Add capacity for indirect

Frank_3

Wayne, what formula are you using to come up with that number? I tried figuring out the btuH needs for a DHW tank once and came up with a ridiculous number, so I knew something was wrong. Never went back to try and find my mistake, though.

Wayco Wayne

How much

capacity should I add to a boiler size if I am adding an Indirect water heater?? Heat load shows 60,000 heat loss for shell of house. Small family. Probably use a 30 or 50 gallon indirect. Will have indirect panel rads and plated rad floor for main level. Has anyone used a Turbomax or Ergomax as a buffer tank in this situation??

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Steve Paul

leave it alone

60,000 btus is plenty for indirect water heating if you use priority for the indirect heater. Use an Argo or similar control with a priority switch. It should work fine.

Wayco Wayne_2

More

My figures say that a 40 gallon water heater would need 26460 btu's to heat 50 degree water to 130 degrees. If I put the indirect on priority that would turn off the heating on a 60000 but output boiler for 26 minutes. If it's cold outside and you need all of your heating you are going to fall behind the curve and not heat enough. Of course not everyone uses 40 gallons of water in the morning, but a 2 gallons per minute shower head a 10 minute shower will use 20 gallons of water and maybe half of that is hot water so 10 gallons of hot water. A family of 4 takes concurreent showers, it's possible to effectively turn off the heat for 26 minutes. In the morning that's not good. Then again it's not good to oversize the boiler either. This job is oil so I can't fall back on a munchkin variable capacity. Maybe I'll just go up as size and add 10000 btu's

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Floyd

If you goin' oil....

get the next biger boiler... the $$$$$ aren't that much more.......

And.... I hate them small nozzles.... notin' but trouble!!!!!

Floyd

kf_2

Formula

Frank,


Gallons per hour needed x 8.33 x temperature rise = BTU/hr. needed.

(8.33 = gallons in one pound of water)

(BTU = energy required to raise one pound of water one degree farenheit) Hence 8.33 x temp rise.

kf

Paul Pollets

Good advice

I agree with Floyd. I always size the boiler for a min of 100K output when trying to recover a 53g tank. I've observed recovery of the Viessmann 53g tank in 10 min. from 50deg to 120deg. using a 15-42 pump. When we install a 92g tank, I look for a min of 150-200K input and use the 26-64pump. The Euros size the boiler for fast DHW indirect recovery. The boiler size is often larger than the heat loss. I also don't care for smaller nozzles or trying to downfire below .8gph Burners simply work better when firing over 1gph

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

joel_14

sorry ,disagree

At least about small firing rates on oil. We have dozens of units running with .50 -.75 nozzels really never a problem. We've gotten away from priority on domestic whenever possable. I don't like what happens when you've got a boiler full of smoking hot water and then you come out of priority and blow that at the system. I prefer to upsize the domestic tank for longer draws and then don't need to prioritize for quick recovery since you can't drain the tank anyhow.
if your using a boiler with large water content though such as a Viessmann, sizing isn't a huge deal as you'll get a nice long run no matter what, which is a priority with oil. Have you tried the vitorond series?? These are cheaper than a Vitola and I think they have a model at around 73kbtu???. Would give you some extra kick. Actually I don't know of anyone making an oil boiler any smaller than that anyhow?? We've put in quite a few and they run very well with the basic kw10 weather responsive controll. A pretty good reasonable cost set up with panel rads. Not as precise as 4way mixing but alot better than a simple bang bang 180 aquastat on a typical boiler.

Warmfoot

Priority

In most of the systems we install, we use a motorized 4-way mix valves. When there is a call for domestic hot water, that 4-way is going to to almost completely close anyway and the high temp water it pulls out of the primary loop is virtually none, due to the high temperatures the boiler is reaching. In my opinion, there is your priority for the DHW without actually putting the controls into a priority setting. just my 2-cents worth.


Ernie Bogue
Master Hydronics LLC
PO Box 779
Keyport, WA 98345
(360) 394-2049

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Bob Bona_4

usually

that 100K is the magic number for 40-50 gallon tanks. Always exceptions, though..I have a Burnham V713 firing .75 running an Alliance 40 gallon on priority at one clients house. Not a problem. Then again, this is one occupant, 2 baths. As long as there is exceptional filtration, nozzles 50-1.0 gallons per hour work great when set up properly-thats what I've personally found (IMO)

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

ScottMP

interesting

Joel I believe that Viessmann, and I know Buderus, have there controls set to accomadate the spike in temperature after DHW production.

With the Buderus Logomatic control the sensor tells the boiler when the tank is close to reach its set point and then shuts off before that, to lower the boiler temp. The DHW pump runs a short time with out the boiler firing, sort of a soft stop. That way the boiler temp is closer to the design temperature of the house and you don't get the spike in heat you refer to.

With a larger system you may be correct that the priority is not as important and the system size can absorb the DHW demand. with a smaller system like Waynes I can see where the house and the DHW both trying to call at the same time can slow the DHW production. Nothing worse to a customer than having not enough hot water.

Wayne I would size the larger tank and still prioritse the tank. I really like the controls that take the system out of priority after an hour. Thats a nice feature when your worried about the house getting stuck in priority.

Scott

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

Frank_3

To kf, re: formula

kf,

The formula I was working with was similar:

btuH = (8.33 * gph * deltaT) / %efficiency

The problem was coming up with a reasonable gph for recovery rate. Do you just pick a number out of the hat?

I came across an ASHRAE table which showed a rate of 3.4 gph per apartment in a multi-family dwelling. Does a single-family house get treated as one apartment? That would be only around 2400 btuH needed to keep the hot water flowing (with an 85 degree deltaT and 100% efficiency).

Using Wayne's results and your formula (which is the formula I have but with 100% efficiency) it would seem Wayne is using a recovery rate of close to 40 gph. Is that more realistic? We hardly let the shower run with only hot water, usually more cold water is mixed in to make things more comfortable. :-)

I actually once tried to come up with an estimate of gallons per minute, then translated that to gph. The btuH numbers were in the 200k range. That's when I walked away and just crossed my fingers.

So, how do you pick a reasonable gph recovery rate that can be used to determine the btuH needed for DHW production?

joel_14

milne

Depends which controll you get with V as to wether you have that capability. We've had issues with older trimatics where the boiler water temp goes high (non mixed water)and then gets shot at something like aluminium panel baseboard that exspand big time and make some noise. through the mixing valve it isn't aproblem ads the valve will compensate. but stuff right off the boiler can get banged pretty hard if the curve needs 110 and it gets hit with 30 gallons of 18o.

clammy

high domestic water use

besides extra storage tanks what can be done for high volumewater use like oversized tubs that do about 12 to 15 g.p.m.the only thing thats worked for me with inderect h.w.t. is to add a plate exchanger with a mixing valve on the outlet of the tank.would like to hear other opitions?

hot_rod

A balancing act

Certainly you need to size the boiler to the DHW load. Hard to go below 65,000, in my opinion. Keeping in mind direct fired water heaters are generally in the 40,000 input range.

Becarefull going too far the other way also. If the heat load is, say 40-60,000 I'd hate to see you double that just for DHW. Depending on the system zoneing, etc, you are opening the door to short cycles on the boiler in light load (heat only) conditions.

OR go with a modulating boiler that can easily adjust to the varying loads. Buffer tanks solve this also, but it is more cost and pieces to "make it happen" correctly.

hot rod

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

hot_rod

Instantanous

DHW heaters for point of use high demand loads make lots of sense to me. The current crop of products from Japan have some huge BTU inputs and modulating burnersb to handle large flow rates like tub fillers.

In some cases instant production makes more sense than storing large volumums of heated water in tanks. Just depends on the job and owners expectations

hot rod

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

JackFre

On-demand dhw

www.rinnaina.com has detailed DHW sizing tables as well as all the tech info on the units. Put in the 2532FFU. .82 EF on Ngas and .87 on lp. Direct vent. 55F temp rise = 5.7gpm, 60=5.2, 65=4.8.....24 hrs per day. The "new" crop of units have the variable input technology to make these units really perform. You guys love Veissman. Rinnai has been making fully modulating gas valves since 1985. The best, most reliable equipment I've ever seen. Dealers must be certified to install Rinnai Continuum's. It is an excellent program and can change your views on hot water.

Paul Pollets

You may be forgetting

that the use of a 4 way or 3 way motorized valve will not allow a burst of very hot boiler water into the system, after DHW recovery. the valves open very slowly, taking into account the higher boiler water temperature. Many of our clients have heavy DHW usage and expect recovery in 10-15 min. If the system is designed right (even with an oil fired appliance) it will not short cycle and still have fast recovery on the DHW side.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"

joel_14

paul

Your correct about the 4 way but i'm talking about systems with a4 way on the radiant and panels fed directly from the boiler. Back befor eyou could do a vitotronic 300

KCA_2

Here's something

To consider. I use the sidearm as a buffer tank. Heat the domestic up to 160deg+ and use a temp-valve for the domestic. The primary pump has to stay on when there's a call for heat. When a call for heat comes the primary pump pulls heat through the coils into the loop and the injection pump takes that heat to the secondary.
The boiler cycles less often. Been using it for years.
Size the primary pump to handle the sidearm, 12' head for a superstor at 5GPM + the rest of the loop. This gives you a heck of a lot of hot water...

Just something to consider. I hope that this pic works OK. I tried to scale it down to fit nicely.

:-) Kca

kf_2

Formula Cont.

Frank,

The part of your equation that you divide by efficiency is correct assuming that your using the BTU/hr input. I use the BTU/hr output having already factored the efficiency. The GPM equation is GPM needed X 500 X delta T. (the 500 comes from 8.33 X 60 minutes)

ASHRAE has standards for most commercial applications however, residential applications vary so much between the small 800 square foot ranch to the 20,000+ trophy homes.

In most cases the largest draw in houses is either a whirlpool tub or these massive shower stalls with multiple shower heads and body sprays. For whirpools you need to know the GPM flow rate for the tub filler and the gallon of the tub. ie. 10 GPM tub filler with 80 gallon tub will use 80 gallons of water (mixed) in 8 minutes. Since 10 GPM's would require approx. 375,000 BTU/hr, storage is required. Now granted some of this water is mixed the actual draw of the 140 degree water is less. (mixed water - cold water/hot water - cold water = % of hot water actually used.

Those big showers are fun. I have actually seen some that use in excess of 25 GPM's, so the duration of the average shower will need to be known and you will have to make a compromise between recovery and storage.

A nice safe formula for houses with whirlpools with large tub fillers is J=T. Jacuzzi size = Tank size. An consider providing enough BTU's to recover the tank capacity in about 20 minutes.


kf

Paul Pollets

BTW

Viessmann controls from the Trimatic MC, RN or Vitotec, have always had the capacity to control the DHW priority settings by either closing the mixing valve or shutting off the "A" pump circuit. If the radiators have TRV,s there would be no temperature change. I like to take advantage of the "casting lag", and in a temperate climate, the boiler can wait to fire while it's still heating the panels.

To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"