Oversize boiler or juuust big enough, based on past utility usage, and other questions

ACK

Oversize boiler or juuust big enough, based on past utility usage, and other questions

TLDR: Is it better to get a boiler that’s just big enough or go far oversized?

We’re considering replacing our ten year old and frequently struggling Bosch Greenstar combi boiler, 151. It continues to have E9 error codes for two winters running, after 3+ replacement parts and two companies looking at it. We bought our home, a 1906 duplex in southern Maine, two years ago.

The error code comes on when we turn the heat up from 50° to 68° in the now vacant but hopefully soon rented second unit on our third floor. The first floor and zone is all original cast iron. The second floor and zone are wall hung radiators. The third floor is an insulated, finished attic with baseboard heat. The boiler is happy to heat just the third floor if I turn off all other zones, and happy to heat the first and second floor with no issue, but errors start not long after zone 3 and another zone have to heat at the same time. I’ve been assuming that because the third zone can heat on its own that it probably isn’t an issue with that zone pump but maybe I’m wrong about that?

Anyhow, in our market it’s hard to find anyone eager to work on Bosches. People I've spoken to prefer Viessmann or Lochinvar, so if we go to replace we will likely go that direction. Viessmann’s combis are sized at 120k BTU & 199k BTU. Using our past utility data and the calculator here https://www.borstengineeringconstruction.com/Existing_Building_Energy_Usage_Analysis_Calculator.html as well as the instructions here https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler our BTU needs during our largest usage on record come to 79,495 BTU/hr.  (426 therms January 2022, 1277 heating degree days, 2° design temp, 89% efficiency.) Given that we had been keeping the third floor at 50° I did a heat loss calculation just for that space and a 15° Delta T, adding an additional 5,470 BTU/hr. The final total need comes to 85k BTU.

Given all this, would the Viessmann Vitodens 100-W B1KE-120, which has a CSA/DOE rating of 113k BTU be adequate?  I mean, clearly on paper it seems to be just about right, but should we be oversizing more? I’ve heard 1.4x as the maximum amount to oversize according to ASHRAE, which for us would mean 119k BTU. But is being within that window enough? If we went with the B1KE-199, the CSA/DOE jumps tu 187,000 BTU/hr, which feels very oversized. 

For anyone wanting a traditional heat loss calculation, I’ve gotten 96,489 BTU/hr using a Weil McLain calculator and 99,479 BTU/hr using USboiler.net, and 97,755 BTU/hr on my own following instructions. We definitely have a lot of low hanging fruit as far as tightening up the house to reduce air changes per hour, and will be making those updates over time. 

Final question: one person we spoke with thought it likely that our old sludgy cast iron radiators may be causing the boiler to have issues. Should we be doing Sentinel x400 or similar before installing the new boiler? https://www.sentinelprotects.com/us/products/central-heating-systems/sentinel-x400-system-restorer 

Sorry for this very long winded post. Any help is GREATLY appreciated.

Hot_water_fan

A combi needs to be sized to the larger of DHW or heat loss. What’s the DHW needs?

Big Ed_4

When you cleaned the heat exchanger did you replace the plate gasket ? No one touched it , replace the gasket and tighten the two bolts . There are also two spacers under the bolts and tucks into the gasket channel . You need them ...

hot_rod

Id certainly work on tightening up the home to get the load lower. Then the 151 should be plenty

pipe a new combi with flush valves to delime the plate hx occasionally

Yes to cleaning and a power flush on the old radiators, add a dirt mag separator with a new boiler

ACK

Hot_water_fan said:

A combi needs to be sized to the larger of DHW or heat loss. What’s the DHW needs?

@Hot_water_fan, thank you for pointing this out. I don't know why I thought DHW would be less of a demand. I went down a small rabbit hole considering this. I think we'd see sometimes two outlets being used at once, rarely two showers at once. I tested our current Bosch Greenstar 151 set up with two showers running full blast and it kept up with pressure, temperature no problem. Specs of the Bosch say that model has a nominal DHW quantity at 140°F of 3.61gpm. Of the Veissman Vitoden 100Ws we're considering, their 120 combi says that the max flow through their heat exchanger is 3.7gpm. Not sure if these two measures are comparable but if so we'd be fine. I have a call out to Viessmann support to see if they can shed any light on this. My gut tells me we'd be oversized with the 199, but that is what the company were working with initially suggested (without doing any calculations). So I wanted to do my homework. 

ACK

@Big Ed_4 I don't believe anyone touched the plate gasket. Thanks for the info. Do you think that could be a factor in an E9 error? ("Heat exchanger safety high limit or flue gas temperature limiter has tripped.") 

Hot_water_fan

To size, you’ll need to know the outgoing minus incoming temp for DHW. The calculation is boiler output / (60* 8.34 * (outgoing minus incoming temp)) = GPM possible. If that’s above your needs, you’re good. Now - that’s for showers. If you have a large load, the combi will go cold. Think bath or clothes washer 

ACK

hot_rod said:

pipe a new combi with flush valves to delime the plate hx occasionally

Yes to cleaning and a power flush on the old radiators, add a dirt mag separator with a new boiler

@hot_rod thanks for confirming the cleaning of the old radiators. I'll ask to see if the guys working on this can do a power flush as well. We have the dirt mag separator planned with the new boiler as well. In your suggestion I'll learn more about flush valves. We have extremely soft water where I am but I'm sure it's worth checking out. Thanks again! 

hot_rod

The hot water output of a tankless or combi has much to do with the temperature rise.

So assume 50° incoming water, raise it to 120F

The formula looks like this for a desired 2 gpm flow

500 (flow) ∆T

500 X 2 X (120-50) = 70,000 btu required.


Input your incoming water temperatures, in some areas water can drop into the 35° range

500 X 2 X (120-35) = 85,000 btu required

So a 120,000 or larger covers most residential applications. Unless you have an 8 gpm body spray shower

PRR

ACK said:

....I don't know why I thought DHW would be less of a demand. ...

FWIW: in Downeast Maine, my on-demand water heater is twice the BTU of my hot air furnace. Yes the air is cold, but water in Maine ground is colder (not in degrees but in heat-suckage).
Yeah, back in New Jersey in a UN-insulated 1830s house the water-heat was a trivial load on a massive steam-boiler, but opposite here.
My recent experience with a "just right" hot air and A/C system is that I coulda used the next size up on both burner and chiller. It works but has to run up to 23 hours/day and won't recover quickly after a deep set-back. Bigger parts would be more idle/standby losses but I can't see it would be unaffordable.
BTW, IMHO: get the place rented ASAP. I think the recent panic in housing is about to turn cold.

hot_rod

I have filled and flushed this old radiator a dozen times, while doing some flow testing. I don’t think it’s possible to get and keep them clean. So be sure to have an excellent filtration device on the system to protect the boiler, pumps and components

ACK

PRR said:

My recent experience with a "just right" hot air and A/C system is that I coulda used the next size up on both burner and chiller. It works but has to run up to 23 hours/day and won't recover quickly after a deep set-back. Bigger parts would be more idle/standby losses but I can't see it would be unaffordable. BTW, IMHO: get the place rented ASAP. I think the recent panic in housing is about to turn cold.

Thanks for sharing your experience! I'll keep all this on mind. For everyone in Maine I hope housing gets easier to come by.