Tankless limitations/ workarounds?

hot_rod

My understanding is tankless get cold supply, heat a number of gpm at a temperature rise to a desired hot outlet.

It’s the incoming water pressure, call it 50 psi that is able to overcome the pressure drop of the small passageways that are designed to accommodate the intended rise and gpm. Fairly simple concept.

When you try pumping that differential, and throw different flow rates, add pressure drop through additional piping, heat exchangers, mix valves, dual functionality, etc. That is where it can go off the rails.

So they engineer a completely different type of heat exchanger, low pressure drop intended to operate at lower delta T, higher gpm, they call it a boiler.

If what you are trying to accomplish was simple, effective, reliable, affordable, etc, I’d imagine someone would already have thought of it?

The advice from a handful of knowledgable folks here from all aspects of the industry is??
Kinda feels like you are trying to pound a square peg in a round hole😳

rbphhc

hot_rod said:

The advice from a handful of knowledgable folks here from all aspects of the industry is?? Kinda feels like you are trying to pound a square peg in a round hole😳

Yup. But imma keep pounding. Thanks!

(When I started this, I understood the advantage of using water pressure to move water through the tankless. I didn't understand how hard a pump would have to work to achieve the same flow, and I didn't understand that boilers have less internal resistance, and more robust HXers. Live and learn.)

BUT. Seems like pumps are used very commonly for recirc and/or direct storage w/ these units, so circulating is not a terrible option.

In any case, we are committed. Everything purchased, installation complete on the DHW side. Incorporating indirect now. Onward!

rbphhc

Decisions:
- Gonna draw DHW off the indirect;
- Gonna circulate indirect through the tankless w/ 006e3;
- Gonna recirc from/to indirect w/ secondary pump;
- Gonna start at 120° and see how it performs;
- If I need 140° and mixing valve, will draw recirc off the bottom of indirect @ 120° in order to bypass mixing valve.
Fingers crossed!

hot_rod

Wouldn't it be nice.

50K of recovery for DHW, 85- 90% efficiency
8K turndown for radiant, with tank as buffer
140° tank for extended DHW draws, legionella protection
Mix valve per code
Recirc that works with 520 tank mixer kit

Sell the tankless on Craigslist

john123

@OP: I am having trouble following that description---I am just a rookie. any chance you could post a hand drawn diagram. This I think is a very interesting topic even though a square peg so-to-speak. Did you ever think of having an electric 1 gallon tank under the sink or beside the bidet! to avoid the wait for the hot water to come through. I know the tankless is much more efficient, but I find this "disadvantage" of the tankless a concern.

john123

@OP: I looked up your tankless but I couldn't figure it out from the on-line specs. Do you think you get 11 gpm out of it? That's what i read. Probably from 70 degree CWS???? I even saw some oblique reference to a pump. does it have a pump? I thought none of the tankless DHW had pumps. Can anyone confirm that?

Larry Weingarten

Hi @john123 , People have tried that for DHW and the effect is that they use electricity to cover the recirculating losses, which can amount to a lot of $$. It’s one reason why I like demand controls for recirc.

Yours, Larry

john123

Your tankless looks like it is propane. What do you estimate your CWS pressure is? HR suggested 50psi. Do you think that is about right?

rbphhc

hot_rod said:

Wouldn't it be nice.

50K of recovery for DHW, 85- 90% efficiency
8K turndown for radiant, with tank as buffer
140° tank for extended DHW draws, legionella protection
Mix valve per code
Recirc that works with 520 tank mixer kit

Sell the tankless on Craigslist

Yes, it may come to that. In retrospect I should have pushed harder for the boiler, the combi, or the Polaris. But price was prohibitive (to the property owner) in each case. They gave me a target budget of $6k, which was half our best estimate for a combi. Hence the tail chase above.

rbphhc

john123 said:

@OP: I am having trouble following that description---I am just a rookie. any chance you could post a hand drawn diagram. This I think is a very interesting topic even though a square peg so-to-speak. Did you ever think of having an electric 1 gallon tank under the sink or beside the bidet! to avoid the wait for the hot water to come through. I know the tankless is much more efficient, but I find this "disadvantage" of the tankless a concern.

Hi John - many sketches in my various posts - if you check through my recent history, you'll find plenty.

Re: CWS pressure, I'm guessing about 60psi, although no gauge on the pump here.

Re: GPM output of the RU199 here in VT, I'm guessing 4 to 5gpm with input temps around 40 to 45 (winter harsh here, and, with ledge, wells and pipes are close to the surface).

However, in circ mode, w/ water temps @ 100°, expecting near max flow @ 10gpm (unless tankless won't run b/c delta too small).

Hope that helps! (Don't do what I do!!!)

hot_rod

rbphhc said:

Wouldn't it be nice. 50K of recovery for DHW, 85- 90% efficiency 8K turndown for radiant, with tank as buffer 140° tank for extended DHW draws, legionella protection Mix valve per code Recirc that works with 520 tank mixer kit Sell the tankless on Craigslist

Yes, it may come to that. In retrospect I should have pushed harder for the boiler, the combi, or the Polaris. But price was prohibitive (to the property owner) in each case. They gave me a target budget of $6k, which was half our best estimate for a combi. Hence the tail chase above.

So the customer does your bidding?

Comes at time when you say no thanks and move on

john123

HotRod: talk to me about the forest as it pertains to your last diagram (please) . You just threw it out there and I'm looking for the background.

The supply from the mod-con would have to be higher than 140F? If OP is using in-floor heating, might that be a little too hot for the best efficiency (return too hot too)?
You set up your last diagram, I gather so that the indirect would get the full 140F as opposed to using CST's on the radiant loop, which would have sent a much cooler supply to the indirect---Could you siphon off the supply water to the indirect first and then send the water to the radiant loop?
I'm not too good on the LLH thing: Would you need hydraulic separation on the radiant loop in your diagram since there is a circulator pump on it? or is that irrelevant?

Be gentle please: because I am a fledgling and a rookie and I am probably making some rookie and incorrect assumptions; but I am anxious to learn. I ask again about my suggestion of a stand-alone regular hot water tank piped in series AFTER the tankless.

I see a mod-con boiler operating at low temperature to heat the slab, on a low power curve and probably with a good algorithm to turn down the gas (8 to 1?) when possible on an outdoor sensor--couldn't get any better for efficiency and if only used in the cold weather=less wear and tear and hopefully less maintenance. It's not broke! Perhaps we should keep it as-is. Simple, straightforward and easily explained to the HVAC technician, if you needed one. If it were used in the summer for DHW through the indirect, the return temperature would be higher if you wanted to maintain 140F in the indirect--hmm? --perhaps not?

Now you have the tankless--once again operating at maximum efficiency, Winter and Summer, regardless of the supply temperature because of the cooler temperature of the CWS. It's not broke either. And it probably works much better in the Summer than the Winter because of the higher temp of the CWS in the summer. Do we know the weather, where the OP is? So maybe we don't even need the indirect in the summer? It's 200,000 BTU and supposed to deliver 11+ gpm.

I'm guessing all the OP wants to do is raise the CWS a bit before it goes into the Tankless so (probably) he can get 2 showers going at the same time in the winter. So he says siphon off a bit of the boiler water and heat the indirect. That is where we get into problems, at least. from the 140F standpoint. And it's easy to swallow a little shower water by mistake. Or would the bugs be killed as they went through the Tankless?

Before we get into a solution using the indirect:

I suggested using a stand-alone (albeit low efficiency) hot water tank (20 gallons perhaps), possibly electric and not too expensive, insulated to the hilt, piped in series AFTER the Tankless, like a buffer tank, which could also maintain the 140F. The advantage I see, is the early delivery of hot water to the faucets and, while not perfect, the opportunity, at least, to get that second shower working in the winter. What I don't know is whether there would be a huge pressure drop and leave the raindrop shower heads just dripping???

As far as selling the Tankless on Craigslist, the OP would probably take a real beating on the price---but the Indirect has no moving parts and it might do better. Sitting on the wall, it is not deteriorating and probably not depreciating either in these times of inflation. It is waiting for its moment in the Sun, which of course would come quickly if my suggestion were tried and didn't work out.

I think early delivery of the hot water is important, year round, but you might pipe in a by-pass and isolate the hot water tank in the summer if you wanted to and save some money.

rbphhc

MikeAmann said:

Can the indirect tank be moved much closer to the shower to avoid the long cold water waste while waiting for the hot water to get there? Then you can take recirc completely out of the equation.

Mechanical room is directly below master bath and adjacent to 3rd bath going in this spring. K sink and guest bath are on the other side of house. I had proposed small electric tankless there instead of recirc in an earlier post (similar to John123's approach, but no room anywhere for a tank).

hot_rod

john123 said:

HotRod: talk to me about the forest as it pertains to your last diagram (please) . You just threw it out there and I'm looking for the background.

The supply from the mod-con would have to be higher than 140F? If OP is using in-floor heating, might that be a little too hot for the best efficiency (return too hot too)?
You set up your last diagram, I gather so that the indirect would get the full 140F as opposed to using CST's on the radiant loop, which would have sent a much cooler supply to the indirect---Could you siphon off the supply water to the indirect first and then send the water to the radiant loop?
I'm not too good on the LLH thing: Would you need hydraulic separation on the radiant loop in your diagram since there is a circulator pump on it? or is that irrelevant?

Be gentle please: because I am a fledgling and a rookie and I am probably making some rookie and incorrect assumptions; but I am anxious to learn. I ask again about my suggestion of a stand-alone regular hot water tank piped in series AFTER the tankless.

I see a mod-con boiler operating at low temperature to heat the slab, on a low power curve and probably with a good algorithm to turn down the gas (8 to 1?) when possible on an outdoor sensor--couldn't get any better for efficiency and if only used in the cold weather=less wear and tear and hopefully less maintenance. It's not broke! Perhaps we should keep it as-is. Simple, straightforward and easily explained to the HVAC technician, if you needed one. If it were used in the summer for DHW through the indirect, the return temperature would be higher if you wanted to maintain 140F in the indirect--hmm? --perhaps not?

Now you have the tankless--once again operating at maximum efficiency, Winter and Summer, regardless of the supply temperature because of the cooler temperature of the CWS. It's not broke either. And it probably works much better in the Summer than the Winter because of the higher temp of the CWS in the summer. Do we know the weather, where the OP is? So maybe we don't even need the indirect in the summer? It's 200,000 BTU and supposed to deliver 11+ gpm.

I'm guessing all the OP wants to do is raise the CWS a bit before it goes into the Tankless so (probably) he can get 2 showers going at the same time in the winter. So he says siphon off a bit of the boiler water and heat the indirect. That is where we get into problems, at least. from the 140F standpoint. And it's easy to swallow a little shower water by mistake. Or would the bugs be killed as they went through the Tankless?

Before we get into a solution using the indirect:

I suggested using a stand-alone (albeit low efficiency) hot water tank (20 gallons perhaps), possibly electric and not too expensive, insulated to the hilt, piped in series AFTER the Tankless, like a buffer tank, which could also maintain the 140F. The advantage I see, is the early delivery of hot water to the faucets and, while not perfect, the opportunity, at least, to get that second shower working in the winter. What I don't know is whether there would be a huge pressure drop and leave the raindrop shower heads just dripping???

As far as selling the Tankless on Craigslist, the OP would probably take a real beating on the price---but the Indirect has no moving parts and it might do better. Sitting on the wall, it is not deteriorating and probably not depreciating either in these times of inflation. It is waiting for its moment in the Sun, which of course would come quickly if my suggestion were tried and didn't work out.

I think early delivery of the hot water is important, year round, but you might pipe in a by-pass and isolate the hot water tank in the summer if you wanted to and save some money.

You could run the tank at 120, but less drawdown, and no bacteria protection as the trade off.
What temperature does the radiant require? If it is a slab, needing below 100, you would add a mix valve at the radiant connection.

You may want to define what the DHW needs or assumptions are for the customer to design to. If not, you are guessing at what to build the system around. The DHW is the larger of the two loads I assume, radiant and DHW?

If they want lots of hot water and fast, then there will be an efficiency trade off.

Look at the spec sheet on the indirect it should tell performance at different boiler temperature.

A large pipe size at the indirect becomes the hydraulic seperation between boiler and radiant pump. You can't use closely spaced tees there or you need to run two circs, the radiant and the boiler.

Expansion on the boiler, thermal expansion tank on indierct.o

rbphhc

hot_rod said:

What temperature does the radiant require? If it is a slab, needing below 100, you would add a mix valve at the radiant connection.

Radiant = in-floor, 90* temps.

If I end up needing 140* at the indirect, I will use 2 mixing valves, one for DHW, one for in-floor.

Will try the mixing valve bypass trick for recirc by drawing off the indirect tank bottom (same line that feeds the tankless).

Understood about legionella risk running 120* (did we always know about this?) But does a tankless system have an advantage here? "Legionella growth is enabled by lukewarm water temperatures, usually in the range of 77 degrees Fahrenheit to 108 degrees Fahrenheit." If the CWS goes straight from 45 to 120, and then is kept at min 120 through circ/recirc, shouldn't it be less likely? I know 140 is preferable if you want to kill it.

Customer does the bidding: tricky because property owners are family friends and the GF's landlords. So I am trying to 'help out' - make things easier on the old family plumber and cheaper for the LLs. Crossing fingers hard the system works well enough,

hot_rod

rbphhc said:

hot_rod said:

What temperature does the radiant require? If it is a slab, needing below 100, you would add a mix valve at the radiant connection.

Radiant = in-floor, 90* temps.

If I end up needing 140* at the indirect, I will use 2 mixing valves, one for DHW, one for in-floor.

Will try the mixing valve bypass trick for recirc by drawing off the indirect tank bottom (same line that feeds the tankless).

Understood about legionella risk running 120* (did we always know about this?) But does a tankless system have an advantage here? "Legionella growth is enabled by lukewarm water temperatures, usually in the range of 77 degrees Fahrenheit to 108 degrees Fahrenheit." If the CWS goes straight from 45 to 120, and then is kept at min 120 through circ/recirc, shouldn't it be less likely? I know 140 is preferable if you want to kill it.

Customer does the bidding: tricky because property owners are family friends and the GF's landlords. So I am trying to 'help out' - make things easier on the old family plumber and cheaper for the LLs. Crossing fingers hard the system works well enough,

I think legionella protection is a personal decision in a home. Any at risk occupants, elderly, immune deficiencies? Less potential if the home is always occupied also.

True the tankless hold much less water for the bacteria to harbor and grow. The expansion tank is an issue, but the tankless get away with a softball size thermal expansion device, so less water content there also.

Yes you will need a mixing device if the radiant needs a 90 SWT.

dko

You need to call Rinnai and find out what their unit does when inlet temperatures are above 70F. The chart with the GPM and Delta T rise is for inlet temperatures below 70F only.

Some units simply do not fire at all above a certain temperature and will just let through the lower temperature water without heating it.

What that does to your recovery is in the air.

hot_rod

We had many issues with tankless not working properly with a pre-heat solar tank feeding them. Some needed diverter valves added to select either solar or tankless source.

I think Takagi finally offered a solar version. I’d hope by now all the brands updated the control logic to work with warm incoming water? Best to check the manufacturer, or a knowledgable rep.

Water in Phoenix can be over 80 degrees in hot summer months. Even higher if the lines run through the attic. Rumor has it over 100 degrees for a short blast!

dko

Called Rinnai because I was curious how their unit works. All I got from the Rinnai tech line was, "it should work." Any technical questions were responded to with RTFM.

Peaked my curiosity as they specifically wrote "call for inlet temps above 70F" as if they had some special insight.

I'm more familiar with Navien NPE units and they will fire up to inlet temp of 140F. With commercial mode unlocked, can use a setpoint of 180F. But in this case will have switch off the exhaust temp limit and make sure to use CPVC/PP/SS, not PVC.

hot_rod

dko said:

Called Rinnai because I was curious how their unit works. All I got from the Rinnai tech line was, "it should work." Any technical questions were responded to with RTFM.

Peaked my curiosity as they specifically wrote "call for inlet temps above 70F" as if they had some special insight.

I'm more familiar with Navien NPE units and they will fire up to inlet temp of 140F. With commercial mode unlocked, can use a setpoint of 180F. But in this case will have switch off the exhaust temp limit and make sure to use CPVC/PP/SS, not PVC.

Rinnai has so many different models and vintages out there, you almost need to drill down on the specific unit considered.

rbphhc

dko said:

Called Rinnai because I was curious how their unit works. All I got from the Rinnai tech line was, "it should work." Any technical questions were responded to with RTFM.

Peaked my curiosity as they specifically wrote "call for inlet temps above 70F" as if they had some special insight.

I'm more familiar with Navien NPE units and they will fire up to inlet temp of 140F. With commercial mode unlocked, can use a setpoint of 180F. But in this case will have switch off the exhaust temp limit and make sure to use CPVC/PP/SS, not PVC.

I did call Rinnai several times on this question. Every tech had a different answer, but I definitely had the impression they have refined operation so they can much more reliably handle lower flow and higher temps. This one is an Ru199iP.

pecmsg

rbphhc said:

What temperature does the radiant require? If it is a slab, needing below 100, you would add a mix valve at the radiant connection.

Radiant = in-floor, 90* temps. If I end up needing 140* at the indirect, I will use 2 mixing valves, one for DHW, one for in-floor. Will try the mixing valve bypass trick for recirc by drawing off the indirect tank bottom (same line that feeds the tankless). Understood about legionella risk running 120* (did we always know about this?) But does a tankless system have an advantage here? "Legionella growth is enabled by lukewarm water temperatures, usually in the range of 77 degrees Fahrenheit to 108 degrees Fahrenheit." If the CWS goes straight from 45 to 120, and then is kept at min 120 through circ/recirc, shouldn't it be less likely? I know 140 is preferable if you want to kill it. Customer does the bidding: tricky because property owners are family friends and the GF's landlords. So I am trying to 'help out' - make things easier on the old family plumber and cheaper for the LLs. Crossing fingers hard the system works well enough,

The legionella bacteria is killed after 140* is reached for over 1 hour. After that the water can sit it’s a non issue. 

hot_rod

From a rep I know

rbphhc

hot_rod said:

From a rep I know

Yes. The Rinnai tech mentioned this too. Based on BTUs, not flow or temp.

So, may be necessary to increase output temp to 140 to get proper BTUs circulating the indirect. And may need to set the lower limit @ 115 or so.

Will tweak in the field to see what works best. Thanks as always, Rod!

nate379

I'm considering piping my combi dhw into a small electric water heater to act as a buffer tank.

This is the first on demand setup I've had and I'm not a fan at all.

My old house had a cast iron boiler and indirect water heater.  Worked great!

As it is now, I can't use hot water unless it's always running.

Example... washing dishes.  Fill the sink, then 30 seconds later pull some hot water to rinse a dish and it's lukewarm, then ice cold, and eventually gets warm.  Takes a good 45-60 seconds of near wide open flow.
I end up with a sink nearly overflowing with ice cold water fairly quickly.

The washing machine ends up with cold water on warm or hot as it pulls water several times before full so it ends up being mostly cold.

Even the bathroom sink which is not even 5 ft from the boiler takes a good 45 seconds before getting hot water.

No idea how people think that is fine?

rbphhc

nate379 said:

I'm considering piping my combi dhw into a small electric water heater to act as a buffer tank.

This is the first on demand setup I've had and I'm not a fan at all.

My old house had a cast iron boiler and indirect water heater.  Worked great!

As it is now, I can't use hot water unless it's always running.

Example... washing dishes.  Fill the sink, then 30 seconds later pull some hot water to rinse a dish and it's lukewarm, then ice cold, and eventually gets warm.  Takes a good 45-60 seconds of near wide open flow.
I end up with a sink nearly overflowing with ice cold water fairly quickly.

The washing machine ends up with cold water on warm or hot as it pulls water several times before full so it ends up being mostly cold.

Even the bathroom sink which is not even 5 ft from the boiler takes a good 45 seconds before getting hot water.

No idea how people think that is fine?

Hence the indirect, in my case.

clammy

Wow didn’t follow through the whole post but it sounds overly complicated and a pita when it comes to having issue that won’t be resolved . It sounds like they want the best but at the cost of being cheap . I hope when they’d do have issue that your around to guide them through the mess or possibly they will come to there senses and stop wasting time and money trying to get some thing that is not designed to do what’s they are trying to do but there gonna attempt to usually every bodies aggravation except there’s own . Personally when they call w no hot water I would not being rushing over . Over complicating some thing that’s usually a no brainer in an attempt to defy the manufactures suggestion being that everyone is smarter then those who designed . I can only wish you lucky for myself I would have shook my head and walked ,personally I have zero time for cheapstakes . I would require them to tell me how to pipe it and provide a detailed diagram so when it don’t do what they except you are not left holding the bag . I never attempt to make some thing do what they clearly are not designed to do , mainly because I know I’m not as smart as those who build it and may i add clearly use it for a purpose it Is not intended for . In the long term how long do you think you ll satisfy the customer w this set up and how long will a tanklesss heater last when used in this manner . Hoping and wishing that some thing will work that Is not designed to what your trying to do in my eyes is a total waste for all concerned and usually at the end the home owner will just say why didn’t you say it won’t work instead of recreating a wheel . Peace and good luck clammy ps as a professional I would never attempt to use a tankless to pull dual duty it s a total waste of time to again try to design some thing that is clearly not designed to do so is a fools choice and most pros may possible do it quick and one thing for sure never return a call when there’s a issue being it’s not a heating boiler it’s a tankless and your using it something it was not designed to do. I would never give any advice on trying to use a piece of equipment for a use it clearly is not designed for any issue you have will be yours . Plus I’m not in business to help a time waster and re creating a wheel when there’s is clearly a product designed to do what is need the only thing is a matter of money again nobody like a cheap stake myself I wouldn’t say I have to get a bigger screw driver and hoop in the truck and leave . Nothing worse then dealing w tools

hot_rod

nate379 said:

I'm considering piping my combi dhw into a small electric water heater to act as a buffer tank.

This is the first on demand setup I've had and I'm not a fan at all.

My old house had a cast iron boiler and indirect water heater.  Worked great!

As it is now, I can't use hot water unless it's always running.

Example... washing dishes.  Fill the sink, then 30 seconds later pull some hot water to rinse a dish and it's lukewarm, then ice cold, and eventually gets warm.  Takes a good 45-60 seconds of near wide open flow.
I end up with a sink nearly overflowing with ice cold water fairly quickly.

The washing machine ends up with cold water on warm or hot as it pulls water several times before full so it ends up being mostly cold.

Even the bathroom sink which is not even 5 ft from the boiler takes a good 45 seconds before getting hot water.

No idea how people think that is fine?

Does your combi have an. Always hot setting for DHW? It keeps the boiler temperature up so that warm up period is gone. It still takes time to get flow to the faucet, the same as the indirect tank.

The heat exchanger in mod cons holds 1- 3 or 4 gallons, much less fuel to keep warm compared to revving up or maintain. A cast boiler.

Intplm.

Have you looked into a thermal bypass valve? Rinnai and other companies offer one.
Installed at your furthest fixture, this item with a properly installed circulator might simply solve this issue.

hot_rod

More and more tankless brands are offering a model with recirc pump built in.

Your fuel cost may increase if you are circulating hot water thru an uninsulated copper loop for example.
You basically have a hydronic heating loop whenever the recirc pump runs.
When you cross to the cold as a recirc, you end up flushing hot or lukewarm water from a cold tap, as a trade off for a cold glass of water

A Viega Manablock might be a good match for a tankless. Small 3/8 or 1/2" lines to the fixtures that flush out in seconds.

rbphhc

Uninsulated lines are DEFINITELY a fixable problem. Does anyone know highest R-value pipe insulation? Cheap black stuff not cutting it in northern VT. TY!

Tom_133

I get fiberglass pipe insulation, sized and R value chosen by me at acoustatherm in Williston Vt. They will install or just sell you want you want.

hot_rod

rbphhc said:

Uninsulated lines are DEFINITELY a fixable problem. Does anyone know highest R-value pipe insulation? Cheap black stuff not cutting it in northern VT. TY!

I think some energy code states require 1" wall insulation on DHW piping. Not a cheap date!

rbphhc

Tom_133 said:

I get fiberglass pipe insulation, sized and R value chosen by me at acoustatherm in Williston Vt. They will install or just sell you want you want.

Howdy, neighbor! That's where I bought my car

rbphhc

https://forum.heatinghelp.com/discussion/comment/1742742#Comment_1742742

Great tip. Went there today and got everything I needed. 👍️