Tankless in-line with indirect DHW

[merim123]

Pondering a different layout for the house. Currently have a HTP Modcon that supports 4 radiant zones and DHW. With family size of 6-8, depending on visiting family in the house, we routinely run out of hot water if all six want to shower or fill up tubs. The indirect is a 80-90gallon capacity. I have the HTP set to DHW for priority and even if I turn off all the radiant zonesand only run DHW, we still run out.

So I was contemplating adding a Rinnai 199 in line with the indirect so that I can take the output of the indirect and feed it into the Rinnai and then have the Rinnai heat the water on demand for the rest of the house. That way, my intake water temp will never be below say 90. But i can control my output hopefully consistently to 120.

Thoughts on if this would work or not?

 

[WorthFlorida]

As done with other type of water heaters, install a thermostatic mixing valve on the DWH and at the Modcon change the domestic hot water temp to 140 degrees and the thermostat on the DWH. This will about double your supply of hot water. The mixing valve would be set for 120º, mixing cold water with 140º water.
https://www.cashacme.com/us/en/thermostatic-mixing-valves/tank-booster

I do not know your unit if it is possible to program it that domestic hot water has priority over the heating system. Buderus tankless will not run the circulators for heating until the direct water heater is at a minimum set temperature.

 

[Sponsor]

 

[merim123]

Had not seen this device before. I have 1" going into my indirect and supply is 2" down to 1". My DHW is already at 140. the showers have thermostatic valves. I'm really down on volume because kids want to fill up bath tubs and rain head shower flows a lot. First world problems I know. that's why I thought about putting in a whole house tankless that feeds from the DHW and then to the house.

To your point about priority for DHW. there is a menu setting on the HTP that says DHW gets 30 min. If you disable that, than DHW becomes priority. At least how they wrote it in the manual.

 

[John Gayewski]

How many btus on the boiler? You should be able to keep up with about anything with a boiler. What's your max output for hot water? Meaning how many gpm of 120 degree (forget the 140 thing for now) water do you need max in one hour?

Adding a tankless could be the way to go but it would be very odd. Unless your boiler wasn't sized to heat your max load. In which case you'd want to see what's cheaper upsizing your boiler or adding a tankless. It could be cheaper labor wise to get a bigger boiler as you may just be able to do a simple change out vs adding another component.

 

[merim123]

199k - HTP-UFT. Installed 2018.

Hard to tell flow need. But if I try to fill up the whirlpool tub, i can't get it filled up with hot water before it runs out.

So typically if i do the rainhead shower, and the wife is bathing one of the kids let's say. And another kid jumps in a different shower, we are done. Lukewarm central.

 

[Fitter30]

Condensing water heater 5.7 gpm @ 70° rise at 199 k btu's. Need to figure out what gpm you're happy with and the temp rise with the coldest incoming water temp.

 

[merim123]

Condensing water heater 5.7 gpm @ 70° rise at 199 k btu's. Need to figure out what gpm you're happy with and the temp rise with the coldest incoming water temp.

So this is how I got my idea. If I feed the tankless from the dhw indirect tank takes my municipal in at 40 let’s say. Then the htp heats the tank to 120. Once I run out and water goes lukewarm then that lukewarm water, say 90 would be my lowest water temp intake. The tankless should easily take that to 120 and give me truly enough hot that will never run out.

 

[John Gayewski]

199k - HTP-UFT. Installed 2018.

Hard to tell flow need. But if I try to fill up the whirlpool tub, i can't get it filled up with hot water before it runs out.

So typically if i do the rainhead shower, and the wife is bathing one of the kids let's say. And another kid jumps in a different shower, we are done. Lukewarm central.

Your boiler is a water heater. You need an actual boiler sized correctly.

 

[merim123]

Your boiler is a water heater. You need an actual boiler sized correctly.

I’m not explaining myself well. See video of system here.

 

[John Gayewski]

I’m not explaining myself well. See video of system here.

I didn't watch the video. If the max output is 199k the unit is the size of a water heater. Above 199kbtu is boiler size. If your using this for your heat and water it wasn't sized correctly for your demand.

Sorry if I'm being short is just I don't see adding another component to makeup for an incorrectly sized unit, but you can of course do that.

 

[Bannerman]

You didn't define which Rinnai 199,000 BTU tankless model you are considering.

Similar to your Modulating Condensing boiler, If the tankless is a condensing unit, it will extract additional heat from the exhaust gases to heat the water. Because the exhaust gases exiting the exhaust vent are so cool, this will result in moisture within the exhaust gases to condense within the unit. Because less heat will be wasted, this will allow the unit to operate more efficiently to heat water. With a RU199iN, up to 96% of the consumed energy is utilized to heat the water.

To achieve this high efficiency, the incoming water supply temperature must be 120℉ or less. When the incoming water temperature exceeds 120℉ as may occur when the incoming supply to the tankless is being pre-heated by the boiler , that water maybe too hot to sufficiently cool the combustion gases, thereby reducing the tankless' heat extraction efficiency percentage to usually low to mid 80%.

When supplied with a continuous supply of fuel and water, a tankless WH is capable of supplying endless hot water. Depending on the incoming water temperature vs the desired water temperature, a Rinnal RU199iN is capable of the following flow rates.

Flow Rate at 40F Temperature Rise = 9.8 GPM
Flow Rate at 45F Temperature Rise = 8.4 GPM
Flow Rate at 50F Temperature Rise = 7.6 GPM
Flow Rate at 60F Temperature Rise = 6.4 GPM
Flow Rate at 70F Temperature Rise = 5.5 GPM

Of course, pre-heated water will reduce the temperature rise, allowing a higher flow rate up to the unit's maximum supported rate while continuing to provide the desired water temperature.

When the temperature rise is too great to supply the desired water temp or, when a single unit's maximum water flow rate is less than needed to supply the flow rate to the fixtures (possibly your scenario?), then multiple identical tankless units will usually be utilized (cascaded) so the flow rate and heat input will be shared over each unit which are plumbed in a parallel fashion.

 

[Fitter30]

Rinnai 199 max flow 11 gpm. Need to talk to the factory for its operation that would take 120° water boost it say if it would lose 10° to 110° how the heater would act. With published literature starting at 40° difference max flow 9.8 between in and out. 60° flow drops to 6.4 gpm. Oter possible problem the higher the water temp the more calcium and minerals are driven out of the water.

 

[John Gayewski]

You should figure out you minimum heating load and your max water demand, then find a boiler that will modulate between the two numbers.

 

[merim123]

You should figure out you minimum heating load and your max water demand, then find a boiler that will modulate between the two numbers.

how would you suggest I do this? is there a template I can use to figure it out. happy to do it.

 

[John Gayewski]

how would you suggest I do this? is there a template I can use to figure it out. happy to do it.

Well I guess it depends on your heating system. If your system has multiple zones with the capability to run independently then your smallest zone would need sized based on your heat loss in that zone. You can use the slant fin heat loss app to help you with that. The max water heating demand I think is more of a simple calculation. I haven't done one in a very long time but I think it pretty searchable. You'll want to know your minimum incoming water temp and how many gpm you want to heat based on how many showers and people you have.

Just as a note there is a limit on what you can do. It could be unreasonable after you calculate everything up, to then go ahead and find or buy a boiler that will do this for you. Smaller gpm shower heads could possible solve your problem, along with not running the washing machine at shower time. Maybe your water demand max and your smallest zone min is too large of a gap and you won't be able to find a boiler to modulate that far?

 

[Fitter30]

Water heater side need to know your max gpm of hot water and coldest incoming water temp. Heating boiler short cycle now at coldest temperature? What is your water temp your running?

 

[merim123]

As done with other type of water heaters, install a thermostatic mixing valve on the DWH and at the Modcon change the domestic hot water temp to 140 degrees and the thermostat on the DWH. This will about double your supply of hot water. The mixing valve would be set for 120º, mixing cold water with 140º water.
https://www.cashacme.com/us/en/thermostatic-mixing-valves/tank-booster

I do not know your unit if it is possible to program it that domestic hot water has priority over the heating system. Buderus tankless will not run the circulators for heating until the direct water heater is at a minimum set temperature.

I called the cashacme folks today. They don't think it will work. I have 1" supply and this is 3/4. I don't want to lose pressure and they think something like this would drop temp even more. I'll just have more lukewarm longer.

 

[merim123]

Water heater side need to know your max gpm of hot water and coldest incoming water temp. Heating boiler short cycle now at coldest temperature? What is your water temp your running?

40 coming in. Running HTP at 160 for DHW tank.

 

[Fitter30]

Any devises for a water system drops pressure. From length of pipe , fittings, all types of valves, filters and anything that water runs through. All have published pressure drops. Pressure reducing valves, backflows, filters can drop pressure from 5 - 12lbs. Look at post 11 Bannerman the flow rate and water temp. The need to have have more temp difference input from output the slower the water has to run through the heater.

 

[WorthFlorida]

....... Smaller gpm shower heads could possible solve your problem, along with not running the washing machine at shower time. Maybe your water demand max and your smallest zone min is too large of a gap and you won't be able to find a boiler to modulate that far?

Merim123, before spending a lot of money for increase is hot water supply, please look at your shower heads and if they are old, they are just using too much water. https://www.epa.gov/watersense/showerheads

John is correct, what you want or need for hot water demand is beyond most residential homes can muster. What will work but it will cost thousands $$$. A three phase electric water heater of 120 gallon size and the use of a mixing valve. It is how hotels meet the demand, however, homes do not get three phase power. There are high output two phase (240v) electric water heaters but the cost is still way up there. Adding inline electric boosters is an option but there is also a limit on water flow vs temperature rise.

Mixing valves do come in 1" sizing. Check out WATTS web site. Cash Acme is only one brand and made for residential water heaters as an easy install. Most times one is plumbed in as needed.

Mixing Valves

www.watts.com

Shower heads by EPA mandate is a max flow rate at 2.5 gallons per minute, however, because of more stringent use by states or city mandates, most after market replacement shower heard are 1.5-1.9 gallons per minute. EPA "Water Sense" rating must be below 2.0 GPM. There is a ton of information on the EPA web site and how standard testing have been adapted for good performance between 45-80 PSI. Most fear that low flow shower heads do not perform.