Small houses more likely to oversize boilers

D107

Through what I heave learned on the wall, I have become an adherent of the 'size the boiler for heat loss and add storage for the hot water load as necessary' school.

However, having met and spoken with various contractors--most in the 'pumping away, we do heat-calcs school'-- for a replacement boiler (47Kbtu heat loss/100K emitter gain) I have heard these opinions:

1. For a house under 100K heat less and for a family with let's say 3 teenage kids where you might have 5 consecutive or simultaneous showers in the morning plus maybe a wash load, you could fall short of hot water if you sized the boiler to the load unless the indirect was massive.

2. The added problem would be that with this heavy morning usage on a cold day, you could have the boiler go on DHW priority for 45-60 minutes, which could give you a significant heating deficit. Especially on a below-design temp day. Some of this depends on personal style. Some families may want unlimited hot water and don't want to have to wait for it. Others may easily avoid morning laundries and not mind waiting a bit for the 4th and 5th showers if their schedule permits. My boiler install is for my mother living alone but someday the house will be sold and I am thinking a little ahead on that and for my own house as well.

3. Some contractors do the heat calc then hedge the results by making indoor design temp 15 deg lower than it should be so, in NY let's say, they know they're covered for 0 deg temps. This also depends on personal style. If it's a bitter cold day in this area, usually the worst outside temps are at night during a setback period when the boiler would be able to keep up. Personally if it's 0 or below when I wake up, I'm not bothered by a temporary shortage of two degrees 7-10 days max a year. But some people would be and might be angry at the installer for the lack of heat. (Hopefully their envelope has been tightened and insulated.)

I am wondering how much outdoor reset will reduce the excesses of an oversized non-modulating boiler.

PS: Mike T some months back posted a wonderful HW heat calc for me and basically figured out that with a boiler with a 75K DOE output for two simultaneous showers, one hot then warm laundry load, I'd need 60 gallons of storage for 55 deg to 130 deg rise, and still the boiler could take 40 minutes to replenish. Of course if you raise the tank water temp to 140 or above, you'll have more hot water but the boiler will be on DHW priority longer. Arghh! How about one indirect and one plain storage tank?

Thanks,

David

Brad White_9

I think you are hitting on something, David

What you are describing makes the case in a low heat loss house to have a separate-fired DHW heater. That or as Mike T. suggests, mega-storage capacity and make it up over time. I can see the cold AM domestic demand coinciding with morning warm-up...

Over-sizing a boiler for DWH when it is indisputably a larger and fixed (albeit sporadic) load is having the tail wag the dog. The notion of using an indirect with a ModCon, especially against the lower end of the modulating range at design cold days has me reconsidering the conventional wisdom...

Otherwise, when the heat loss is up there, the combination of ModCon and Indirect is hard to beat.

Your notion of a plain (I assume separately fired) and an indirect has merit- may make sense to use the indirect firstly as a pre-heater (to make a ModCon more efficient by lowering the return temperature), then finishing with the separate fired heater.... Just what I needed, another mental project

D107

good; it might make me re-think this.

There probably are some direct-fired 75 gallon tanks out there that could handle that simultaneous 3-4 showers two washload scenario or close to it. Or of course a new family moving in could just put in a bigger direct and leave the boiler alone. I know directs don't last as long and some efficiency is lost but maybe made up by not oversizing the boiler. And of course if the boiler's down you still have hot water.

Or, if outdoor reset provices a kind of defacto modulation, perhaps oversizing the boiler within reason is not a big issue.

Thanks,

David

hr

If in fact the DHW load

is that much bigger than the heating load, and if the heating loads fall within the mod con efficiency operating temperatures.. Maybe a condensing water heater would be the next best way to provide a lot of DHW. Perhaps an 80 gallon version with a 120 or 140K input.

Or lessen that large DHW load:) Water shortages may help that issue.

Doesn't the 50K Munchkin kick up to an 80K on DHW call?

Or a two stage mod con boiler.

Actually I believe that is how the large Aerco's do it. Solonoids step in various burner stages which then modulate around a 5-1. So 300K to 1 million.

I still feel a modulating large volume boiler would be the ideal appliance. Take a Polaris or Voyager and build it around a modulating, and two step burner. Then wrap a coil around it for radiant loads. Now you have DHW and buffer to handle micro hydronic loads.

hot rod

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Guy_6

Bingo

Hot Rod has nailed it: The design of today's houses are requiring less heat but more hot water. The Mod-Con boilers are the perfect fit, operating only in line with the load put on them, be it heat or hot water. Most have programmable priority times, so that the scenario listed above can be avoided.

Supply House Rick

Sizing

If I work up an estimate for a contractor, approx 60% say "better give me the next size up, just to be sure". I will say being in oil country that most are oversized to WAY oversized. This doesn't benefit the poor homeowner paying for the oil.

Rick

Mike T., Swampeast MO

Have been thinking about this all day while enameling woodwork--tedious and time-consuming but great for reflection...

I really suspect that if you use the indirect I suggested that would allow two concurrent showers followed by a load of laundry, that you won't have any problem--particularly in the non-heating months. Remember--consecutive showers aren't a problem. After two concurrent showers, you could have any number of consecutive, but no further concurrent... Two provisions: 1) Standard low-flow shower heads. 2) Indirect capable of instantly absorbing the full boiler output (again, not all indirects are created equal). If the boiler cycles on high limit due to insufficient heat transfer ability in the indirect, you may well have problems.

Regarding heating season operation:

1) With three teenagers and two parents, I agree that five rapidly successive showers are a possibility. I'm an only child so never had this problem, but time in squad bays in the military (and life after) tells me that the world is pretty well evenly split between those who prefer to shower before bed and those who prefer to shower in the morning. Unless all three of your teenagers have a private bath, I'm willing to bet that at least one will prefer to avoid the morning shower rush.

2) As long as your home is reasonably insulated with reasonable weatherization, I'm not at all a fan of significant daily setback of space temperature. Debate rages (and I still haven't heard back from Danfoss), but I consider such contrary to efficiency with a condensing/modulating boiler. As long as you're not trying to recover from setback at the same time that everyone is doing their business, an hour or so with little space heating shouldn't be too much of a problem.

3) Provided the boiler/control system allows such you could certainly boost the DHW temperature to quite high levels early in the morning in anticipation of the rush. (A DHW mixing valve should be considered a requirement.) This could cause conflict if you're also using daily setback, but such can be avoided by boosting the DHW shortly after setback begins and using a so-called "intelligent recovery" thermostat to have space temp up to normal slightly before the earliest riser...

---------------------------------------------------

If you want to use a smaller indirect or if peak load is a problem, it's certainly possible to add a stand-alone water heater in series with the indirect. (The stand-alone comes first.) Unless electricity in your area is utterly outrageous compared to your boiler fuel, I suspect it would be most efficient and least costly to use an electric and install a timer so that it heats in anticipation of any rush(s) and then shuts down.

Steamhead (in transit)

If you need to prioritize DHW only at certain times

another way is to use a multi-zone circ relay unit with a priority jumper that is held by screw terminals. Wiring a digital timer across these terminals will switch the DHW priority on and off. The Erie relay we used on the last job we posted had this capability.

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D107

Thanks for the great strategies

1. I am hoping the logamatic can handle what the multizone circ relay with jumper cable does as posted by Steamhead. (I'm checking with Buderus.) If I didn't mention it before, we'll be installing either G-115/21 or G124x/25. No mod con here unfortunately.

2. From Joe at Buderus: "Buderus recommends that our boilers can be upfired by 10% without any issues or warranty concerns. This can be accomplished with nozzle changes and/or pump pressure adjustments. For firing rates above 10% it is suggested to contact us directly..." This could be a little heat-calc insurance. And by the way, he says they CANNOT BE DOWNFIRED. He has also told me that the ST-200 HX can handle the G124x or G115 full firing rate.

3. Mike T's suggestion for bringing the space up to temperature BEFORE the first riser in the morning (DHW priority) is precisely the antidote for a long period of priority. My mother happens to keep temp at 70 day and 60 night as a matter of personal comfort.

The reasons for the oversizing I think go beyond contractors fears of inadequate heat/customer complaints. One, it's just easier to oversize and walk away. But also a true heat/emitter calc --not even mentioning pipe run measure--takes mucho time. At least two hours to go through the whole house, open up a few convectors (hoping they're in working order.) Add in transportation, software input time and printout and analysis, talking with customer I'd think it could easily be four hours. More if you have to open up a wall to verify insulation. Even then you might want to add 10% for what you couldn't see, insulation or rads that had degraded over time, de-rated btus for last emitters in the line, etc.

The average installer might worry they'd lose the job if they included that time in their price. But I'm one homeowner--of many I hope--who would greatly value the calc --and proper circ sizing--as a way of saving fuel dollars longterm by installing a smaller boiler. In fact there should be a book/software package out called 'How to Do Your Own Heat Loss Calc' because many of us would welcome the chance to learn about our homes. The IBR Handbook H-22 is a little bit too dry for most consumers I think.

As Mike T has pointed out there is built-in padding even on the existing heat calc software. Not counted is occupancy, solar gain, less infiltration since it can only happen from two not four directions at one time. Also with a properly insulated and humidified home, one might feel comfortable at 65 not 70 degrees.

The house, though small in heat loss, is I think quite common and a good representative of alot of what's out there. Maybe a majority; not everyone's out there building the big ones. There were five of us (we got pets for the added btus--Dad was an engineer) in a house of less than 1900 sq ft; 300 sq ft was later added as a porch. So I think it's important to have these strategies for these so-called 'small' houses.

Thanks again,

David

Weezbo

fast recovery ...

a plate exchanger as a seperate zone tied to a temp sensor on the discharge pipe from the indirect, with a recirc back to the checked cold of the indirect...will allow you to fill fairly large hot water demands rapidly,and only function should the temp of the water drop below a given set point. dont forget a anti scald devise *~/:)

CC.Rob

ditto -- this is probably very common

David, as a fellow HO, I enjoy your posts, and to the pros, I value the feedback you provide. Thanks.

My house (according to Slantfin HE2) is ~47k BTU/hr loss on a design day, and continues to drop somewhat as the envelope is improved. Has ~64k BTU/hr of fin-tube baseboard at 180F. The indirect would like to see about 105k BTU input for rated performance. So we're in the same boat -- DHW demand is double the space heating need. Present boiler is somewhat oversized for even the DHW load....

[Our particular problem is the control/reset/priority strategy is essentially nonexistent until we can afford to address it properly.]

I would not be too concerned about being cold on 15F below design if your radiation is that large. Winter of 04/05 we had a run of 15-20F below design days and we had no problem keeping the place warm, and that was before about 6k BTU/hr worth of envelope improvements so it was ~53k loss to 64k emitter output.

I think Manual J, like many things engineering, is quite conservative.

D107

steamhead can you translate this reply from Buderus?

re my question about the logamatic being able to prioritize DHW at certain times. I understand that in terms of instant demand, those two boilers' output is about half the 149K required for the ST-200. But I don't see that as a problem given Mike T's calc/recovery time etc. But I'm not sure if Buderus is saying the Logamatic can do what you proposed with your relay/jumper cable etc.

Re: Logamatic boost DHW temps just for early morning?

"The R2107 logic looks at the domestic hot water 30 minutes before going into the "Day mode" and will bring the tank up to the set point if needed. Based on the information provided, the boilers G115/21 or G124X/25 with a ST200 indirect tank, will require a longer recovery time due to available Btu's from the boiler. Piping and circulator sizing can also effect preformance. Options such as raising the tank temperature setting and using a tempering valve or add a storage tank should be considered. It is suggested to have the technician contact us to review the application.

If the multi-zone relay board is wired into the R2107 as we suggest, on a call for domestic, warm weather shut down or pump logic, the multi zone relay board is de-energized. Using the priority zone on a multi-zone relay board for domestic is not recommended because the relay board does not control the temperature in the boiler. The R2107 controls the boiler temperature and will operate on the reset curve in the space heating mode."

Thanks,

David

Steamhead (in transit)

I think they're saying

yes, it will make sure the tank is up to temperature before switching from night to day house temperatures. But with the tank and boiler you want to use, the tank will take longer to come up to temperature since it is being used with a smaller-than-usual boiler.

If someone out there who is more familiar with this control system has a better translation, we're listening.

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D107

Seems like DHW priority is also on setback at night

Indirects being new to me I had assumed that the indirect would maintain the set temp (130deg or so) all the time. Seems that before day space heating temps kick in around 6am, dhw will be brought up to temperature. I had been thinking the opposite order, i.e. first bring back space to day temp, then raise indirect temp maybe to 140-150 if HW load was high.

On a design or lower day with a 5-6 degree night space heating setback, boiler might have to start firing around 4:30am to reach temp by 6am. If DHW priority has to be established BEFORE that, then it might have to start at 3:30am or so AND maintain that until 6am when the HW usage starts. Given my ignorance of the DHW priority, I'm assuming it will kick in --and interrupt any heating cycle--whenever the indirect water temp falls below the established setpoint. If it's nighttime and only has to make up slight temp drops from standby losses, then that water heating should only be for very brief periods.

Seems Buderus is saying not to use a relay setup.

Thanks,

David