New Boiler: Size, Brand, Fuel

[Tom Sawyer]

I didn't realize it was electric but that's even better. System 2000 uses a standard 40 gallon electric tank for storage so his current tank would do nicely. I've put a pile of 2000's in and many on small homes with very little radiation and surprisingly, they really don't short cycle. No, you don't get constant cycle times either but certainly long enough that it doesn't destroy efficiency.

 

[Dana]

I'm not sure there are any good duct design web-tutorials out there, but you can try searching for them. The gold standard of duct design is ACCA Manual-D (which is written into building codes) , and there are Manual-D duct design software products out there, none of which I'm familiar with.

They want $45K to bring the gas-grid to you? OUCH!!

For $45K you could probably tighten up the house a bit more and buy enough solar panels to get close to "Net Zero Energy", if your heat load is as low as you estimate.

 

[Sponsor]

 

[Tom Sawyer]

I seriously doubt the 22k heat loss. I'd bet it's at least twice that.

 

[Dana]

I seriously doubt the 22k heat loss. I'd bet it's at least twice that.

BS.

See the intial post:

"When I installed the mini splits I have done heat loss analysis and it came up as 20k btu for the entire house at zero outdoor temperature and 7700HDD. This was confirmed by my oil bill history for last 4 years."

If it was twice 22K he'd be freezin' his tuchus off whenever it dropped below 20F outside.

It's a 1500' house above grade, with an undisclosed amount below grade, only 500' of which is finished and actively heated. His outside design temp is -3F. A tightened-up sub-code 2x 4 framed house with clear glass single panes + storms will typically come in at ~15BTU/ft (x 2000' = 30K) @ 0F, but a 2x6 house with low-E windows would come in more like 12K (x 2000'= 24K) @ 0F. If most of the basement is below grade it could very easily be under 20K.

The 20K estimate may be on the low side of reality at -3F at an indoor temp of 72F, but no way is it anywhere near 44K.

 

[Alex Frost]

I am not an expert but this the data and the calculation I have used. Even my oil consumption history over 4 years supported it which made me feel comfortable with the results.

See attached file.

 

[Dana]

Looks right to me, assuming the place was actually heated to something reasonable in the mid to high 60s using only the boiler, with no auxilliary heating over that period with say, a wood stove or something. At 72F indoors, -3F outdoors you'd be at about 22,000 BTU/hr. If you keep the place at 55F or something you'd have to use a much lower base temperature than 65F for your heating degree-days, and it would skew the numbers considerably (but not 2x). It's better if you tracked the true oil use against the actual heating degree days over a winter time period rather than annual oil use over a 25 year average annual HDD type number. The shoulder seasons skew the numbers a bit, but probably not more than 15%.

Assuming that it IS skewed by 25% you'd be looking at a heat load of about 25K, which is 12.5 BTU/hr per square foot of conditioned space. If the 500' in the basement is mostly below grade the load per square foot there is very small compared to above grade, which may explain why it's coming in quite a bit better than 15 BTU/ft for the 2000 ' of fully conditioned space.

 

[Dana]

Another sanity check: Running it through the NORA Oil Heat's FSA calculator, setting it up the boiler description as 83% efficiency with 100K of output and a 1% idling loss, at 475 gallons/year it comes up with a heat load of 22,289 BTU/hr @ -2F if it were in Portsmouth, or 22,286 BTU/hr @ -6F if it were in Concord. Even if you set it up as an 83% 100K boiler with a completely unrealistic 0.1% idling loss its still in the 24Ks, not more.

The ~22K number is definitely in the right ball park.

 

[Alex Frost]

Does anyone know how can I determine if my boiler is what is called "cold start" boiler? Also, is it good practice to let the boiler cool down completely if there is no for heat in that time frame?

 

[Dana]

Consult the manufacturer as to whether it's cold-start tolerant.

If it's a cold-startable boiler, it's fine to let it cool between calls for heat.

If not rated for cold starting it has to idle at about 140F to limit corrosion on the fire side of the heat exchanger plates.

Peeking at page 15 the PF-series manual, under "COLD STANDBY" and looking at figure 6 on p.14, if there is a boiler bypass or system bypass branch in the near-boiler plumbing to mix boiler-output water with the return water from radiation, it's a cold-startable boiler.

 

[Alex Frost]

Hi,


Can someone explain or clarify how Honeywell L8124 Triple Aquastat should work. Here is my view:

Current setting: Hi Limit set to 180F. Low limit set to 160F with differential set at 20F.

If there is no call for heat the boiler temperature would drop to 160F before burner would fire up…correct? If there is a call for heat and the temperature is 180F should the boiler fire up? If not when should it fire up (at what temperature) while there is continues call for heat by either zone?

I would think that even if there call for heat that burner would not fire up until temperature drops to 160F due to 20F differential.

I am trying to understand how this should work so I can determine if my Aquastat works properly.

 

[Alex Frost]

Dana,

I am researching installation option for 12RLFCD and I was thinking to use it for our 3 bedrooms and basement (which would only used occasionally and much lower temperature about 60-65F). I would put unit in the basement and run the ducts thru the floor to each bedroom and one duct in basement. If I do this, where should my return be: basement or somewhere in the hallway outside the bedrooms (if even possible).

Thanks
Alex

 

[Tom Sawyer]

20 degree differential is too much. Drop it to ten. You have its operation correct. Remember, it's old, it's analog and it has a margin of error. Don't expect pin point accuracy. Where the sensing bulb is located has an effect on its operation too.

 

[Alex Frost]

Is it recommended to keep low temperature at 140F? I guess it would probably never reach this low before the burner kicks in on thermostat call.

 

[Tom Sawyer]

If you have a coil for domestic hot water and turn the low to 140, your wife will hate you. This is because at 140, a heat call with drop the temp way below comfortable shower temperature and take the burner awhile to recover.

 

[Alex Frost]

I do not have domestic water tied to my boiler. I have separate heat pump water heater.

 

[Tom Sawyer]

If you're not getting domestic hot water off the boiler than the low limit should be totally bypassed. It's an ultimate boiler, you can cold start it. There's no need to maintain temperature at all.

 

[Alex Frost]

What kind of aquastat would you recommend for this purpose. Like single high limit aquastat?

Any potential condensation issues as result of cold start? Below is the excerpt from manufacturer manual about running it as cold start and they recommend some sort of modification to avoid condensation.

1. Cold standby is suitable only when thermostatic
union is used with piping bypass shown in
Figure #6.

2. A change in control wiring is also required. To
prevent excessive condensation, please contact
factory before making this change.

3. Owners of non-tankless boilers often object to the
burner starting when space heat is not required.
To avoid this, an optional outside thermostat may
be used to shut down the system totally when
outside temperature is above a specific limit set
by the homeowner. See wiring diagrams.

 

[Dana]

There's no such thing as the differential being too small when there's 2x as much radiation as it takes to actually heat the place, as is likely to be the case here. At 10F it'll short-cycle on zone calls.

For a ducted mini-split you can use the hall as the return path as long as you use jump ducts of adequate size that whether the door is open or closed doesn't much affect the flow. It's pretty easy to use a partition wall stud bay as a jump duct, with a 12-14" wide grill near the floor on the bedroom side, and the same sized grill near the ceiling on the hallway side, which improves the privacy issue.

 

[Alex Frost]

Dana,

You are rigth, my boiler is cycling like crazy. It won't let the temperature drop much below set high limit before the burner kicks in.

Would I be ok putting return air grille in one of the bedrooms (the largest one)?Most of the time the bedrooms would be open except during the night to keep the temperature in the bedrooms steady. This would be easier installation then in hallways due to limited space and aesthetics reasons.

 

[Dana]

No matter where you put the grille to the return plenum, for every supply register there has to be a corresponding cross section of free return path. Sometimes partition wall jump ducts are easiest, sometimes ceiling joist bays can work (though it has to be on the opposite side of the room from the supply, or sometimes a sufficient door cut at the threshold. Without the return path the room pressure differences driven by the air handler will use the "Great Outdoors" as part of the return path, which becomes both an efficiency and comfort (= dry outdoor air) problem in winter.

These suckers aren't exactly a thing of beauty, but they are one way of providing a return path through a closed door without impinging on privacy.