Replace Electric heat with combi in duplex

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odi3

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I want to replace electric heat in my duplex with a combi boiler here in London Ontario.
Building is built in 1880 and a side by side duplex, each side 700 sq ft, with poor insulation and dirt basement.
I have a gas fitter to install the combi boiler, but he doesnt want to do the plumbing, so i will be doing the waterline work myself using uponor(wirsbo) fittings. I will be installing a basic 2 zone (one for each unit) setup as described in the installation manual. (I cant seem to find the pic at the moment but will post when i find it)

Current Heating looks like this:
Unit A
Living Room 750W 48" Register
Kitchen 750W 38" Register
Small Bed Room 750W 38" Register
Large Bedroom 750W 38" Register
Bath None

Unit B
Living Room 750w 48" Register
Kitchen None 750W 48" Register
Small Bed 750w 38" Register
Large Bed 750w 38" Register
Bath currently none, but want smalls register added

Here is my proposed Equipment list, I will update as i figure out new items

Current Proposed Equipment list:
Navien Combi Boiler NCB-180E ?
Navien Prebuilt manifold (i forget official name)
3/4" Pex A uponor for water source
1" Pex A uponor for closed loop lines (must be oxygen)
3/4" uponor shut offs unknown quantity
1" uponor shut offs unknown quantity
backflow preventor for feed water source
condensate stabilizer/filter
aquastat (i dont know what this is)
2" pvc input/output
pressure relieve valve (this may be included)
2x Taco Viridian 1816 or a Grundfos Alpha 2 15-55
2x Ecobee thermostats?
relat for thermostats taco/unknown model (2 zone model)
dirt separator model unknown?
Air Eliminator model unknown?
expansion tank size/model unknown?
9x radiators size/model unknown?
7 TRV Valves size/model unknown? (install all rooms except where thermostats are)



Some questions:
1) How do i determine combi boiler size? I want to just basically replace the current electrical registers with radiators in same spot and size. Since building is about 1400 sq ft, looks like my requirement is about 60,000 btu (2000x30) So since my house is old and poor insulation, and live in cold climate, would a NCB-180E work for me? which is 14,000 - 80,000 BTU/H

2)How do i determine new radiator sizes? is there an easy way to convert the electric sizing to water radiator sizing? or are water radiators listed in W?

3)If i install with 2 circulators, (one for each init) How do i size the circulators?

4)How do i size the expansion tank? and does any of the other equipment need sizing?

5)Any other tips, advice or equipment selection is greatly appreciated!

Thanks!
 
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Dana

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Looking for a "Hydronic heating design 101" course, are you?

Design-by-web-forum is fraught with pitfalls, but...

1) How do i determine combi boiler size? I want to just basically replace the current electrical registers with radiators in same spot and size. Since building is about 1400 sq ft, looks like my requirement is about 60,000 btu (2000x30) So since my house is old and poor insulation, and live in cold climate, would a NCB-180E work for me? which is 14,000 - 80,000 BTU/H

Heat loss/load is not a function of square feet of conditioned space. It's all about the exterior surface area by construction type, insulation levels, air leakage rates, and the temperature difference between interior & exterior. The heat loss at the "99% outside design temperature" is a standard reference point for the outdoor temperature to use when designing a system, and using 68F/20C as the indoor design temperature is enshrined in some building codes.

To get the radiation/heat-emitter sizing consistent, start with a room-by-room IBR or Manual-J type heat loss calculation for EACH room.

But the boiler sizing can be determined by wintertime energy use from prior seasons, measured against local weather data using this methodology.

It is highly unlikely that even a poorly insulated 1400' house needs anything like 60,000 BTU/hr to stay warm even at the record cold temperature in your location, assuming you have doors that close and there is glass in the windows. The load at the 99% design temp is likely to be no more than about half that much in most of southern Ontario, perhaps a bit more in western or northern Ontario (where are you?).

It's highly unlikely that the electric baseboards are correctly sized for the room/zone heat loads. But 750 watts = (750 x 3.412 BTU/watt=) 2559 BTU/hour. You have a total of 8 of the 750W heaters, so if the house has been able to stay reasonably warm, the design heat load for the whole house can't be more than 8 x 2559 BTU/hour = 20,472 BTU/hr, not 60K.

If some of the rooms run colder than 20C on the coldest days the design load is more than that 20K, number, but it's unlikely to be as much as twice that.

ALL combi boilers are oversized for the design heat load, but getting the best efficiency out of them is all about sizing the radiation relative to the minimum-fire output of the boiler, as explained here.

But the most important factor with a combi is the domestic hot water end. The NCB 180 is probably enough for a 2 bathroom house in cold-water country (= pretty much all of Ontario), but could be really marginal if there are large soaker tubs to fill, or if there are more than two low-flow showers running at the same time. The NCB 240 gives a bit more margin, but may not really be necessary if it's only standard size tubs, no gusher-showers, and only one bath on each side.

2)How do i determine new radiator sizes? is there an easy way to convert the electric sizing to water radiator sizing? or are water radiators listed in W?

Most hydronic radiators specify the heat output at one or two average water temperatures. To get condensing efficiency out of a combi boiler it needs to run no more than 140F even at the 99% design temp, and cooler the other 8673 hours in a year. Using an outdoor sensor the water temperatures can be automatically adjusted up or down as the outdoor temperatures fall or rise, a function referred to as "outdoor reset".

With the individual room radiation designed/specified to deliver the 99% design heat with 140F water and adjust the outdoor reset parameters correctly the room temperatures will be stable, the burn times long and efficient, and the seasonal average efficiency can be well north of 90%. But without the room by room load calculations you're shooting in the dark.

As stated previously it is highly unlikely that the electric baseboards are correctly sized for the room/zone heat loads, and they are probably not proportionally sized for the loads. Even a sloppy IBR or Manual-J will usually get the proportional sizing right. Just be sure that total heat emittance on each zone can emit the full (or at least 3/4) of minimum-fire output of the boiler (~13,000 BTU/hr for the NCB-180) with 120F water so that it can operate at condensing efficiency without short-cycling.

3)If i install with 2 circulators, (one for each init) How do i size the circulators?

To do it right you'd have to calculate the flow where the temperature drop from one end to the other is 10-15F, calculate the pumping head at that flow, then spec a pump with a curve that's about right. But these days it's easier to just spec an adjustable "smart" pump such as a Taco Viridian 1816 or a Grundfos Alpha 2 15-55 and tweak it after it's installed. You pay more for the pumps, but they pay for themselves in lower electricity cost, lower noise, higher comfort over time.
:)

4)How do i size the expansion tank? and does any of the other equipment need sizing? [/QUOTE]

The total system water volume has to be calculated. Then calculate the difference in water volume from room temp (where it will be during the summer season) and the highest system temperature (at the highest output temperature of the boiler), and be sure that the expansion tank is sized to accommodate it. Several expansion tank manufacturers have handy online calculators and/or instructions on how to make those calculations.
 

odi3

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Wow! thank you very much! I live in london ontario. I will review all of this closely!

On sizing the boiler: Both units have regular bath/showers, but both sides will have their own in-suite washer/dryers.
If i get the NCB-240, to avoid hot water problems, will this cause any problems due to having too high heat output? Or alternatively, should i install 2 separate ncb-150 units? one on each side?
Both sides have separate water/gas/electric meters. My current thoughts are to just cover heat/water since each unit only uses like 3 meter cube where min usage is 7.
there is a minimum 50 each meter minimum. gas is somewhere similiar.

Great idea on the variable pump. I will get those.
 
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Dana

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The 99% outside design temp for London Ontario is +3F/-16C.

The NCB 240e's minimum fire output is 17,000 BTU/hr, and it's up to the task of two full bathrooms in your climate.

With the NCB-240e the amount of radiation needed to guarantee no short cycling on zone calls with 120F water (cool enough to deliver 95% efficiency) would be about 30% more than with a pair of NCB-180e, but it would be a simpler installation to do just one combi-boiler. You might sketch out the design and compare the upcharge cost of the up sized radiation vs. the upcharge for two complete combi-boilers.
 

odi3

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Thanks a lot for the expertise!
I have scrapped the combi boiler idea and thinking of just putting in one regular boiler and hook both units to the single gas 40 gal hot water tank.

by scrapping the combi idea, i can better fit the boiler to the heat requirements of the house, avoid hot water demand problems by using a tank and simplify each system incase of problems.

Here is my gas bill for one unit, gas hot water heater only.

Customer Charge $22.50
Delivery to You $1.87
Federal Carbon Charge $2.04
Gas Supply Charge $3.83
Gas Price Adjustment $0.49
Delivery Price Adjustment $0.22
CR Federal Carbon Price Adjustment $0.34
Storage Charge $0.27
Charges for Natural Gas $31.12 +HST HST* $4.05
Total Charges for Natural Gas $35.17

Its basically 12.50 for heating water for one unit which is basically nothing.


If the electric baseboards max out at 20k i think the smallest navien boiler will be way more than enough at 55k.
nhb-55
 

Dana

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If the electric baseboards max out at 20k i think the smallest navien boiler will be way more than enough at 55k.
nhb-55


The min-fire output of the NHB-55 is about 7500 BTU/hr at condensing temperatures. It takes about 35-40' of typical fin-tube baseboard to emit that much at condensing temperatures. As long as EACH zone has that much baseboard it should be pretty easy to prevent short-cycling.

FWIW: An indirect fired water heater running as the "priority zone" off the boiler will recover faster, and will usually have at least twice the lifespan as a typical standalone gas water heater. Like it's little brother, the NHB-80 also throttles back to about 7500 BTU/hr in condensing mode, and it would provide even better hot water performance than the -55 (or a standalone.) The upcharge for the 80KBTU/hr burner isn't very much, and it doesn't change the system design or average efficiency at all.

The NHB series boilers all have water-tube heat exchangers, and MUST be plumbed primary/secondary (unless you can do all of the design math to make it work, which usually requires changing out the internal pump.) The purpose made manifold for that series is more idiot-proof, though other (and cheaper) hydraulic separators can still work if you know what you're doing.

There are similar sized boilers with fire-tube boilers that have very low pumping head and tolarate a wider range of flows through the boiler. The Westinghouse WBRUNG-80 (the same boiler under the paint as the HTP UFT-080) is a much simpler boiler for a DIYer (or pro) to design with and install than an NHB series boiler. It's easy to set it up to be pumped-direct (one pump, no primary/secondary). That boiler also has a second port already plumbed into the boiler to support an indirect water heater (which WOULD need it's own pump), along with some controls built-in to manage the water heater. The wall-hung versions run about USD$1500-1700 from distributors (they can even be ordered through the big orange box store in the US, not sure if that's true up in the wild north...)
 

Fitter30

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Have you checked with your utility companies if they offer rebates or energy audit that might run a load ? Windows , doors and insulation don't cost they pay every month.
 
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