. Wired to its own circuit this will heat up the hot water to desired temp if it starts to run out. No simple fix here, however make certain that the boiler is turned up high enough to provide ample hot water. And if you turn it up higher you have a scald risk and you must install a tempering valve to protect yourself from scalding.
Why don't you call in a pro and get an estimate to deal with the problem. They could be more specific than I can without seeing everything.

 

[Snagglefoot]

How many gallons is your water tank? I have a boiler and it heats the water in the water tank using heat exchanger coils.The temperature in the tank is 138 F. My shower valves are Delta Auto types which hold the temp at 98 F. (I might adjust those higher. ) Do you have a dedicated pump to feed the boiler water of the heat exchanger of the water tank.? I suppose you could add a water tank and run it in series with your tank but it would need additional heating. How about just adding an electric water heater?

 

[fcjeep]

By a "passive" hot water tank do you mean an "indirect fired" hot water tank?

I have oil heat with a 65 gallon Bradford White indirect tank with its own circulator. We have sometimes had 2 showers, the dishwasher, the washing machine and maybe a sink or 2, all going at the same time and never ran out of hot water.
The 65 gallon tank was here when I came in. I think it's a little overkill. A 40 gallon tank would probably be enough for us.

 

[jayjr1105]

Sorry if I wasn't clear. No tank whatsoever right now, it's pretty much on demand from the boiler. There is an expansion tank up in the ceiling but I don't think that "holds" hot water but more for pressure relief.

I had quickly chatted with my HVAC guy about an indirect hot water tank (sorry I messed up the terminology and said passive tank) and it seemed like a cool concept that doesn't require additional energy consumption from electric or gas.

 

[walterjay]

Yes an indirect tank on a seperate zone will do the job. 40 gallon tank is sufficient for most households. Figure $3,000 installed price. I have done a few of them.

 

[jayjr1105]

This is exactly what I need (I think)

https://www.youtube.com/watch?v=1YljdhInklo

 

[walterjay]

Yes. That is the system that I am talking about.

 

[Cujet]

You may also be able to increase the BTU of the burner. Some models tolerate a metering jet change and airflow adjustment for a fairly wide range of BTU outputs.

Also, consider a water flow restriction on the input side to the boiler. So the water flow cannot be faster than the boiler can heat. Done correctly, it won't affect showers, as the BTU heat flow into the water is about the same. But during tub filling, the flow rate never exceeds the threshold of comfort.

 

[pkunk]

Why not go modern and install a 'on demand' water heater. With the boiler as the preheater, the on demand will provide as much hot water as you can use. Cost for a 6gpm one is under a $1000 with venting

 

[Deplorable]

^^^^^ This. I have a tankless water heater and the only way you can run out of hot water is to run out of gas or water. The gas models are a lot more efficient than electric. Mine only heats to 120* but you can't keep your hands under it. And it uses nothing when not in use. The one I have is a Ranai and I wouldn't go back to a tank type. Mine is mounted in the outside wall and vented without additional venting. Very small footprint.

 

[Wolf359]

Basically the issue is the flow rate. Same as getting a tankless on demand hot water heater. You need to figure out the flow rate of all the fixtures and if the flow rate of the boiler is higher than the demand of all the fixtures, you never run out of hot water. On the other hand, if the demand from all the fixtures exceed the rating of the flow rate of the boiler, then you're going to run out. A simple thing to do would be to get a lower flow rate shower head. Those storage tanks will last a lot longer than a regular gas hot water tank, but as others mentioned, it's going to cost a lot more. Around here, installing as gas hot water tank is in the $1k range including the hot water tank. Those should last 6-12 years or more with proper service. Having the indirect tank might make it last 20+ years.

 

[irad]

I would cap the boiler water heater and install a gas water heater. If you have three children go for at least a 75 gallon unit. I had three at home and as they got bigger the showers were longer. I have a 100 gallon unit. Boiler generated hot water is much less efficient than a separate water heater. Get the simple style gas water heater, the ones with no electronics. They work in a blackout and are less expensive.

 

[Kira]

A friend has a wood/oil furnace. There's a coil of 1/2" copper pipe which grabs heat and stores it in a plumbed 30 gal. tank which feeds their propane hot water heater. Come warmer weather when the furnace is not used the pre-heater does nothing.
It works well. Alas, it looks real homemade.

 

[Cujet]

Originally Posted by irad
Boiler generated hot water is much less efficient than a separate water heater.


That may not be so. Conventional natural gas water heaters waste a lot of energy due to the flue transferring heat 24/7. That's why tankless are 25-40% more efficient. As there is no heat loss when not in use. It's not unusual for a conventional tank type, natural gas water heater to be 55% efficient in real world use.

A condensing water heater is efficient, as it has a powered exhaust and transfers nearly all the heat of combustion to the water. And can use a 2 inch PVC exhaust.

But as mentioned above, the problem is the flow rate. Tankless heaters typically restrict the flow to a rate that they can handle.

You can do the same.

 

[Pete]

I've had my Vaughn Top Performer in for about 10 years now. I think it's a 35-gallon. These have a type of cement lining, interestingly. We've not run out of hot water, even in summer, when the boiler hasn't run in a couple of hours.

If you have gas available, a tankless system should be considered.

 

[Wolf359]

Originally Posted by Pete
I've had my Vaughn Top Performer in for about 10 years now. I think it's a 35-gallon. These have a type of cement lining, interestingly. We've not run out of hot water, even in summer, when the boiler hasn't run in a couple of hours.

If you have gas available, a tankless system should be considered.


I've always been against tankless systems and I'm a real estate broker so I see lots of hot water heating systems. The tankless loop off the boiler is also somewhat common. The tankless hot water system you only see in really new construction or high end places. Basically the problem with the tankless gas hot water system is that you normally need a 3/4 inch gas line and a higher flow rate for the gas meter. That can be solved with a new gas meter but you also need to vent the tankless system out the side of the house so that limits the install options. It basically costs a lot more and if you do the math on how much you're saving on the hot water bill, it's not really that much. It's normally about $10-15/month for the gas bill on a regular hot water tank. If you have to pay a couple thousand extra for the tankless option, it will take a long, long time to recover your costs. And those can easily go away because most tankless systems only have a 5 year warranty. And there aren't too many plumbers who know how to fix them, and you might not be able to get the parts for it. Worst case on a water heater is that it dies on a Sunday and if you like your hot water, you can just run to Home Depot for a new hot water tank and install the same day. However when your tankless goes, then you need the plumber to show up and diagnose it, then order whatever parts it needs. Probably explains why I don't see too many of them, sounds great in theory but once you do the math, it's not quite there yet. And that's why I haven't installed them in any of my 10+ rentals.

 

[pkunk]

I got 20 years out of my Bosch Tankless and it was still working but needed a real cleaning. Replaced it with a Rennai. Very happy. Vents through the roof as the Bosch did but with their piping, no need for a wall vent. Much more efficient than heating a tank and we've never run out of hot water. The Bosch is now thoroughly cleaned and am saving it as a backup in case the electronics in the new Rennai go south. The Bosch was standing pilot ignition rather than all electronic. 10 year warrantee, & looks to be well made. The Rennai is top rated on Amazon and with piping & extras was still a bit under $1000