Indirect or Combo Boiler with Recirc DHT?

TerryR

DHW Load?

Thanks, Snowmelt,



You said "What they want you to do is do a heat loss for the house and your domestic..." That's what I'm trying to do. But so far as I can tell the heat load for domestic is inconsequential - much less than any error in the heat loss calculation. The biggest load is two standard showers in an hour once a day. Seems to me any system with domestic storage of any reasonable amount, or putting the domestic on priority, easily handles that.



But even if some additional boiler capacity were required, I don't see how adding a tankless helps. I keep being told that tankless doesn't play nice with recirc, which is the whole point of my inquiry here from the beginning.



Terry

TerryR

Sizing with DHT

Thanks, Heatpro,





That make sense - ignore the DHW for boiler sizing but size the indirect correctly. 



I don't understand your point about modulating boilers and matching the smallest load. Could you explain please?



Terry

SWEI

Adding a tank

allows the recirc to work without firing the tankless.  The tankless takes over as soon as flow increases to the point where its sensor tells it to fire.



The bottom line is that tankless water hears are not the be-all and end-all that the salesmen want you to believe.  They are fantastic at what they do, but for many customers a tank-type heater (especially an indirect) can actually be a better option.  The extreme ∆T conditions experienced by their heat exchangers is a poor match with hard water, most especially if they are not regularly maintained.  We recommend tankless when it makes sense and are big believers in them for the right application.

TerryR

"...not the be-all and end-all... "

Yes, that's what I've concluded. Thanks.











Terry

4Johnpipe

Flame vs Hydro

Terry I would definitely recommend the Hydro with a low water temp radiant heating system coupled with hot water recirculation...100%

TerryR

"low water temp radiant heating"?

What is the significance of your term "low water temp radiant heating"? I don't know the design temperatures of my radiant system, but most of it - which is tubing stapled to the subfloor - runs at boiler temperature best I can tell. Only the basement, where the tubing is embedded in the slab, has tempering valves. There is also a tempering value on domestic water feed from the boiler's domestic tank.



Looking at the temperature gauge on the boiler, it seems to fire at around 160 degrees and shut down at 210. Those numbers don't seem compatible with the Hydro if I'm understanding the design correctly. Or do I misunderstand how it works?





Edited:

I just looked again at the Hydro - I guess I misunderstood that it was maintaining the tank at domestic temps. I see that it uses tempering valve to deliver domestic, and can deliver up to 180 for heating. So the holding tank is obviously at 180 or above.

Terry

Snowmelt

That's bull

If you go to the navien site, which is also advertised, they have a pump build in on a aquastat and timer.



For other tankless you have to install an air seperator, expansion tank and a aqua stat.

RobG

Absolutely

As Snowmelt said. Terry, just go to the Navian site and they will tell you exactly what you want to hear. I for one always trust manufacturers claims : ).  



If it were me, I would go with the indirect and the recirc and be done with it.



JMHO,



Rob

Snowmelt

Not so fast

Nothing wrong with the navien as long as you install it correctly, but that goes for any product. Let's face it all these space heating or domestic water heating companies play by the same rule book as everyone else.

That being said. I do like the navian but I also like C.I. Boilers with indirect water heater.



I install what ever the customer wants.

4Johnpipe

Water Temps

Yes the Hydro should always be installed with a thermostatic mixing valve for the DHW. The tank temperatures are programmable as well. We can set a target temp, max temp and low temp. The space heating module works in cadence with a variable speed circulator that will control the amount of hot water that actually flows across the heat exchanger with an outdoor sensor. We have a maximum space heating water temp of 130 degrees and our basement slab is the lowest required water temp. By memory it is about 86 degrees. Tube spacing and floor covering also adds into determining all of this. I do not see a reason that even under mount radiant should be running at water temps near 200 degrees. We have a product called Sun Board above the floor throughout with light weight over pour in the tiled areas / bathrooms. We also have a section of radiant wall for the 2 story staircase.

Tim Potter

A FEW MORE QUESTIONS,

Terry,

I reread the whole thread, & I think you have more to do to get the max efficiency & more importantly comfort from your system.



Please post Pictures of your boiler & near boiler piping please.



If I read your descriptions correctly, I think you have boiler temp water circulating through radiant tubing. This is bad if its so. Max temp for tubing is in the 140* range, & you reported boiler temps in the 160 to 200* range.



that is how my system was piped when it was installed, it was not kind to the boiler or the tubing.



The goal would be to get just the right temp through the tube with a low wattage pump in constant circulation.



Tim

TerryR

Radiant temps

Tim,





Yes, that's exactly what I said. The majority of the system operates at boiler temps, excepting only the zones in concrete slabs. From what I'm seeing temps up to 180 are recommended for staple up installations, and some of the zones are under carpet, so require fairly high temps. But I wouldn't doubt that  the temps are higher than needed - I wouldn't say the original installation was exactly "designed."



The boiler manual says the cutout temp is 210, which is what I observed on the temp gauge on the boiler. 



I can post photos if you think it would help, but that's exactly how it's piped.



Terry

heatpro02920

Mini tank

Its not really counter productive to have the mini tank for recirc, it actually works very very well, I have done enough of them to be confident its the way to go.... Including my own house...

I have one going in this week, I actually build them as a kit at the shop and then just bring them to the site ready to install, if you like I can post a picture of the one sitting in m truck rite now... The tanks hardly use any power, they literally just run enough to make up for the heatloss of your recirc loop... They dont even run on startup since you can just run the rinnai to fill it with hot water, and every time you run the hot water it fills the tank... its a really nice setup, and when you use the rite parts it can be very convenient...



Let me know and Ill post a pic of what it looks like...

TerryR

Mini Tank

Sounds interesting. Yes, I'd like to see a photo. It's a small electric tank? So the makeup water is heated by the gas heater, and the electric just makes up for the loss in the recirc system? 

 What happens if you shut down the recirc pump at night - I guess you reheat with the electric the overnight loss in the mini tank and in the recirc loop?





Terry 

heatpro02920

Just a simple tiny water heater

I like the Bosch but I have used others in my systems, the first one I designed was a 6ga rheem I believe... It worked well, any small electric water heater will do it...



I have plugged in kilo watt meters and asked customers about their electric bills, the meter showed about $6 a month to run the tank {I got a different reading every time I did it, lowest was $3 highest was $9} and all the customers I asked about the power usage said they never noticed a difference....



This is the more expensive of my installations, not all of them are done like this, you can do the same thing with out using all of them fittings, I have a picture of it done much simpler, I will look for one...



This shows the grundfos pump {built in a-stat and timer} with the check valve flanges, plus webstone isolation unions and a 3/4 cross tee, ect them parts add up, I believe cross tees alone are $15 or so... Ill find a picture of a simple install I did, I have one in mind, it was a VERY tight budget....



So the pipe going into the right side of the tank is coming from the tanklesses hot line, the one coming straight down through the top goes to the recirc line, and the left {hot} goes to the houses hot water supply.. pretty simple, the center ball valve stays off unless you want to by pass the mini tank...



PS Sorry for the dirty stairs in the picture, I took all of this out of my service truck and stuck it together on the stairs to my garages attic...

heatpro02920

OK, I found a pic {not a good one}

of a similar install, this was done after the water heater was installed, it was a mess, started with a simple Rinnai install and then ended up wanting recirc, Its a terrible picture, but you see the ss pump and the a-stat in the recirc line, the rinnai unit piped in above the mini tank. So done for a lot less in materials, a lot more simple, just pipe it through and add a circ with a-stat, I used the a-stat from his water old water heater, you can use any aquastat or none, but I prefer the grundfos pump with all the bells and whistles... I like the timer, since I set mine so it doesnt recirc from 10pm to 5 am and then from 8am to 3pm so my recirc only runs around 10 hours a day and the tanks are small, tight, and very well insulated so they have hardly any stand by losses, the low amount of surface area and the small amount of water make them efficient...





Anyway, as you can see you can make it as simple or complicated as you like, I did one with 2 rinnai ru98's, a bosch es8 tank {the on in the pics}, a ss 009 pump on the recirc with an aquastat that was slave to a series of power sensing modules, one on each bathroom light and one in the kitchen, so when the bathroom or kitchen lights were on, the pump ran until the a-stat was satisfied, it was new contruction the home owner was an electrician, it came very nice..

TerryR

Mini Tank

Thanks, Heatpro,







That looks very slick. I can see how in new construction you can do a lot with occupancy sensing approach. 





Terry

heatpro02920

I have tried many different ways to tackle DHW

I just don't see the sense in running heating equipment year round, I don't like tanks {around here if you get 8 years you can count your self very lucky}, I don't see the sense in running hot water 24-7 when we use it for an hour a day, and I have had great luck with tanklesses, happy customers and good results all around... While the initial install can be costly, in 20 years or so when you have to replace it, you can do it yourself, 3 unions, plug it in, and hang it on the wall... I see this is the near future, your boiler breaks, you just plug in a new one...





For recircs I like the mini tank setup, another thing I do is run the recirc line as small as possible and make sure its insulated as well as your hot line where ever the recirc goes... http://www.homedepot.com/p/SharkBite-3-8-in-x-100-ft-Insulated-PEX-Pipe-U855I100/203316939?cm_mmc=shopping-_-googleads-_-pla-_-203316939&skwcid&kwd=&ci_sku=203316939&ci_kw=&ci_gpa=pla&ci_src=17588969

TerryR

Tackling DHW

I hear the argument about running heating equipment year round, but really there is no choice if you want hot water. The question is only which heating equipment to run year round. 



I don't see the value in running recirc 24/7, but we do use the hot water every couple of hours all day, from 7:30 am to 12 midnight, so an on-demand system makes no sense to me. We do shut it down with a timer outside those hours, and I think an smaller pump and a working aquistat would also help.



The insulated PEX you mention looks great for new installs. Our existing copper lines are insulated, at least as best I could convince our plumber to do it, but with 1/2" rather than 3/4" insulation. I suspect it wouldn't hurt to add another layer to the exposed sections



Terry.