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Best Options for Retrofit of Heating System?

We have a oil burner/hot water radiator heating system in a 1500 sq ft house that is approximately 60 years old. This system also provides the hot water for the house. Furnace is probably 15 years old and appears to be ready to fail. Also have AC and duct work for first story which is probably at least 15 years old as well. Rarely use the basement, but it does have one radiator for the hot water system.

We would like to use a state loan program to replace at least the heating and hot water systems (capped at $15K). I have an energy audit scheduled for a couple of weeks from now which I am sure will focus on the need to seal and insulate. We definitely want to do this as well, but need to replace the heating system before it dies. Any thoughts on best retrofit options?

Asked by Jeff Lucas
Posted Tue, 11/13/2012 - 11:03

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3 Answers

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Jeff,
First of all, if you are planning to perform air sealing work and insulation improvements, that work should be performed before you replace the heating system. Otherwise you will needlessly invest in an oversized heating system.

Once you've done your envelope retrofits, there are many options open. If you want to have air conditioning on every floor, you may want to remove your hydronic heating system and replace it with a forced-air furnace and new ductwork.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 11/13/2012 - 11:24

2.
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Jeff,

Where is the house located? Do you have gas service available (I'm assuming not, if you're on oil), or do you want to stay with oil? How do you feel about electric heat pumps? There's lots of info that goes into this decision, and many ways to go about it.

One thing I'd suggest thinking about is separating the hot water from the heating system. I've seen & heard recently that such system are very inefficient in the summer when the building doesn't need heat, and the boiler is firing only for hot water. A basic electric tank may actually be better, depending on your usage - and electricity pricing.

I'll second Martin's comment that air sealing and insulating really ought to come first - then the heating system can be smaller/simpler. But I'm not a fan of forced air, there are other ways to get AC and better ways to heat (you're already set up for hydronic), depending on your climate, building, comfort preferences...etc.

Answered by Cramer Silkworth, PE
Posted Tue, 11/13/2012 - 14:34

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Don't just yard out a high-mass radiator system and replace it with a hot-air furnace willy-nilly. If the heat load at the 99% outside design temp is low enough after insulating & air sealing that the radiator system has sufficient surface area to deliver the heat at the 99% outside design temp with water at domestic hot water temperatures or low your options get a lot broader. The replacement heating system is best targeted for the "after" picture on insulation/weatherization, which can be dramatically lower BTU requirements than the "before" picture, since oversizing can lead to both lower comfort and lower efficiency.

Climate and location count too, especially when considering air-source heat pump options (forced air or hydronic.)

Un-insulated basements represent a much larger heat load than most people think, and can easily be 25% of the total if the rest of the house is reasonably well insulated, even if you're not specifically heating it to the same temp as the rest of the house. The above-grade portion of a poured concrete foundation has a U-factor of about 1, and the band joist is only somewhat lower than that - call it an average U-factor of ~0.8 from grade to the underside of the sub floor. In a 1500' house you probably have about 160' of perimeter. with say 2' of exposed foundation + band joist, for 320 square feet of U 0.8 wall. In a 55F basement when it's +25F outside that's a direct heat loss of (55-25) x 0.8 x 320= 7680 BTU/hr. When it's 0F outside that's 14,080 BTU/hr. And that's not counting the air leakage. But if you insulate the basement to R15 (say 1" of foam between a batt-insulated studwall and the foundation) those losses are cut by more than 90%, to nearly inconsequential levels.

A typical decently insulated 1500' timber framed house circa 1950 with OK double panes (or tight storm windows over single-panes) and not too much glass area will usually have a 0F heat load of about 30,000-35,000BTU/hr, with an uninsulated foundation, but should come in well under 25,000 (or even under 20,000) BTU/hr with the basement insulated.

If the radiator system was designed for an uninsulated 1950 house with single pane windows the odds are pretty good that it can deliver the design-condition load with 120-130F heating system water, at which point condensing fossil burners can run efficiently. Even hydronic air source heat pumps can run pretty efficiently (down to outside temps of about -4F/-20C.)

If your 99% design temp is 15F or above better class air source heat pumps (particularly ductless mini-splits) can heat at about 1/3 or less the cost of heating with better class oil boilers. Got a ZIP code? (For weather and design temp estimation.)

And how much oil have you been burning per year with the beastie-boiler? (For estimating the "before" picture heat load, using the boiler as the measuring instrument.)

Answered by Dana Dorsett
Posted Tue, 11/13/2012 - 16:41

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