Condensing boiler system design....

Mitch_4

remember that because the AFUE tests at 140 out, 120 in, the 92-95% efficiency rating for condensing /mod cons is fairly accurate, but unrealistic for CI boilers that typically run 180, 80% at 140/120 might drop to 70 or 75% at 180.

Aslo using lower efficiecny requires more btu for the same effective transfer, and it takes less btu if you design the system around 140/120 for a mod con as you save 1 btu/pound h2o /degree (less btu's to heat to 140 rather than 180)

AFUE is only a start point.

Dr Pepper

Condensing boiler system design.....

OK, so the boss sez, don't reinvent the wheel, but how much more does it cost to design & build a NEW building with a condensing boiler and heating coils sized to take advantage of the low temp. water (140 - 110*F??), still get a reasonable payback, and keep first cost reasonable?
Here is the building: Stand alone single story medical facility. 3 million btuh +/- total heating load. Perimiter heat with reheat VAV probably. Radiant is out due to cost. The building will be either the same old way 180*/20*T< or low temp.
Are there studies, comparisons, available for this question?
Has anyone gone thru this question already?
Thanks,
HVAC engineer in a corner...!

Brad White

Where it $hows up

beyond the "boiler to boiler comparsion", is in the emitter cost. Using 4-row rather than 2-row coils, triple-element fin-tube rather than single. Radiant panels are a bonus in a medical facility due to ease of cleaning by the way.

Point of fact, in many cases, the 2-row terminal box coils work fine at 140 degree entering water. 4-row would be reserved for high heat areas so you may not spend any more in that category.

The cabinet unit heaters in the lobby/vestibules put out half of what they do when you use 180 F water so you install two instead of one.

Those kinds of things.....

Cost out the boilers of course, compared to traditional fire or water tube or cast iron models. We normally see a premium of about 50% over conventional over cast iron but this has been narrowing in the past few year as condensing boilers become more mainstream.

Often it included on-board controls for stand-alone operation. The venting cost is higher but often smaller and if indoors, less insulated. Cost thoese parts out.

Once installed you are essentially done. Savings are yours provided setpoints are maintained and not over-ridden. The only "penalty" might be higher air pressure drops of deeper coils in the airstream and the cost of running more fractional unit heater motors.

In the big scheme of things, the costs are incremental and tend to get lost in the budget of a larger facility.

Check for utility rebates and other benefits (state, local, federal) as may be availble for condensing over conventional boilers too.

ALH_4

efficiency

Brad, would outdoor reset be practical in this type of application? My hvac experience thus far has been limited to residential construction.

One thing about modulating boilers is that they can be very efficient even at high supply temperatures when the burner is at lower firing rates. So even if 180°F is the supply temperature, modulation might let you run at 95%. On the Vitodens, the combustion efficiency difference between 120 and 180 at low fire is something in the neighborhood of 1%. Firing rate is more important than the supply temperature, and because the heat load is rarely at design, high efficiency can be achieved even at high temperatures. So even if you do not increase the emitter sizes, burner modulation is extremely valuable.

jp_2

??

andrew, my guess its unversal that heating with the lowest possible 'heat' wins everytime?

though the vito maintains high efficiency at higher temps, seems the building as a whole suffers at higher temps, that is, higher temps mean higher losses where losses occur(loosing heat where you do not want it lost).

but maybe the higher cost of more radiation losses here.

Brad White

J. Paul

Your point that the building suffers at higher temperatures due to higher temperature = higher losses presuposes that the setpoint has no upper limit. I mean, you are right of course but that is a fundamental principle of control..

The point of using deeper coil rows and more radiation is not to make the building warmer but to make the building the same temperature you want it to be with cooler heating water.

ALH_4

losses

I agree completely. I'm just stating that combustion efficiency is affected more by modulation than by supply temperatures. Relating to operating cost, I think getting a modulating condensing boiler into a project is the highest priority. Reducing the supply temperature is next. Relating to comfort, outdoor reset is first, though mod/cons tie directly into this.

It is sometimes easy to forget (for me anyway) that everything we spec is sized for the worst case scenario. Even emitters sized for high temperatures at design are "oversized" most of the year and therefore can run with reduced supply temperatures.

Brad White

I agree in principle, Andrew

but I think the spread in efficiency at those temperatures has to be more than 1%. One condenses, one cannot so the stated efficiency spread just has to be higher than that- at least 5% I would say. If 180 degree water can run at 87% (my experience, no tables before me), condensing typically starts about 92% and higher.

I do agree that modulation is key and would take modulation over condensing every time, assuming I could not get both.

Keep in mind though that a larger building such as a medical facility would have a number of smaller boilers (N+1 at least, such that one out of commission will still heat the building).

That said, boilers in that size range tend to have modulation even if not condensing. (I get my wish or at least the half I want.)

Most areas have hours of heating operation which allow below-condensing temperatures. The bulk of hours at which water temperatures are above the condensing range are relatively few. Still, the emitters are an investment bought once. I want condensing year-round if I can get it, but low temperature use regardless, condensing or not.

Where reheat is used, (medical facilities have ventilation loads often well above cooling load requirements) you have an inherent low-temperature load to deal with year-round. Taking 55-60 degree air up 5, 10 or 20 degrees can be done with 100 degree water! Perfect for condensing even in summer.

Your core question of outdoor reset: Depends on the facility. Patient bedrooms maybe. The ER has to be maintained 24-7 as do the OR's for emergencies. But at 30 ACH supply rates or higher, recovery is quick even if set back... Besides, those are interior rooms usually.


Keep in mind the skin of a larger building is a minor heating load compared to overall floor area or building mass. Ventilation loads are the clear dominant load.

jp_2

simpler case

Ok,my comment was very elementary.....

not perfected worded either. I was trying to say that supply lines in a large structure are going to suffer more loses the higher the supply temps need to be, which of course very elementary....

higher delta T to the surrounds... but sometimes i think this basic is forgotten.

such as higher loses from a radiator against the wall to the outside the higher that radiator temp needs to be. along with the loses that will take place all along that radiators supply line.

jp_2

brad?

in this statement:

Keep in mind the skin of a larger building is a minor heating load compared to overall floor area or building mass. Ventilation loads are the clear dominant load.

are you saying that air handlers(hydro coils) need to take up more of the biulds load than wall radiators?

a friend of mine was pondering this same question at a medical building, "which takes more of the load, the air handlers or the wall radiators, both are in same areas, same rooms?"

ALH_4

Thanks

Thanks, Brad. There is quite a bit more to these larger systems than I have had the opportunity to work with first hand.

Apparently Viessmann thinks 167°F can be a condensing temperature if the heat exchanger is sufficiently large relative to the flame?

Brad White

I am not sure that

the 167 F (75 C) temperature is necessarily a harbinger of condensing more than it is, in celsius, a nice round-ish kind of number

You are correct, the absorbtion of heat from the flue gas is a function of surface area of the HEX and mass flow. Less mass flow, more surface area = more retention time hence more transfer.


The return temperature is where it starts so by designing to a 140 F maximum supply temperature and at least a 20 degree delta-T if not more (I use 30-40 on commercial/institutional work), condensing is assure.

What is more, the relatively few hours when the 140 F is required due to colder weather still condenses on the back side of the flow cycle. With modest reset at the boiler level, condensing can occur entirely, supply to return such as on an RFH system.

ALH_4

sizing

I love wide dT's. 20°F seems useful only for doing the math in one's head. Calculators should have rendered the 20°F dT obsolete 30 years ago. :-)

I just pick 75°C because it is the maximum possible with the Vitodens, so their curves do not extend past this number. But at 167°F with a 20°F dT, the Vitodens is theoretically capable of 97% efficiency at 25% modulation, read off the postage-stamp-sized plot in the flyer.

This brings up a question in my mind... Suddenly I am envisioning 6-24 burners in 15-60 heat exchangers. Would this have a significant effect on efficiency at higher temperatures? I do not think designing around this idea is proper for a new system, but what about a retrofit? How do boiler manufacturers arrive at the heat exchanger size relative to burner size, particluarly in a mod/con? I realize I am getting very off-topic here.

Brad White

One has to work it out, J. Paul

but think of it this way:

In a typical detached 2-story house every room has an exposure or at least very few truly interior rooms. The external surface area relative to floor area is fairly high, maybe 1:1 or greater.

In a commercial or institutional building with a larger floor plate and multiple stories, the exterior 15 feet or so along the perimeter is all that really needs to be heated relative to the exterior heat losses. The ratio of external surface area to total floor area might be 1:5, 1:10 or more.

The interior spaces "know no season" and in fact require cooling year-round. Constant internal gains and no external losses, you can see this.


Take a typical single-exposure hospital patient room, maybe a 5 foot high by 8 foot wide low-E window and a 12 foot by 12 foot gross wall area with a 9 foot ceiling. 0.5 ACH infiltration will get you about 3,500 BTUH of heat load. (Zero outside, 72 F. inside although code says 75.)

Ventilation load at the greater of 20 CFM per patient or toilet make-up might get you to 60-80 CFM for that room. The ventilation load to heat that air, originally from zero to room temperature is 4,700 to 6,250 BTUH, significantly more than the skin losses plus infiltration.

A surgical OR at 600 SF, 10 foot ceilings and 30 ACH ventilation rate gets you to 3,000 CFM. If 20% of that is outside air and the cooling load only requires 2,000 CFM, one has to reheat the air to the tune of over 20,000 BTUH and with no external heat losses to speak of.

Just two examples.

jp_2

thanks brad

that does make some good sense.

Dr Pepper

Wow, thanks for the come back...

First off I'm in northern Illinois, so it gets cold and stays that way for a while. Natural gas has 100,000+/- btu/therm, the building system is going to use 80% or 95+% to heat the building and lose 5 to 20% up the flue. That only gives a theoretical window of 10-15% efficiency to do something with. Your right the heating load is driven by occupancy loads for the building. Once the architects meet the building Fed/State insulation guidelines, I get what's left to deal with. I got some "contractor cost" prices for similar sized condensing 95% (Aerco), Hi-effiency 88% non-condensing (Burnham) and 82% Bryan Flex-tube. The price doubles from the 82% to the 95%. About $40,000 + the added cost of HEX surface area that I haven't added up yet.

I understand this owner isn't interested in Green Buildings except from the income aspect. So, how do we make it worth doing?

Brad White

Lifecycle Cost

You have to give them a lifecycle cost analysis using net present value numbers. This will take into account the estimated cost of a base system and comparitive cost of the proposed system reduced by any rebates.

Properly done, the capital cost, borrowing cost, escalated fuel cost and service labor cost against savings. All of this is weighed against anticipated life cycle and cost of replacement at that time. A cast iron boiler may outlive most others properly maintained but may not have the efficiency. Modern ModCons have a lifespan not currently known but has been assumed at 15-20 years depending on the make. Projected of course. Europeans have better data in this regard.

Assume escalation of fuel and service at a certain rate per year or if you elect "simple payback" the assumptions will be cleaner.

The AFUE numbers are steady-state figures. They are not really accurate in predicting operating costs; modulation has to be taken into account. AFUE is better than nothing until you know more