Dealing with an undersized tankless DHW heater

Stenty

Hello,

I may soon end up managing a property with what I suspect has an undersized 150k btu tankless DHW heater.
It serves two apartments...so that's two showers/tubs, two kitchens and dishwashers, and one clothes washer.

Under a heavy draw, I suspect flow will suffer and create a problem. How can that be solved other than replacing the unit?

1) Perhaps a jumper cold water line that reconnects after the tankless with a spring check valve that keeps it closed off until the tankless scrubs off enough pressure that the valve is overcome. A storage tank for the not-as-hot water to mix in.

2) Pipe the tankless like a primary-secondary. The cold line doesn't pass through the tankless. The tankless is connected with two closely spaced tee's and a variable speed circulator controlled by a temp sensor after that connection. At above some flow point the tankless will not provide hot enough water but at least the flow will be maintained. A storage tank afterwards.

This scenario has occupied my head all morning and it's not an issue yet but it has me thinking.

Any other interesting solutions?
Any caveats to 1) and 2) ?

SWEI

The limit is on BTUs delivered, so primary-secondary will merely trade less hot water for more warm water. You either need more BTUs or a storage tank.

Stenty

I currently have a Bosch from many years ago at my house and it chokes the flow if you draw too much.
It seems focused on maintaining temp over maintaining flow.

Stenty

I just called Rheem and according to tech, a Rheem will choke the flow to maintain water temp instead of allowing full flow at reduced temps.

SWEI

Most tankless do. What happens if you lower the setpoint?

What are your groundwater temps like there? Is this a condensing or a noncondensing unit?

Stenty

SWEI said:

The limit is on BTUs delivered, so primary-secondary will merely trade less hot water for more warm water. You either need more BTUs or a storage tank.

I already contemplated the hot-warm water trade and the need for a storage tank or more BTU's

It's about flow.
I'm looking for an idea to maintain flow other than my two ideas
or unforeseen issues with the ideas I originally posted.
Or a product that addresses the issue.

kcopp

I would install a storage tank and pipe it the same way you you would install a booster tank on a tankless coil.

http://www.vaughncorp.com/products/aqua-booster/#howitworks

Stenty

My concern is keeping the flow rate from getting choked by the tankless.

I have no concern about maintaining temperature. The concept of and need for a tank after the tankless is in my original post.

Do either of my ideas concerning maintaining flow have any potential issues ?
That is my question.

Or are their other solutions?

Leon82

You can cascade another one,assuming the control supports it. This is the nature of the beast with tankless. Mine is 199k and will run 2 simultainius showers in the winter. If you open the sink the flow drops.

Stenty

In parallel or series ?

Stenty

If in series, flow will still choke.

SWEI

Stenty said:

I have no concern about maintaining temperature.

What happens if you set the target temp at 100°F?

Firecontrol933

What I have in my house is an electric water heater that I use strictly as a storage tank. The cold water is piped to the bottom of the tank and the hot out the top so the only flow issues you will have is the restriction of the tank and it's fittings.
To heat the tank I used the bottom thermostat to cycle a stainless steel circulator that circulates the cold water from the bottom of the tank through the tankless heater and back to the top hot outlet of the storage tank. This effectively takes the flow restriction of the tankless out of the picture as far as flow to the fixtures.
You could series multiple tanks if needed to get capacity so that your tenants won't run out of hot water during heavy use.

This "design" was brought about when the wife was not the least impressed with the lack of flow when she used more water than the unit wanted to provide. She was also not impressed with the minor fluctuations in outlet temperature.

Leon82

If the heater has cascade control ability it would run series then the first one would preheat the water for the second one

Stenty

In general terms, not a specific scenario or product, a tankless has a max flow.

In a situation where the max flow of a tankless falls short of demand and swapping a unit out or adding another is not an option, what ideas do you guys/gals have for maintaining flow regardless of any temp drops ?
Temps issues can be handled with a tank.

Leon82

Either set the output temp lower of temper it down on the outlet.

If its an apartment put in low flow shower heads

Stenty

I like your suggestion Firecontrol.
The manual for a Rheem warns against allowing heated water to return to the supply side of the unit but ...

I'd like to see a condensing tankless get only cold water on the supply side ideally.

Stenty

Considering that your only options are 1) or 2) in the top of this thread, with no concern about the resulting temp, is either clearly not feasible?

I'm just trying to see if what popped into my head has flaws that I don't see.

Larry Weingarten

Hello: I normally respond with technical info, but here I'd like to question some assumptions. Having two apartments on the single undersized heater seems a mistake to me. It only can cause finger pointing trouble between tenants and also the owner. That alone is reason to have two heaters. The approaches you're suggesting feel like engineering band-aids to make up for an inadequate heater. If you install a second heater, (tank or tankless) choking down of the flow is less likely to be a problem and you wind up with a system that actually delivers hot water as people like it, without interference between apartments, which is the point. If we don't deliver the service, nobody will be happy.

There is a lot you can do with low flow fixtures, but with tubs in the mix, you'll have a hard time ever making the present heater deliver satisfactorily. Them's two cents!

Yours, Larry

jacobsond

"Under a heavy draw, I suspect flow will suffer and create a problem. How can that be solved other than replacing the unit?"
I would listen to Larry. The time and effort to modify a poor design will give you nothing but troubles. I would do the simplest. Either install one new unit to handle both or just install a second unit. One for each apartment if that's possible.

Jack

Larry is correct. You cannot push the flow/temp rise around. it is what it is. By the time you try to add hardware to the system you will be close to the cost of the second unit. Also, replace the shower heads with good low flow units and check to see that the flow restrictors are in place on the faucets. You can't make thee unit make more. Try to control the flow.

bob eck

What are the shower heads GPM? Install 1.5 GPM shower heads. Also look at the aerators in the kitchen sink faucet and lava faucet. Reduce the GPM for each of them. This would be the easiest and cheapest to try first.