Boiler selection

[Timothy Graham Work]

Hey guys, new to the forum here but seems like a great resource for this sort of stuff. I'm just finishing my first home build and have installed pex tubing throughout the house. It's approx 2600 sq Ft including the garage (which I also put tubing in and plan on heating). I am located in BC Canada withing zone 6 with a HDD of 4000-4999. I just had a heat calc done by the local heating and cooling engineer and my heat loss is coming in at 49,000 btu/hr. The engineer also states that I need a 20% back up and therefor my boiler needs to be at 59,000 btu/hr. My plan is to get a combi boiler from veissman (vitodens 222-f) but it comes in two different sizes. The 19 which has a mbh of 12 to 68, or the 35 which has a mbh ranging from 19 to 125. I know that the real killer is over sizing your boiler but I also don't want to end up undersized either. The local plumbers who will be installing aren't a ton of help here so any advice would be great. Thanks.

 

[DewPoint]

How did you decided on the configurations (length, spacing, size, zone) of the pex runs before having heat loss data?

Both boilers are oversized for your heating needs - I think you know that. I assume that the 222-f is your choice of boiler because of the domestic hot water but you didn’t mention your domestic hot water demand expectations.

 

[Sponsor]

 

[Timothy Graham Work]

How did you decided on the configurations (length, spacing, size, zone) of the pex runs before having heat loss data?

Both boilers are oversized for your heating needs - I think you know that. I assume that the 222-f is your choice of boiler because of the domestic hot water but you didn’t mention your domestic hot water demand expectations.

I had an original heat calc done through my plumbers, they outsource it to a heating supply shop one town over. The problem was they didn't ask much as far as type of window or doors or really much for insulation. We ended up doing a rigid foam insulation on the exterior which wasn't accounted for and they didn't heat the garage and a fan coil on the upper level (which we didn't want). Basically they had 12 inch centers throughout the house and a navien 240c combi boiler (which after some research I wasn't sold on) I spoke with the veissman representative in the area and he suggested 6" inch centers through the rather than just in the bathrooms. I ended up doing 9" centers because the plumber felt 6" centers was overkill.

I am going with the combi boiler because of the direct hot water supply. I feel like we will be fine with either the 1.8 gpm or the 3.6 gpm (after the 40 gallon auxiliary tank is emptied). We don't have a ton of high flow showerhead or anything and I take pretty short showers. Both the plumbers and the heat calc engineer are suggesting I go with the more powerful boiler for some reason but I'm just not sure. The price difference is only 300 dollars so it's not a matter of cost at this point.

You say that both seem overpowered for my heating needs although really I'm only 9000 btu/hr over with the smaller boiler so I don't really know if I can get much closer. Any suggestion or advice would be much appreciated.

 

[Dana]

Hey guys, new to the forum here but seems like a great resource for this sort of stuff. I'm just finishing my first home build and have installed pex tubing throughout the house. It's approx 2600 sq Ft including the garage (which I also put tubing in and plan on heating). I am located in BC Canada withing zone 6 with a HDD of 4000-4999. I just had a heat calc done by the local heating and cooling engineer and my heat loss is coming in at 49,000 btu/hr. The engineer also states that I need a 20% back up and therefor my boiler needs to be at 59,000 btu/hr. My plan is to get a combi boiler from veissman (vitodens 222-f) but it comes in two different sizes. The 19 which has a mbh of 12 to 68, or the 35 which has a mbh ranging from 19 to 125. I know that the real killer is over sizing your boiler but I also don't want to end up undersized either. The local plumbers who will be installing aren't a ton of help here so any advice would be great. Thanks.

It's extremely unlikely that your design heat load is 59K unless you just built one of the air-leakiest/over-ventilated new houses in BC. With 2x6 framing, decent windows and decent air tightness most houses would have a heat load of under 50K @ -25C. (What is your 99% outside design temperature?) Most heat load calculation tools overestimate by a bit, and quite a lot unless all of the input parameters are aggressive (as recommended by the instructions for ACCA Manual-J,) rather than conservative (as entered by HVAC pros overly concerned about under-sizing.)
Manual-J something of the the "gold standard" in the US (even though energy modeling tools such as BeOpt are better), but most HVAC pros and even some engineers mis-use it with way too conservative assumptions, particularly about air tightness or ventilation rates. If you have a detailed copy of the load calculations you may be able to spot a pinky (if not a full thumb) on the scale.

When in doubt, go smaller. ASHRAE recommends 1.4x oversizing from 99th percentile heat load. If your actual load is ~50KBTU/hr,(more likely than not), a 1.4x multiplier would put you at ~70KBTU/hr, which makes the 19 the clear choice. If it can really thottle all the way back to 12 MBH you should be able to dial it in to where it's modulating rather than cycling, most of the time. The bigger version's 19 MBH output is still probably less than half your design load, but probably well over a third, which adds up to a lot more burn cycles, which adds wear & tear on the boiler.

 

[Timothy Graham Work]

It's extremely unlikely that your design heat load is 59K unless you just built one of the air-leakiest/over-ventilated new houses in BC. With 2x6 framing, decent windows and decent air tightness most houses would have a heat load of under 50K @ -25C. (What is your 99% outside design temperature?) Most heat load calculation tools overestimate by a bit, and quite a lot unless all of the input parameters are aggressive (as recommended by the instructions for ACCA Manual-J,) rather than conservative (as entered by HVAC pros overly concerned about under-sizing.)
Manual-J something of the the "gold standard" in the US (even though energy modeling tools such as BeOpt are better), but most HVAC pros and even some engineers mis-use it with way too conservative assumptions, particularly about air tightness or ventilation rates. If you have a detailed copy of the load calculations you may be able to spot a pinky (if not a full thumb) on the scale.

When in doubt, go smaller. ASHRAE recommends 1.4x oversizing from 99th percentile heat load. If your actual load is ~50KBTU/hr,(more likely than not), a 1.4x multiplier would put you at ~70KBTU/hr, which makes the 19 the clear choice. If it can really thottle all the way back to 12 MBH you should be able to dial it in to where it's modulating rather than cycling, most of the time. The bigger version's 19 MBH output is still probably less than half your design load, but probably well over a third, which adds up to a lot more burn cycles, which adds wear & tear on the boiler.

So the 49,000 btu/hr is calculated on the coldest day possible for the area. I think she said - 28 degrees Celsius. We might see that temp one or two days a year at most. She didn't really tell me what the 99% day calc was but I feel like my region is somewhere between - 4 and -8 on avg throughout the winter.

 

[Dana]

Somehow I'd read that as 59K for a heat load (not enough coffee, mayhaps? )

A load of 49K @ -28C (probably the 99.6 th percentile temperature bin, which isn't going to happen every year, just most years) is a credible number. (In my ramblings I was hazarding it to be somewhere in the neighborhood of 50KBTU/hr @ -25C, not too far off the mark for a WAG.)

So with the 68 MBH max boiler you're already well covered and reasonably sized (almost the ideal 1.4x oversize factor, per ASHRAE) . The 125 MBH version would be extreme overkill (unless you're anticipating a cold snap that reaches -100C? )