Indirect DHW tank fed off boiler that may be too small

[msimm15]

Sorry for the long post but here it goes - any help is greatly appreciated.

Little background first:

We recently (2yrs ago) converted out heating from oil/steam to gas/hot water. My house is 4 stories - 18'x34', (basement, 1st floor,2nd, & finished attic) and is attached on both sides. I sized up the system with heat loss calculations myself and with the help of some people here. I had a Williamson GWA-070 boiler installed and we did cast iron sun rad radiators throughout, and the house heats up very nicely.

The DHW is an indirect Burnham stone lined 40gal from the previous owner - probably about 6 yrs old fed off the new boiler. In sizing up the boiler I didnt pay enough attention to the rate of return on the DHW tank though my plumber said the 70k should be fine. I probably should have gone with the 105k model.

Long short - the DHW works but not for that long on regular use (2 adults and 4 kids). it works OK but will run out of steam. We recently installed a 4th bathroom in the attic so we now have 3.5 baths. Typical use durring bath time is 2 showers + a bathtub. When running 2 showers shortly after having filled a bathtub there is no hot water left.

Is there anyway (now that I already installed a new boiler) to squeeze more hot water out of the system or to speed up the recovery rate.

I changed the shower head in the basement to a low flow but the ones on the 2nd and and attic dont have enough pressure to work with the low flow heads. I am planning on changing the DHW pump to get better pressure on the other floors when the other showers are running. I raised the temp on the DHW tank to 140, and the boiler to 190 (a bit high).

Is there anything else I can do to get more hot water. I understand that raising the temp of incoming water will also speed up recovery rates but I dont see a good option for that here.

Your help and advice is greatly appreciated.

Thanks
Mark

 

[msimm15]

One idea I also thought of was to install a separate electric instant hot water heater in the attic for just that shower and have that as an isolated system not fed from the main boiler. I ran wires for a 30 amp circuit in case I needed to go that route but that is a last resort

 

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[Dana]

Upsizing the boiler for higher domestic hot water performance is almost always a mistake. The vast majority of the time it's better to up-size the volume of the indirect. A 70K boiler is enough to run a single 2 gpm shower pretty much 24/365, so the rest has to come from the tank if running simultaneous showers.

The first thing to do is to crank the aquastat on the indirect up to it's maximum, not a mere 140F. It SHOULD have a thermostatic mixing valve on the output mixed down to 115-120F or so to limit scald risk. Most indirects can be operated up to 180F, (or at the very least 160F), which will improve the apparent capacity for 105F showerhead output by quite a bit.

Electric water heaters have abyssmal performance issues in climates with incoming water temps as cool as yours. Don't waste your money.

If it's showering performance you're after, spend the money on a drainwater heat exchanger 4" x 48" or taller.downstream of the main showers would roughly double the showering capacity, making it possible to run two 2.5 gpm showers nearly 24/7. It won't help with tub-filling though. But in showering mode it also doubles the fuel efficiency.

The "right" one is always the biggest and fattest that actually fits, since the marginal cost of the unit is more than paid back by the increased energy return efficiency, and the installation labor is about the same for a 24 incher as it is for an 80 incher. From this vendor a 4x 48 incher runs ~$900, delivering 47.5% of the energy back under NRCAN test conditions, whereas a 78-incher runs ~$1200 delivering 62.4% return efficiency under standard test conditions. The difference is that with the 48 incher you'll be extending the apparent capacity of your indirect in dual-showering mode by quite a bit, whereas with a 78 incher it'll be hitting "endless shower" territory for two simultaneous showers when backed up by a 7oK burner.

This is DIY-able, easier if you have plastic drain pipe rather than cast iron.

 

[msimm15]

Thank you for the response Dana, unfortunately the drain heat exchanger isn't ideal in my case because I would have to rip open walls to get to the drain which is no HUB piping. I raised the temp on the indirect up to 160. The limiter was set at 150 so I had to move it.

The system has on it a thermostatic mixing valve - a VTA321. But to be quite honest I don't understand the point of it. Why would I want to heat the water to 160 degrees then cool it down with the mixing valve. I have it turned all the way down (clockwise) but I'm really couldn't find any usefully paperwork on the VTA321 so I don't really know what it's doing at the setting.

On another note. I recently redid the main shower (2nd fl) and installed a delta 17t thermostatic shower trim with a new shower head (flow restrictor removed) and it would go cold before the end of the shower. I had to keep adjusting the temperature knob to keep the temps up. I thought the idea of the T series was to keep the temp set. Any ideas ?

Thanks

 

[Dana]

"Why would I want to heat the water to 160 degrees then cool it down with the mixing valve."

First off, you can't deliver 160F water to the taps without extreme scalding risk. But the reason for raising it now is to increase the amount of buffered heat, and thus improve hot water delivery perforamnce.

The thermostatic mixing valve pre-mixes in some cold water, so it's drawing much lower flows out of the tank than is being delivered to the hot water distribution system. As the tank temperture drops with flow it mixes in less cold water, and when the tank temp hit's the mixing valve's setting it's mixing in NO cold water, all of it's coming from the tank.

The napkin arithmetic:

A 40 gallon tank x 8.34lbs/gallon has 334 lbs of water in it. Raising 334lbs of water by another 20F stores and "extra" (334lbs x 20F= ) 6680 BTU. A 2 gpm shower is (x 8.34=) 17lbs per minute.

With a 70F rise (mid to late winter incoming water of 35F, showerhead temp 105F) that shower is using 70F x 17lbs= 1190 BTU/minute, so that "extra" 6680 BTU of buffered heat is buying another (6680/1190=) ~5.5-6 minutes of showering time. If you started out with a 120F storage temp, you've bought yourself more than 10 minutes of showering time at the absolute coldest part of the winter by raising the storage temp. You're probably only looking at about a 60-65F temperature rise right now, so it's really more than a 12 minute gain.

 

[Jadnashua]

One thing that hasn't been mentioned that I saw...make sure that the indirect is plumbed as a priority zone verses an equal zone to the rest of the house. When the tank needs heat, it would then get 100% of the boiler's output verses having to share it with the rest of the home's heating task. Unless that need is huge and long-term, you'd almost never notice the house cooling off if the indirect was calling for heat.

Having a larger boiler and keeping it as an equal zone to the rest of the house only would make some sense if you were running a laundromat or spa where you needed a continuous use of hot water.

As Dana said, when using hot water in either the shower or tub, you're not drawing 100% from the tank, you're mixing it with some cold. If the tank is allowed to get a lot hotter, you need to mix in more cold to keep the same temperature. Because of safety, it's best to use that mixing valve at the tank so the outlet isn't super hot, but you're mixing it down to useable temperatures in steps...essentially making the tank 'look' bigger.

 

[Dana]

More napkin math:

A typical tub-fill takes 30 gallons (~250 lbs) of 110F water. With 35F incoming water that represents 75F temperature difference, so a tub fill is going to take 250lb x 75F=18,750 BTU out of the tank. Filling the tub at 6 gpm takes 5 minutes, and in that 5 minutes the boiler is only adding 5000 BTU, for a net draw of ~13-14,000 BTU out of the tank.

With 334lbs of water in the 40 gallon tank that 14,000 BTU represents a drop of (14,000/334=) 40F. So if the tank is set to 140F at the end of a tub fill you should have 40 gallons of 100F water, a bit on the tepid side for showering, but if set to 160F you should have ~120F water, which is more than enough to start showering. The boiler is still delivering 1000 BTU/minute, and the draw-down rate even with a couple of 2gpm showers going will still take quite awhile before it drops below 105F, which is still a decent showering temp.

 

[Jadnashua]

To keep from having to constantly adjust the shower valve to maintain your desired temperature as the tank cools off, this would be a good time to consider a thermostatically controlled valve that will do it for you!

Make sure your indirect is wired/controlled as a priority zone, or, when the house is calling for heat, the WH won't get all of the heat available.

 

[msimm15]

Jadnashua, thanks for the response - that was exactly the reason why I installed the Delta 17t in the first place. - which doesnt seem to be doing its job. If I am correct I thought the 17t is a thermostatic valve.

The indirect is not wired as a priority - the plumber was so sure it wouldnt need to be based on the heating load of the house, Its very rare for more then one thermostat to be calling for heat at a time - but I will make that change myself.

 

[msimm15]

I installed the
LAHARA®TempAssure 17T Series Shower Trim

 

[Dana]

A thermostatic mixing valve on the shower is a nice feature, but it has to be fed sufficiently hot water to make it work. If what's coming out of the tanks is 105F and the mixer on the shower is set to 107F you'll never get there.

Depending on your actual heat load & radiation for 60K-out boiler the indirect may not need to be set as the priority zone to deliver reasonable hot water service. That's 2x the burner output of a typical 50 gallon standalone tank, and if the radiation can only emit half...

But if you're trying to maximize hot water performance, giving priority to the indirect is certainly one of things that makes a difference (more of a difference with some heating systems than others.)

 

[Jadnashua]

A thermostatically controlled valve can never make the water hotter than what comes in...it will max out at all hot, but keep things relatively constant until that supply cools off too much.

So, to summarize, two things to maximize available hot water:
- start with it hotter than needed, so you end up using less from the tank. To be safe, temper the tank's output.
- if needed, make the indirect the primary zone so it can get all of the heat available from your boiler when needed

The third, you said you didn't want to deal with, would be to add a drain water heat exchanger. BUT, that would only help with taking showers, not filling up a tub. By the time you drain the tub, there's no water flowing to the WH, so no boost, and no energy is recovered...it all goes down the drain.

 

[Bgard]

Is the indirect plumbed with the proper size piping and pump on the boiler water side? If the flow rate is too low you will not get the 70k available from the boiler. You will need at least 7 gpm. Does the boiler cycle when the Dhw is operating?

 

[msimm15]

Thanks for all the responses.

Bgard - in response to your questions - the indirect is piped with 1" supply and return to the boiler and it's a got a good pump on it so that part should be fine.


Jadnashua - regarding the drain heat exchanger - I would love to do it but it would involve too much demo of the walls to get to a usable section of the drain and it's a cast iron pipe...

Interesting to note - I touched the cold water source to the boiler tonight and it was actually HOT, not sure why but it's probably just standing water being next to the hot water line and is of little use once the shower starts.

I am still fighting a low water pressure problem to 2 of the bathrooms at the same time. 2 of the bathrooms are fed of a single 1/2" pipe instead of 3/4" so that isn't great either