Parallel on-demand water heaters

Chuck_7

Propane fired on-demand water heater for a large single family house. Large house with large family. I am sure that one 180mbh unit will not work out. If we go with two, would you pipe them in parallel or zone the house (i.e master bath on one and rest of the house on the other (the master bath has two showers and a large tub))?
I would parallel them but I question how the two would operate together under low flow.

Chuck_7

By the way.

The systems will have recirulation. (another arguement for parallel)

Constantin

This may be a dumb question...

... but considering that you have a recirculation system combined with a hot water on-demand system, aren't you going to have some serious short-cycling? For that matter, will the recirculation system be able to maintain the minimum flow required to keep the WH safe?

Lastly, why do the WH have to be installed in paralell? If you were to install them in series with the same temp setpoint, the first one would carry most of the load, most of the time. Whenever the flow requirements are so great that the first one cannot raise all that water to temp quickly enough, the second one will kick in and supplement. From a control, wiring, etc. perspective, this is likely to be a simpler solution than trying to load-balance two on-demand units in paralell.

For a more even load-sharing option, set the first WH at ½ of the total temperature rise, the second one at the final ½. For example, if the incoming water temp is 50°F and the desired water temp is 110°F, set the first WH to 80°F, the second to 110°F and then have the recirc system feed into the piping between the two WH. Check valves and expansion tanks will be needed but this is simple to implement as long as both WH can modulate to meet flow vs. temp rise requirements.

There may be good reasons (like maximum flow issues) that speak against using on-demand WH in series. I just don't know them and look forward to being educated!

Chuck_7

Max. flow/buffer tank

Rinnai, which has a maximum flow, has a detail with parallel heaters, recirc. and a small tank to buffer the low loads.
But I would still like to look at alternatives.

hr

Why not just

buy a condensing water heater. a Polaris, Voyager, AO Smith or BW would sound more like what you need. Large outputs with many different tank capacities. Sounds like that is what you are trying to build with two instantanous and a buffer tank?

In some cases there is no substitue for a container of hot water for a dump load. Like large tubs! A well insulated condensing water heater give you high efficiency and low standby loss. And a lot less maintenance than a miniture tube style HX, especially if hard or high TDS water is in the mix. Buying that much horsepower to instantly heat a 6, 8. 10, or so GPM rate sounds like the long way around.

Still I would do a calc to pin down exactly (within reason) how much hot water you need to generate. Better than guessing most all the water heater manufactures have tables to size to large loads, some online.

hot rod

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Chuck_7

Believe me, I have been draging my feet on the on-demand water heaters. They will be propane. I think an oil fired tank heater (or indirect heater) would work out better (the boiler will be oil fired). The owner really wants the on-demand heaters.
Actually I think the large tub is the best reason to use on-demand.

Constantin

It all depends, no?

As I see it, large dump loads like tubs are precisely the reason to use a tank as a buffer. That way, your customer can enjoy the jacuzzi and a more-properly sized WH the 95%+ of the time the customer isn't using the jacuzzi.

When I look at some of those "tropical-downpour" showerheads (at a reported 12.5GPM) I say no to tankless heaters and yes to small dedicated potable hot water boilers like the one made by HTP. But even with that variation of a Munchkin, I'd use a small tank to buffer and TRV the output to get an even water temp, no matter what I threw at the boiler.

Jack_23

Have you measured the actual flow?

Given that (I think, and no offense meant) you do not have a lot of experience with these on demand units, it is comforting to do the leg work and measure the actual flow at the various fixtures. You then know precisely what you are designing to. The best reason to not parallel the units is to avoid the recirc, assuming you can locate the units fairly close to their point of use.

You would want them in parallel and not series to prevent the pressure drop which series piping would create.

Rinnai's (which I represent)in parallel with the MSA controls will unify the operation of the multiple units. it will modulate the burners and rotate the firing rate. With two units you would need .4gpm thru each unit to get them to fire. With one unit the min flow rate is .6gpm to get ignition.

Large showers or tub flows are really the ideal situation with on demand units. Once you know the flow, you design the system to accomodate that peak demand. For a 15gpm shower I'd need three 2520FFU's or 2532FFU's in New England. What would you need to handle that type system with a tank/ind. After all, the people who buy these types of fixtures want to be able to enjoy them. That means they want to stay in there as long as they like.

The advantage of the on demand high flow system is the customer can go borrow the money for the system and put it into a home improvement loan at fairly low interest rates. They then consume energy only when the system is operating. The tank alternative is high capacity tanks storing water 24 hrs a day waiting for that maximum load. The utility will thank you. Rinnai's EF on LP is .87

There have also been commercial systems I have done with multiple Rinnais and storage of various sizes. The idea is to get the max recovery by the Rinnais with the min storage possible. This type system works well in apt buildings where you can have a high flow occasionally. Feed the tank, pump to the Rinnais and bring your building recirc back to the tank. Immersion A-stat controls pump. use the 2532 model in this system for reduced pressure loss. A 009 will give 5gpm on this system.

In the above system you feed the tank to prevent a possible system depressurization. I just went thru this with an engineer who used two Rinnai's and a 50 or 80 gallon (can't remember)storage tank serving 10 apts. He drew it so the CWS went to the Rinnai's first and fed the storage which in turn fed the apts. Had the system been built that way and the system saw high flows (exceeding the ability of the two Rinnai's to supply the set water temp) the Rinnai's flow control valve would slow down the flow and system depressurization could result.

Constantin

Interesting Viewpoint

Has anyone got the specs on how to compare AFUE vs. EF?

I'd like to think that a dedicated Munchkin will beat the Rinnai in terms of thermal efficiency due to its condensing HX and the 5-1 modulation that the gas valve allows. As I'm not a heating tech and have no firsthand experience with the unit, I don't know if it needs a IDWH as a buffer or not. However, thermal losses from a IDWH can be very, very low.

Which system performs better in the long term is probably is probably a question of usage. A constantly-inhabited house/commercial project is likely to benefit sooner from a condensing HX than a vacation home in the boondocks. Nevermind some ventures like commercial laundries where HX's on the DWV pipe to pre-warm the incoming water make sense.

Either option should be a big step up from traditional gas water heaters that heat the environment about as much as the water within them. Nevermind the standby losses that Jack alluded to.

Nick W

Learned the hard way.

You can't pipe them in series because the second unit would be fed with hot water.
No good, doesn't work.

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Jack_21

Turndown ration on the Rinnai is 12:1

15,000-180,000. I too, would like to see a EF vs AFUE. I have done several laundromats in the NE area over the last four years. Have used anywhere from 3-8 units with no storage. Working very well.

As to EF vs AFUE, I too would love to see some info. You have to understand that conventional tank type WH have been so inefficient that they have been granted special dispensation. I guess as a reward for their low tech. When Rinnai and the other on demand WH manuf went for Energystar for their units they were unable to qualify. It was not that they did not deserve it, just that because the tank WH's were so inefficient no Energystar water heating catagory was ever written. You cannot get rated for what does not exist. Exactly the same situation exists on Rinnai's Energysaver DV Wall Furnaces. Mid 80's vs 60-65 for the competition, so no Energystar exists. I believe that Energystar gets re-written next year.

Chuck_7

Recirc.

I actually have to recirc. because I can not get the units near the use. I do not really want to use two recirc. systems (with separated heaters).

larry

With a condensing boiler do you get it condensing when creating DHW? I was under the impression condensing boilers rev up to higher than normal supply temperatures when they are making DHW. If they are not condensing, the amount of hot water created per BTU consumed is probably very similar between the boiler and the Rinnai.

Constantin

Correct

...when a boiler is feeding a IDWH and the water temps in the supply loop exced 140°F or so, the amount of condensation will go down with time.

However, the Munchkin I was referring to is the VWH Series, which is conceived essentially as a tankless water heater that happens to be based on a condensing boiler. They offer 4:1 turn-down rates and two capacities of 200kBTU and 400kBTU. You can generate a lot of hot water with these puppies!

Considering that the incoming potable water seems to be used in the condensing section first, I doubt this boiler will ever not run in condensing mode. Besides iceland and a number of other volcanic locations, ground water tends to be below 140°F in temperature. Besides, if the groundwater is that hot, you don't need a water heater anyway.