New Boiler: Size, Brand, Fuel

[Alex Frost]

Hi All,

I have an old (25 year) oil fired Ultimate boiler PF-04 that will require replacement soon as I don't think fixing it and throwing money into it makes sense.

With that said I am looking for some help/advice as to what replacement boiler to get in terms of size, brand, or even fuel type (oil or LPG).

My current house is split entry house with about 1500sf on upper level and about 500 of finished space in lower level. The house is well insulated with newer windows and doors.

I also have 2 mini split heat pumps (in LR, and DR/Kitchen/Hallway, none in bedrooms) that I have been using for last couple of months to heat most of the house .

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.

Not sure what other information I could provide to help someone steer me in the right direction.

Thanks
Alex

 

[Dana]

There are no oil boilers that fire down to under 20,000 BTU/hr. Even the smallest are over 50K. A hydronic heating system based on an oil-fired hot water heater (which is inherently self-buffered by the thermal mass of the water in the tank) would cycle a lot less, and would still have plenty of capacity left over for heating hot water.

How many zones (I'm assuming it's at least two, one upstairs, the other down) and how much radiation on each zone?

What is your 99% outside design temperature, approximately?

You may be better off spending the replacement boiler money on a ducted mini-split to cover the doored-off low-load bedrooms that are't well served by your current mini-splits. A 1.5 ton Fujitsu 18RLFCD mini-duct unit could probably heat your whole house at 0F, but a 3/4 tonner would cover the loads of a handful bedrooms with margin, whereas separate half-ton multi-split heads is likely to be overkill, oversized, and less efficient. Running room-by-room load numbers on the rooms you still need to heat would be a good start, but you might be able to ball-park it by the amount of hydronic radiation there is in each bedroom.

What are your current mini-split make/model?

 

[Sponsor]

 

[Alex Frost]

Hi Dana,
Even if I would go 100% with mini split heat pumps I would need secondary heat for extreme cold and potential power outage.

I have 4 zones (3 for upper level - 1 zone for large living room, 1 zone for dinning room, kitchen hallway, 1 for 3 bedrooms and bath.

I live in Manchester NH so it's -3F ODD

Total heat loss for 3 bedrooms is 5500 at -3F ODD

My 2 current models are MSZFH09NA

 

[Dana]

A Fujitsu 9RLFCD would be able to cover the combined heat load of your three bedrooms, and is specified for operation down to -5F. It'll keep going below that temp, at an unspecified output, but it should pretty much cover it, even during polar vortex events due to it's oversizing factor for your actual loads. (It's nominally rated for 12,000 BTU/hr at +17F, but still has more than enough at -3F.)

If the combined heat load is ~5500 BTU/hr @ -3F, the combined heat load at +47F (one of the HSPF test criteria) is about 1500 BTU/hr, and about 3000 BTU/hr @ +28F. The minimum output at +47F is about 3000 BTU/hr, somewhat less as the temperature falls off, which means the thing will begin to cycle a bit whenever it's above freezing, but should modulate with load whenever it's below freezing.

The key is finding somebody who is capable of installing it in a reasonable manner. Most installers want free reign to stuff it in the attic above the insulation, which is a terrible place to install it since it requires punching holes in the pressure boundary of the house, and has the additional heating/cooling load of the cold/hot attic surrounding the cassette and ducts. If possible to install the thing UNDER the ceiling in say, a closet, with short duct runs built into soffits, or in a basement space below, but most HVAC installers can't be bothered to deal with the other construction build-out necessary to do it "right".

The smallest 3 zone multi-split compressors from either Mitsubishi or Fujitsu have a minimum modulated compressor output of ~6000 BTU/hr @ +47F so they would pretty much be cycling rather than modulating all the time. They would heat the place, and may be easier to install than a mini-ducted solution, but the excessive cycling means they would be only about 2/3 as efficient as your FH09s.

The FH09 is great unit, since it's minimum-modulation is ~1600 BTU/hr which give it a great modulating range, and can deliver something like half your whole house design load at -3F. But Mitsubishi's SUZ/SEZ mini-ducted units suffer from rapidly falling capacity below +15F or so, and an unspecified output at your 99% outside design temp. Their mini-duct cassette has blower capacity issues cramping the duct design aspects too. The floor plan can make it easy or hard to retrofit a mini-ducted solution, depending on the particulars.

With four zones on the hydronic system it will definitely benefit from a thermal mass buffered approach, if you go that route. You probably don't have enough radiation in the whole house to balance boiler output with radiation output, let alone balance boiler output with any one zone, which is a recipe for short-cycling the boiler in to low efficiency. I'm not familiar with the Ultimate PF-04 (couldn't find specs online either), but assuming the -04 indicates a four plate boiler it's BTU output could be north of 150 BTU/hr, which would take a couple-hundred feet of fin-tube baseboard to balance reasonably. The PFO-4 can be throttled down to half that, but do you even have 100 ' total?

How much baseboard or other radiation is there on each of those four zones?

 

[Alex Frost]

Hi Dana,

I was planning to add in spring Mitsubishi concealed slim cassette, but now with your input I might go with Fujitsu. I have a guy who installed my current two units for very cheap as he works full time and did this on side. I helped him and I also did all electrical work. Also, I am planning to put indoor unit in one of of the kids closets and run flexible duct thru attic to two other rooms.

The model is PFO-4 . It looks like gross BTU's are 159MBh and net IBR 64MBh.

Here is breakdown by zones
Zone 1 - 550sf living room - it has about 64ft of baseboard (on all 3 outside walls)
Zone 2 - DR, Kitchen Hallway, Entry Area - 29ft
Zone 3 - 3 bedrooms and bath - 22ft
Zone 4 - Lower level (barely used) 16 ft

How would you throttle down the boiler? By using smallest possible nozzle? I do feel like my boiler is short cycling this season. I didn't notice it much last year, but I could have problems with my aquastat and temperature gauge as it does not give accurate reading based on aquastat setting. In other words, one or the another is bad.

 

[Dana]

Yes- it's possible to down-fire the boiler by reducing the nozzle size. It's not clear which nozzle you have, since 159MBH (the DOE output using with the maximum sized nozzle) is not consistent with net IBR of 64MBH (which would indicate the minimum sized nozzle.)

The PFO-4 can be fitted with a 0.60 gph nozzle, reducing it's output to 74MBH. If it isn't already using the smallest nozzle, it should be swapped. This should be done by a competent tech and tuned using a combustion analyzer. If the flue is way oversized for the lower firing rate or doesn't have a stainless liner there may be limits to how far it can be throttled back, but even at 74MBH it's ~4x oversized for your reported load.

Looks like you have 131' of baseboard. At 74 MBH that's 74,000/131= 565 BTU/hr per foot, which would balance at about 180F average water temp (190F out, 170F back) with typical fin tube baseboard when all zones are calling for heat at once. If just Zone 1 is calling for heat that's 74,000/64'= 1156 BTUhr per foot, which won't even come close to balancing even with 210F AWT- it will be cycling on/off during a continuous call for heat, but probably not too insanely. But when any one of the smaller zones is calling for heat on it's own it'll be cycling very rapidly.

Assuming a heat load of 20,000 BTU/hr, with 131' of baseboard that's 153 BTU/ft. That much baseboard could deliver the heat (with margin) at domestic hot water temperatures, which makes an oil-fired hot water heater solution pretty easy. In that sort of configuration the heating water is isolated from the potable water side with a plate-type heat exchanger, with a bronze pump on the potable side of the HX, and the standard circulator pump(s) on the heating system side. When any zone is calling for heat it only activates the pumps, not the burner. The burner is agnostic of the state of the thermostat controls, and is only controlled by the aquastat on the hot water heater. If you set the tank to 140F, even with a 10F drop across the HX (it'll probably be less, if properly designed) you'd have 130F water entering the heating system, and probably ~120F water coming back on the biggest zone, delivering over 200 BTU/hr per foot.

You may have to bump the storage temp a bit if it's not enough to keep the bedroom zones warm but maybe not. The reported load is 5500 BTU/hr, over 22' of fin tube, which is 250 BTU/ft, which would be delivered at ~130F AWT.

Since the burner never "sees" the zone calls, it only turns on/off in response to the temperature of the tank. If you used say, a 50 gallon Everhot with the 0.95 gph burner (~117,000 BTU/hr out), that's 420 lbs of thermal mass. With the differential on the aquastat set to 20F, it's minimum burn is 420 lbs x 20F= 8400 BTU. At 117,000BTU/hr that takes 8400/117,000= 0.072 hours (x 60=) 4 minutes, 20 seconds just to heat the water even if NO zones were drawing heat during the burn, which means the thing simply can't short-cycle.

If going with a mini-duct cassette solution, hard-pipe it with mastic-sealed joints and seams, not flex. Flex imparts too much duct impedance, and will give inconsistent results. Be sure to seal the duct boots to the ceiling gypsum, and mastic seal the boots too. Any duct leakage into a vented attic generates an air-handler driven pressure difference between the indoors and outdoors, using the great outdoors and random air leaks in your exterior walls as the return path. Do the math on the duct design too, or oversize it and use balancing vanes to adjust room-to-room temperatures. In the latter approach, a short fat plenum at the cassette with home-run pipes to the rooms makes it pretty easy to tweak, with minimal interaction with other rooms with the vane settings. Be sure to think about access for filter changes, service, and for routing condensate drain for the air conditioning too.

 

[Alex Frost]

Dana,

The current nozzle size is 0.75-0.85 gph according to the paper tag left by heating tech. If chose this route I will see what needs to be done.

Your idea of oil fired water heater is intriguing to me. However, I do have separate water heater (State Water Heat Pump) 50gal for my domestic water. With this in mind would oil fired water heater still make sense?

What is the HX you referred in your response?

Lastly, would any of the propane codensing/modulating boiler make sense in my case?

Thanks
Alex

 

[Dana]

HX= Heat eXchanger, in hydronic-speak.

With an ~0.8 gph nozzle it's pumping out ~94,000 BTU/hr, divided by 64' (your biggest zone) is about 1500 BTU/ft. That's going to cycle quite a bit even on the big zone, and go nuts on smaller zones, which is probably what you've noticed. Looks like the manual (see p.8) recommends the Delavan .60 X 80 A nozzle which is 0.65 gph, which would drop the output to about 76,000 BTU/hr. The manual also recommends setting up the aquastats to 180F hi, 160F lo, 20F differential. The burns will be longer and more efficient if you bump up the high limit an increase the differential (if possible with the aquastat controls it came with), to give the baseboard more output per foot, and use the thermal mass in the boiler to it's maximum by pushing it through a greater temperature range. (Doubling the differential will roughly double the burn times.)

If you were going to keep it for more than a season it's worth installing a heat purging economizer control like the Intellicon 3250 , to maximize the temperature swings of the available thermal mass, lengthening burn times and reducing short-cycling. With this sort of retrofit control you can program the low-limit to about 140F (below that it becomes a flue condensation problem), and set the high limit to the maximum. The control purges heat out of the boiler on a new call for heat until it reaches the programmed low limit, then lets the temp run to the max on a continuous call for heat. It "learns" the system, and anticipates the end of a call for heat by cutting the burner, which allows it to finish the call by pulling heat out, parking the boiler at a lower temp. In your system it's probably good for about 15-20% in fuel savings, and would cut the number of burn cycles by more than half. Many newer oil boilers like the MPO-IQ come with smart heat purge controls, but it won't fix the cycling for your stubby 16'-22'-29' zones as well as a buffering thermal mass such as a tank of water would.

There may be propane mod-cons that modulate down to 7500 BTU/hr or so, (HTP's UTP-80 gas unit goes down that far, don't know if there's a propane version available) but on your shorty-zones 7500/16'= 470 BTU/hr per foot. That balances with boiler output at an average water temp of 160F or so, which is well above the condensing range. To get much condensing efficiency without short-cycling the mod-con would take a minimum of 30' of baseboard (40' better) on any single zone. A condensing propane HW heater could work but it's probably not going to be worth it. The per BTU fuel cost of propane is high (higher than oil) and very volatile compared to heating with mini-splits.

 

[Tom Sawyer]

System 2000

 

[Dana]

He's heating his hot water with a heat pump water heater (at a lower cost than heating hot water with oil.) without the indirect as a heat dump the System 2K will short cycle as badly as anything else. A Bock or Everhot is inherently self-buffered, and he has enough baseboard to deliver the heat at DHW temps, for ultra-low standby loss. It'll beat a System 2K on both upfront cost and system efficiency with his heat emitters and low load. It doesn't HAVE to be used for domestic hot water (unless code requires any hot water heater used for space heating to be a combi-system, which is true in some locations.)

 

[Alex Frost]

Dana,


I think my long term solution is to install another mini split heat pump for our 3 bedrooms (Fujitsu9RLFCD) and probably another 7k wall unit in my lower level.

In terms of boiler as a secondary heat option, I am thinking my best option is to try to use smallest nozzle possible along with one of the fuel economizer. I know you suggested Intellicon HW+, but what about Beckett Heat Manager or Hydrostat 3250?
Also, do these economizers replace aquastats or they are tied to existing aquastat. The reason I am asking is because there is something funky going on with my boiler and I can’t figure out what. The triple aquastat (Honeywell L8124A) is set for 160F upper and 140F lower limit with 10F differential. Boiler gauge shows 180F and 30PSI. It appears the temperature does not drop much below 180F at any time and boiler keeps firing to keep it at 180F (if my gauge is correct). There is no excess water squirted out of the pressure relief valve. I also noticed, burner keeps firing often and sometimes is on for only 1-2 minutes.

Any idea what could be going on?

 

[Dana]

The Beckett Heat Manager is just a slightly dumbed down Intellicon 3250 (and manufactured by Intellicon.) You can get either for under $150 if you shop around on internet auction sites, etc. The installation of either requires only some electrician skills, and it uses some the existing aquatstat controls, but is in series with the connection to the burner, which has to be cut & spliced into. Download and read the installation instructions.

The Hydrostat 3250 has it's own temperature sensor that needs to be installed in the sensor well currently occupied the original aquastat.

If the boiler is cycling in a narrow temperature band below 180F and the L8124A is set for a hi-limit of 160F, there's something screwy going on with the L8124A. Sounds like there's a separate safety temperature limit aquastat (?) set to 180F, with an internal differential of a few degrees. It may be as simple as re-installing the sensor for the L8124A into the well on the boiler so that it's sensing the temperature correctly. Check to see if the sensor is loose (on either end). If the L8124A is sensing a temp lower than the low-limit temp due to faulty sensor it could keep the burner on indefinitely.

 

[Alex Frost]

So, by installing Hydrostat 3250 (replacing L8124A) would potentially resolve this issue and also provide more efficiency...am I correct in my thought process?

 

[Dana]

Read the instructions carefully, but yes, you would probably be able to replace the L8124A with a Hydrosat 3250 and be done with it. (I've never personally installed one of those.)

BTW: There are no 7KBTU wall heads that are standalone. A 2-zone multi-split for both the mini-duct cassette and the basement zone could work, but the minimum-modulation on the multi-split compressors is 6000-7000 BTU/hr which means in your setup it would almost never modulate. You're likely better off installing another FH09NA for the basement zone or finding some way to run a duct down to the basement (and maybe going with the 12RLFCD) so that the mini-duct cassette can still modulate rather than be forced to run at a higher speed by the modulating limitations of a multi-split compressor. The 12RLFCD has the same 3100 BTU/hr minimum modulation of the 9K unit, but has a bit more power in the air handler, and a bit more capacity on the high end (not that you'd really need it.) You may have to seasonally tweak a register or balancing vane for the basement zone to keep it's temperature in range, since the heat loss/gain characteristics of basements are a bit different from upper floors.

 

[Alex Frost]

It seems like there are lot of good feedback online on Intellicon HW+ which seems to be easy DIY, but in my case since I might need a new aquastat then I would need to buy both: new aquastat and Intellicon HW+.

The best option would be to install single 12RLFCD in the basement and run the ductwork to the bedrooms. This way one unit would cover all other areas.

I thought with the basement and 3 bedrooms I would need 6000-7000 BTU/hr. Basement was never part of my "heat load" calculation as we do not use it often (if at all), but it we were to heat it I am sure we would use it more. Hence, the plan to add zone to the basement.

 

[Alex Frost]

Dana, I forgot to ask in my prior post if placing the "air registers" in the floor (coming from basement) is better option compared to ceiling?

 

[Dana]

Dana, I forgot to ask in my prior post if placing the "air registers" in the floor (coming from basement) is better option compared to ceiling?

Doesn't much matter. In a house that's reasonably air tight the stratification issues go away. Traditionally air conditioning was installed at the ceiling, heating at the floor, but duct use changes with the season. Being a highly heating dominated climate, having the heating registers (or mini-split heads) nearer the floor are more traditional, and it works well, but it works just fine a the ceiling too.

There is some enhanced efficiency if the RETURN duct registers are near the floor, which is where the cooler air stratifies. With 2-5F cooler air entering the mini-split's interior coil the temperature difference between the outdoors and coil is lower, which means the compressor doesn't have to push as hard to get the heat there. It's a pretty subtle distinction though- it's not worth re-installing the FH09s near the floor or anything.

 

[Tom Sawyer]

So use the plate exchanger and the existing water heater. Shut off the gas during the heating season and let the 2000 make hot water and turn it back on in the summer although truthfully, the 2000 will make hot water cheaper and faster than the gas will. Still, he'd have back up.

 

[Dana]

So use the plate exchanger and the existing water heater. Shut off the gas during the heating season and let the 2000 make hot water and turn it back on in the summer although truthfully, the 2000 will make hot water cheaper and faster than the gas will. Still, he'd have back up.

The existing water heater is a State heat pump water heater. You can't heat a house with a heat pump water heater that's drawing half or more it's heat from the house even if it COULD deliver 20,000 BTU/hr in heat pump mode (to the water, that is.)

And there's no way a ridiculously oversized System 2000 is going to make hot water at a lower cost than a heat pump water even at current electricity & this year's noo-improoved lower oil price.

Nobody is talking about a gas or propane water heater solution here either. There is no gas service to this house, but there may be propane(?). An Everhot or Bock oil fired water heater is what was being discussed as the self-buffered solution, not a propane fired combi. If there WERE gas service to the house, a condensing gas hot water heater solution might beat the existing State water heater on operating cost, and a condensing gas combi would be slightly cheaper to heat with than the mini-splits, and a lot cheaper to heat with than an oil fire System 2K. There's merit to simply finishing off the mini-split solution to cover the whole house, and keeping the oil burner around as backup.

 

[Alex Frost]

Dana,

Thank you for clarifying my options and I wish there was natural gas on the street, but no plans for Liberty Utilities to do so, however they quoted me in the $45,000 range to do so like I would not have better use for that money..lol.

Back to 9 or 12 RLFCD unit. I am thinking to install in the lower level (aka basement) and run 3 ducts toward each bedroom and leave the 4th one for basement (assuming it can be done for 4 separate ducts). What is the best tool or website to figure out best duct size for each room?