Design Temperature
Design Temperature
It seems like most systems are designed around maintaining comfort on the coldest day of year. This can mean that the heating system is specified for 60,000 BTU but your average BTU requirement is 12,000 BTU. The mismatch likely adds quite a bit to the price tag of the boiler or possibly necessitates a boiler instead of something more economical?
Why wouldn’t a secondary heating system that can add heat to cope with the few days of the year where the heat requirements are at their highest be a more economical design?
So instead of designing for the coldest day, design for the average winter temperature day and then add a secondary heat source say electric baseboard to deal with the days where the temperature is outside of that range.
Thanks for any thoughts!
Mike
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Replies
Mike,
These days, most quality heating appliances, including high-quality furnaces and ductless minisplits, are modulating units -- meaning that they have a variable heat output. For most of the winter, they heat your house at a lower output than the BTU/h rating on their nameplate.
First, most houses in the US have a heat load well UNDER 60,000 BTU/hr at the 99% outside design temp (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf ), even older way-sub-code houses like mine: A 2400' of 2x4 framed 1.5 story bungalow with 1500' of insulated basement. Heat load: ~35,000 BTU/hr @ +5F, the local 99th percentile temperature bin.
Typical IRC 2012 code min new construction for a 2500' house will be less than 30K. Tighter & better (but still not super-insulated) 2500' houses will often come in under 25,000 BTU/hr. If taken to the "Pretty Good House" standard, design heat loads under 15,000 BTU/hr aren't rare.
The marginal equipment cost adder to the price tag of oversizing hot air furnaces rather than being "...quite a bit..." is actually " fairly small", and it also has nearly zero impact on operating efficiency. This has led some tract-home builders to take a "one size fits all" approach to their housing developments. Being 3x oversized with a hot air furnace may have no effect on efficiency, but it has a strongly negative impact on comfort & duct noise due to much higher air volumes than necessary. A 2-stage hot air furnace that's 3x oversized isn't really much better. Keeping the oversizing factor down to the ACCA recommended 1.4x (or less) works pretty well though, even for a single-stage burner.
The marginal equipment costs for oversizing hydronic boilers is also quite low, but oversizing the radiation can be expensive. But unlike the hot air furnace case, oversizing boilers by more than 1.7x begins to cut into operating efficiency signficantly, especially (but not exclusively) non-modulating higher-mass boilers. Worse still, oversizing the boiler but NOT similarly upsizing the radiation leads to excessive equipement wear & tear, shortening the service life of the boiler due to excessive on/off cycling.
With either boiler or mini-splits, there are limits to modulation. There are very few with turn-down ratios greater than 5:1, and many that only have turn-down ratios of 3:1. Even with modulating equipment it's best to not oversize output capacity by more than 50%, which will avoid most of the comfort, efficiency, & longevity issues associated with oversizing. With boilers the max output capacity is the same across all outdoor temperatures, but with air-source heat pumps like mini-splits max capacity drops with outdoor temperatures, so it's important to figure out what it's capacity is at your 99% outside design temp to neither under or oversize it by very much. This isn't always easy to figure out, but most manufacturers will specify it's maximum capacity at a few temperatures, as well as a minimum operating temperature. The capacity is almost but not exactly linear with temperature, but drops off more rapidly when it nears or passes the recommended minimum outdoor temp.
With mini-splits that have higher turn-down ratio oversizing by 25-50% can even result in higher average efficiency, since they're more efficient at part-load than when running full blast, and oversizing modestly means they never run full-tilt. The only down-sides being the elevated minimum cubic feet per minute of the blower at it's lowest speed, and a few hundred dollars in additional up-front cost. Oversizing by more than 1.5x leads to more pronounced comfort hits. With a low turn-down ratio mini-split oversizing by more than the minimum necessary leads to a lot of cycling on/off during the shoulder seasons, leading to modestly lower efficiency, but more comfort issues due to bigger indoor temperature swings when it's mostly cycling rather than modulating.
Your balancing cost with comfort, you could have a dozen heat sources that are each 10% of the design load and activate each one as needed (with 20% reserve for breakdowns and below design temp). It would cost ungodly amounts of money but if done properly should be much more comfortable (assuming you have the space to install all of them).
But you have diminishing returns, a 50% modulation is probably sufficient for adequate comfort, 25% in steps may make things a bit better, 20% in steps may make a slight more improvement and 10% in steps may make so little improvement you won't even notice.
From a bigger-picture perspective, if everyone had undersized primary heating systems and used baseboard electric on the coldest days, the demand on the electric grid would be very peaky, leading to higher electricity costs for everyone or to utilities instituting special charges on one sort or another that would make that solution unattractive.
What type of heating system do you presently have ?