Heating systems for new home

[Slickcraft]

Originally Posted by FlyingScot

I have never had mini splits in the same home as oil, so unable to do a direct comparison, but I am confused by your math or maybe by terminology. If mini splits are, say, 30% (or more) more efficient than oil, and they are operating at 85% efficiency at -13, aren't they still more efficient than oil at that temp? Thanks

The math is straightforward. At minus 15 deg 85% efficient means 100 watts in gets only 85 watts of heat out. Over 0 deg temp the efficiency is over 100% so the heat pump puts out more watts than what is required to run the unit.

The above has nothing to do with comparison with oil heat. That said, the advantage over oil is temp dependent and numbers like 30% are on average. At 10 below zero best to run the oil burner.

Alan

[FlyingScot]

Originally Posted by Slickcraft

The math is straightforward. At minus 15 deg 85% efficient means 100 watts in gets only 85 watts of heat out. Over 0 deg temp the efficiency is over 100% so the heat pump puts out more watts than what is required to run the unit.

The above has nothing to do with comparison with oil heat. That said, the advantage over oil is temp dependent and numbers like 30% are on average. At 10 below zero best to run the oil burner.

Alan

Yes, this is why I am confused. I understand that a minisplit on warmer days is better than a minisplit on colder days. But the question is whether a minisplit on colder days is better than oil on colder days. If a minisplit is 30% better than oil in general, and oil is assumed to be always at 100% of its own efficiency, then doesn't the minisplit need to fall to 70% efficiency before oil is equal, and 69% before oil is more efficient? If all of this is the case, then the minisplit on a -13 day at 85% is still the best choice

[ishoot308]

Originally Posted by FlyingScot

Yes, this is why I am confused. I understand that a minisplit on warmer days is better than a minisplit on colder days. But the question is whether a minisplit on colder days is better than oil on colder days. If a minisplit is 30% better than oil in general, and oil is assumed to be always at 100% of its own efficiency, then doesn't the minisplit need to fall to 70% efficiency before oil is equal, and 69% before oil is more efficient? If all of this is the case, then the minisplit on a -13 day at 85% is still the best choice

Isn't the efficiency of an oil burner on average around 85%?? I realize some of the newer ones are better but for the most part 85% is pretty standard is it not?? Which would mean the mini split at -5 would be far more efficient would it not??

Dan

[John Mercier]

Originally Posted by ishoot308

Isn't the efficiency of an oil burner on average around 85%?? I realize some of the newer ones are better but for the most part 85% is pretty standard is it not?? Which would mean the mini split at -5 would be far more efficient would it not??

Dan

If it put out enough BTUs at the lower efficiency.

[DickR]

Originally Posted by Slickcraft

The math is straightforward. At minus 15 deg 85% efficient means 100 watts in gets only 85 watts of heat out. Over 0 deg temp the efficiency is over 100% so the heat pump puts out more watts than what is required to run the unit....

I think the term "efficiency" is being misused here (for a minisplit). The performance of a heat pump in heating mode generally is indicated by "Coefficient of Performance" (COP), that being the ratio of energy delivered inside (as heat) to energy input as electrical power (3412 BTU/hr per KW). A reasonable seasonal averaged COP might be around 3, but this will be substantially higher when the outside temerature is say in the 40s or 50s. The COP also drops as outside temperature drops. Think of it as being like pumping water uphill; for a given amount of pump power, you get a lower flow as you pump farther uphill.

That 85% number for the minisplit likely indicates how much of full rated capacity (BTU/hr) is being delivered from a very cold outside temperature. In general, except for some losses along the path, a heat pump delivers as heat all of the electrical energy input to the compressor plus whatever is absorbed from the source (outside air) by the evaporation of refrigerant on the cold (low pressure) side. Thus the COP will always be above 1.0 and substantially so.

It should be noted that heat pumps installed in heating climates generally are sized according to the expected heating load at some design minimum heat loss rate, which can be calculated sufficiently well for any house. On the other hand, most fired heat systems installed over the years typically are grossly oversized relative to expected demand, often by a factor of 2 to 4. Such a system has a lot of extra capacity, which leads to short cycling in normal use but which does let a cold house recover rapidly after a power outage or other period of deep setback. In contrast, a properly sized heat pump does not have that huge reserve of overcapacity for rapid recovery.

[Biggd]

My wood stove has been blasting out heat for two straight days now and I'm just able to keep the house at 68 degrees tonight, and it's a well insulated home. I have oil heat and I don't think that would keep up at these temperatures. I doubt a Mini Split would keep up tonight, it's -12 right now in Meredith.