Combi unit or not for DHW and Radiant

[iowa4x4dieselman]

As the title states, I am looking for pros and cons on installing a combi boiler that will do the DHW for my house vs having 2 separate units. From what I have read is that the DHW side on the combi units doesn't have as high of GPM as 2 stand alone units would be. Our master bath will have a 6ft jetted tub and a shower with 2 shower heads. the in floor heating will be 2 zones one for the house ~1800 sq. ft. and garage ~1200 sq ft.

Just don't want to end up cutting myself short on hot water requirements for running 2 shower heads at once or filling up the tub. The tub faucet is capable of 18 gpm @ 80PSI per deltas website

I know our shower heads are labeled as 1.75 for the hand shower and the "raincan" shower is 2.5 gpm.

 

[kj_mustang]

or, boiler with indirect hot water tank. I have this setup with a 46 gallon indirect tank and it works great. We have never run out of hot water but I don't have a big tub yet. It is rated for a continuous draw at 185 Gallons/Hr @200F.

 

[Jackfre]

The better combo's deliver 4gpm @70* rise. They are normally in the 160-199,000 btu range. Your fill spout flow rate is not really relevant to the discussion, yet. What is the vol of the tub. 7.48gal/cu ft. How fast do you want the tub to fill and what is the flow rate of the showers, # of showers and people in the house?

 

[chinboys]

I have a name brand combo condensing unit for 4 years now. It works great to make hot water for my heating circuits and dhw. But I had to switch out the internal mix metal dhw tank due to leakage at rust joints at the lower seams and outlet return for an external all stainless steel tank with ss coiling a year ago.
Just make sure you research what your unit is made up of in materials via the web before committing to such... they are expensive to buy and time-consuming to fix.

 

[iowa4x4dieselman]

Thanks for the replies so far.

The house will have 3 bedrooms on the main level with provisions for 2 more in the basement. Currently there is only 3 people in the house, but we plan to have more children.

The flow rates of the showers are 1.75 and 2.5 gpm for the master bath, and the main bath tub shower has max gpm of 2.
Whirlpool tub is rated @ 70 Gallons max fill.

Chinboys -

with the issues you had mentioned. the cost for 2 separate units would be about the same price as the combi unit, just want to make sure I am getting quality as you stated.

The unit I was looking at was a Rheem - RCBH199DVLN 18k-199K BTU/h.

 

[chinboys]

I like this combo high efficiency "boiler and DHW" you are thinking of buying.
It looks to be a continuous DHW maker on steroids.
You shouldn't have the same corrosion problems I and my ex-brother-in-law had with our units.
But you will have capacity DHW issues due to demand and time of year DHW use... I use to have a Takagi continuous hot water maker and during the winter where the temp of the input water is colder.
The dimensional size of the unit is nice too.
And heating of the home or the heating of the DHW cannot happen at the same time... the system will ask for heating or DHW priority.
Good luck and success if you plan to go with this. And let us know how it works out.

 

[iowa4x4dieselman]

I hope to hear back from the plumbing company on there experiences thus far with these types of units.

Heres another one I have been researching - Noritz CB199-DV-NG

https://www.noritz.com/product/cb199/

 

[kj_mustang]

Pay close attention to the gpm at your preferred hot water temperature rise point. Do you know what the supply water temperature is for that location?

 

[iowa4x4dieselman]

Pay close attention to the gpm at your preferred hot water temperature rise point. Do you know what the supply water temperature is for that location?

I do not know the pressure off hand. I'm not quite sure how I would know, unless the city would have a rough idea.

I do know we were told that all of the houses are supplied with 1" water lines so a sprinkler system can be installed.

I have been researching the indirect water heater option and it does seem like another viable solution. The only downfall I can see thus far is it will take up more space than the single wall unit.

 

[kj_mustang]

Supply water temperature, not pressure. If your supply water temperature is 50F versus 70F, then you will get less gpm of HW at a delivered tempature of 120F because you have to raise the temperature more. Check the charts in the specs for the equipment you are considering, usually labeled temperature rise.

 

[yeldogt]

When big tubs were all the thing it was not uncommon for people to install dedicated "on demand" units just for the tub. Simple static units -- no standby loss -- sized for the need. Previously people needed to have large storage tanks just for the tubs. The need for max efficiency was not important -- the savings was eliminating the oversized tanks -- not the the savings going with complex modulating condensing water heaters. I have a simple static "on demand" NG unit to supply my outdoor shower at the beach .. the rest of the house is a tank. Supplying one devise -- on demand sized just for that.

The Combi units keep getting better -- the newest modulating units seem to be quite good.


Is the whole house going to be radiant head in slabs -- one temp water.

 

[Jackfre]

I would suggest that you look at the Rinnai Model I boiler. It will give you the 4 gpm you need and handle the heat as well. It is a fairly new unit but was designed for simple service. I have been following it with a couple contractor friends using it and it is getting excellent marks from them. I am pretty sure that both Noritz and Rheem are re-branding a Korean Kyungdong boiler. The leader in the wall hung boiler field has been Navien. By way of full disclosure I both represented and consulted with Rinnai for 27 yrs. I no longer have any business or monetary affiliation, but I like the hell out of their products.

 

[yeldogt]

Jackfre ... what your thoughts on HTP. People love the Superstore SS indirects ... I'm actually thinking of using a standard w/ indirect. VS the Viessmann I have

 

[Jackfre]

HTP is a good outfit. While I know the company, I am not really jammed up on their individual products, other than their indirects. For a "standard" I am surprised you aren't in the Buderus fold. They are hard to beat for a conventional boiler...as you know! Second to that I'd look at Weil. Curious what it is that interests you in the HTP boiler.

 

[yeldogt]

HTP is a good outfit. While I know the company, I am not really jammed up on their individual products, other than their indirects. For a "standard" I am surprised you aren't in the Buderus fold. They are hard to beat for a conventional boiler...as you know! Second to that I'd look at Weil. Curious what it is that interests you in the HTP boiler.

Sorry -- poor choice of words. "standard" = straight boiler ...not a combi.

Yes -- I do love the old CI Buderus. But this project does not have that possibility -- no space and no way to vent. I don't love the Bosch/Buderus line of wall units. The Bosch has limited vent length ... Buderus has had some issues.

Viessmann was always tops w/ propane -- but people are really complaining about the Customer Service and support in the past few years.

Like the OP I need radiant and DHW -- My preference is boiler w/indirect -- distribution of DHW using a manifold and home run Pex. 40 gal indirect is perfect with two bath 3 people that I have. For a little more output you can increase tank temp or go 52 G. The standby loss on those tanks is impressive.

Even w/ a large CI boiler having to heat in the summer for DHW -- the NG gas cost for cooking/dryer/DHW is under $25.

 

[iowa4x4dieselman]

When big tubs were all the thing it was not uncommon for people to install dedicated "on demand" units just for the tub. Simple static units -- no standby loss -- sized for the need. Previously people needed to have large storage tanks just for the tubs. The need for max efficiency was not important -- the savings was eliminating the oversized tanks -- not the the savings going with complex modulating condensing water heaters. I have a simple static "on demand" NG unit to supply my outdoor shower at the beach .. the rest of the house is a tank. Supplying one devise -- on demand sized just for that.

The Combi units keep getting better -- the newest modulating units seem to be quite good.


Is the whole house going to be radiant head in slabs -- one temp water.

The basement and the garage will be heated with radiant with separate zones. I will have a furnace for the main level/A/C as needed. I hope the radiant takes the brunt of the heat and the furnace is hardly needed.

Rinnai to me is one of the pioneers in tankless, I do like the navien NCB-240E as well. My neighbor has a navien boiler for his garage heat, but no DHW tied into that.

 

[Jackfre]

Yeltdog, check out the new Rinnai model I heat only. I agree that the Bosch/Buderus line has been poorly handled vs the competition. To our OP I would suggest that you skip the furnace for the main floor and go with air handler(s) You can run them off the boiler and still have your AC. Going with smaller AH allow you to zone very effectively, but you may have ac couple condensing units to cool. You could also do a staple up radiant in the main house and use mini-splits for H/C.

 

[yeldogt]

The basement and the garage will be heated with radiant with separate zones. I will have a furnace for the main level/A/C as needed. I hope the radiant takes the brunt of the heat and the furnace is hardly needed.

Rinnai to me is one of the pioneers in tankless, I do like the navien NCB-240E as well. My neighbor has a navien boiler for his garage heat, but no DHW tied into that.

My problems with combi units: They need to be sized for the DHW ... In tight/ insulated houses .. some of the turn downs are not enough for the needed output for heat. The unit is cycling ...

Straight heat can be better matched ...

 

[iowa4x4dieselman]

My problems with combi units: They need to be sized for the DHW ... In tight/ insulated houses .. some of the turn downs are not enough for the needed output for heat. The unit is cycling ...

Straight heat can be better matched ...

So would you recommend purchasing a boiler and a tankless unit separate?

If so I have to ensure I have a large enough supply line for the BTU load including

Stove
Furnace
Water heater
Boiler
dryer

 

[yeldogt]

iowa ... it's all about math.

With the "on demand" be it a separate unit or combination. When you turn on the water -- the unit will turn on and there is a wait for the hot water. To some this is not a bother -- others hate it .... if you do the "on demand" .......... design the supply system to take water volume in the pipes .... into account.

You need to figure out what the tub is really going to need ... or live with what the system can give you. Filling an average size tub in a reasonable amount of time should not be a problem ... it's wanting to use the tub and others taking showers. That's where the "on demand" works out great.

The combination systems keep improving -- previously they could be a bit of a pain (maintenance) .. and did not last as long as other types. I don't see the benefit if they really can install two units for the combi cost... what type?

We have been renting a house over new years for the past 4 years -- big w/ 5 bedrooms. 5 full baths and 2x 1/2 ... it has 3 on demand units. The owner recently replaced them all -- for this type of house and use ... they work. But you stand in the shower .. wait for the water and don't play with it.

I don't know -- just don't love them.

You also have to think about the future -- will the one combi be enough when you build out? You don't want to get stuck down the road

There is no one answer --

I'm a boiler w/ indirect guy. Size the boiler for the house and size the tank to the load -- as houses have become tighter and the heat loads and boilers matched better .... boilers are getting smaller. With smaller boilers -- the indirects need to be sized based on recovery (it's published). Most 40g tank water heaters only have about a 30k burner -- that's fine with one bathroom.. maybe two w/ 3 people in the house. An indirect has a higher capacity boiler behind it -- so it's able to heat the water faster as it's used. With smaller boilers -- this capacity is lower ... so people use slightly larger tanks ...52 gallons and maybe hotter water in the tank. Indirects are not cheap $1100-$1500 ...So no huge savings over having a tank sitting next to the boiler -- they just last longer as they are SS today. Also -- the standby loss of the tanks is very low today.

Some people use two tanks .... like they have duel hot water heaters.

With the future build -- plan for that water need. You don't want to have a situation where you need another indirect t and don't have an easy way to pipe or enough gas.

NG or propane ?

 

[iowa4x4dieselman]

We currently have an on demand setup, and I agree it takes a few before the water gets there, but the "endless" is where it really shines. I can go wash my car and take a shower without worrying if I will have enough hot water.

Looking at the indirect option you have mentioned, the way I understand it, it is a "endless" system so as long as you don't exceed the amount of water that it can make up and overrun the makeup capacity. Whereas a standard tanked water heater has hot going out and cold going in, causing a temp drop. The indirect is heated before being placed in the storage tank.

I will be on city natural gas.

The expansion would be just adding an additional bathroom in the basement, and a bar sink. Total occupancy of the house would be 6 people with the 5 bedrooms.

 

[kj_mustang]

An indirect tank that is heated by a boiler has a cold water inlet from the main and hot water outlet. There is a coil or outer jacket tank that has hot water (usually 180 F+) from the boiler loop that heats the domestic hot water supply. Since it takes awhile for the heat to dissipate out of the boiler loop in the indirect, the DHW in the indirect tank can get hotter than what you prefer at your faucet. You usually put a antiscald type mixing valve on the DHW supply outlet from the indirect. This usually means a large supply of hot water from the indirect. I can notice when my boiler hasn't run in a good while that the hot water will be a little cooler but it isn't much. Once that boiler kicks on it takes no time to feel it hotter again.

 

[iowa4x4dieselman]

The unit our plumber is going to install is a Rinnai I120CN. He is only hooking up the DHW side and leaving the raidant alone. So i am seeking help on designing a radiant heating panel for the 2 zones. I have heard that theres a possibility i only need one circulation pump with 2 zone valves. Does anyone have a similar setup. Most i can find only have one zone.

Thanks,

https://www.rinnai.us/product/boiler/i120cn

 

[kj_mustang]

From the Rinnai manual located here,
800000111-I-Series%20Condensing%20Combi%20Boiler%20Installation%20and%20Operation%20Manual.pdf

Go to page 56 for the diagram, you would just be coming off the loop with your supply and return lines to the two zones with zone valves on the supply side. You need to size the pump for all the pressure drop of the zones and the primary loop.

 

[whatuusay1]

i second the recommendation for an indirect tank off the boiler. I have a 40G Indirect Weil Mclain tank off of our Navien 80k boiler using DHW priority and it works great, endless hotwater! I've yet to run it cold It kicks on and has no problem keeping the tank at temp during heavy usage. My wife made it a point to run a hot shower & laundry after i set it up to try and tax it and the tank quit calling for heat as soon as she ended her shower (tank was already at full standby temp). Its pretty impressive, and if we needed more we could always raise the standby temp of the tank and mix the output down using an anti-scald valve but i cant see that ever being required. We use the boiler for basement and garage heat as well, and its had no problem keeping up. For reference we're a family of 3, and have a fairly large master tub (I don't recall the fixture rates). My wife told me we better not run out of hot water (like a friend of ours deals with on their new house) Challenge met!

 

[jmiller_2308]

i second the recommendation for an indirect tank off the boiler. I have a 40G Indirect Weil Mclain tank off of our Navien 80k boiler using DHW priority and it works great, endless hotwater! I've yet to run it cold It kicks on and has no problem keeping the tank at temp during heavy usage. My wife made it a point to run a hot shower & laundry after i set it up to try and tax it and the tank quit calling for heat as soon as she ended her shower (tank was already at full standby temp). Its pretty impressive, and if we needed more we could always raise the standby temp of the tank and mix the output down using an anti-scald valve but i cant see that ever being required. We use the boiler for basement and garage heat as well, and its had no problem keeping up. For reference we're a family of 3, and have a fairly large master tub (I don't recall the fixture rates). My wife told me we better not run out of hot water (like a friend of ours deals with on their new house) Challenge met!

Can you confirm what I think I read. Namely, you have a 40 gallon Weil McLain indirect water heater being fired by an 80,000 btu boiler?

I was interested in Weil McLain indirects but every model that I look at seems to want significantly more MBH than what an 80,000 btu boiler could provide. For example, the Navien NHB-80 lists its input max of 80,000 BTU with an output of 73 D.O.E and 64 MBH. Looking at Weil McLain the least MBH input model they advertise is the Aqua Pro 30 and it requires a minimum of 115 MBH.

I have a larger Weil McLain boiler that has 105,000 BTU in and outputs 88,000 D.O.E. with 73 MBH and when I asked Weil if I could use it to fire one of their indirects they gave a confusing response that really only told me that the All Gold/Ultra Plus 40 (which is what I think you might have) was replaced with an Aqua Plus Model 45 which according the Wiel's chart needs 150 MBH.

Amtrol and HTP seem to be the only indirects I have seen that advertise different performance numbers for different MBH inputs but Weil has yet to make any such info available to me. So if you have actual performance data with a boiler that might be a bit smaller than mine it would be great news for me to have at least anecdotal evidence that a smaller boiler would work.

 

[jives]

We have a combi DWH/radiant heat wall hung boiler. Offhand cannot remember the brand, but the first unit we used when the house was built 10 years ago failed in 2 years. It has supplied all the heat/hot water for a 4BR/3BA house and 8 people. Maintenance is a bit of a pain, as the propane regulation is iffy. Could just be the brand.

 

[Jackfre]

That model 120 I is rated at 5.1 gpm @ 70* rise. Having an 18gpm fill rate on the bath faucet it will look like it is peeing water into the tub, but it will fill it. As well, once in the tub it will be able to peak the tub temp as needed. Frequently with tank type water heaters you may be able toil the tub, but recovery becomes an issue, so once in the tub enjoying a soak as the tub temp drops you may not have the hot water available to peak the tub. You will with the on-demand. Also, from my experience a 2.5 gpm shower head with 55* ground water need 2.1 gaps of hot mixed with .4 gpm cold to give my shower temp.

 

[whatuusay1]

Another vote for indirect off of a boiler. I have a navien 80k btu boiler with 40g Weil McLain indirect tank with priority and use both for radiant floor and dhw. It’s a killer setup. Unlimited hot hot water. My wife loves it. It heats up so quickly. I timed it when we first set it up and the tank was “full and hot” 90 seconds after the shower/bath stopped. I have an grundfos alpha pump moving 3-4 gal a min through it @ 200 degrees. My only complaint is the navien is a little louder at full tilt than I expected, that said it doesn’t run that often.

Tankless units run city water through the heat exchanger on their boilers, a closed system indirect setup can run distilled water (which will significantly prolong their life and limit maintenance).

Also I know rarely does cost or operating expense get mentioned but I have about 4500 in the radiant/dhw setup (did it all myself) and our gas bill last month (Jan/Feb) was $128.00 (gas furnace, dhw, 1800sqft radiant garage/basement, gas range and gas fireplace. I’m located near Kansas City, it’s been plenty cold this winter I have 2” XPS under the basement and garage slabs.

 

[Jackfre]

My experience with "distilled water" is that it is actually a pretty aggresive fluid. In the distillation everything is removed. Just as nature abhors a vacuum, distilled water will not stay that way in the presence of metal. The water gets what it wants from the surrounding material. I know that de-ionized water is used in some systems.

 

[whatuusay1]

I hadn’t heard that about distilled water, but some searching does appear to support that. I’m going to buy a tester and check out the water. I appreciate the heads up, don’t want a nasty surprise after all that hard work ��

 

[Jackfre]

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