New Propane Boiler Options

There are 3 separate BTU numbers on each boiler. Prior to 1968 there were only 2 but when the government got involved with the department of energy they needed to have an equal footing to measure furnaces and boilers for those yellow Energy Guide stickers on each heating appliance to make it easy to compare the different systems. But I digress

Back to the three different BTU numbers.

1. Gross Input aka Input aka GPH on oil fired equipment = How much energy goes in
2. Output or DOE output is how much energy gets put into the home (maybe). Or better described as what is left over after what is lost out the vent pipe.
3. NET output aka I=B=R NET and now known as DOE NET. This is the number that you are supposed to use when you know the actual heat loss of the building. It is actually a 15% lower number that takes into piping and pickup consideration on a water boiler. (it is 30% on steam boilers)

So when you know that you need 46,000 BTU per hour on a design day (that is what we consider the average coldest day of the year for an area) Then you use the NET number which is much smaller that the Input that the boiler usually is sized by.

Here are three different rating tables from three different boilers

You can clearly see before there was a Department of Energy in the 1950s boiler rating were dome by the American Gas Association (AGA) and the Institute for Boiler Ratings (I=B=R)  and all the boiler manufacturers used these ratings to rate their boilers.  The Weil McLain model D boiler that has an I=B=R Gross output of 90,000 was about 80% lower than the Input of the boiler at 112,000  

When the 1970s oil boiler was rated they used GPH for the I=B=R input and Gross output was changed from I=B=R to DOE output. That would make that oil burner about 84% efficient since one gallon of fuel oil is equal to about 140,000 BTU of energy.  At this point there are no yellow AFUE stickers yet but the Department of Energy has been formed to keep the cost of oil from getting out of hand.

In the current gas boiler you can see that the numbers are still the same but they all are under the direction of the DOE.  

• Input
• DOE heating capacity or output, and
• DOE NET

Still the same numbers as in the 1950s but the names have been changed to protect the innocent. So look at the NET number when the boiler says 70,000 it is really going to do a good job all the way up to 56,000 BTU heat loss on a design temperature day.  I believe you will be just fine with the GV90+3

do we have enough information about the life of the refrigeration system in hp water heaters to work out if the tco is less than an indirect or gas direct fired water heater? with an indirect the important thing for efficiency is to make the tank large enough that the boiler only fires for larger loads.

@mattmia2 That is a fair point. HPWH manufacturers have not been doing themselves a favor as recent generation seems to be less reliable. LG is out with some newer stuff and their engineering tends to be better.

A larger indirect tends to make the install BOM cost even worse as they tend to be significantly more expensive than smaller ones. A larger tank that can stratify with a dual coils can improve efficiency but again, but the cost math won't work out.

@Roger I've been running an indirect with a plate HX fed before by a modcon before (now an air to water heat pump). I have not done official measurements on efficiency but have had instrumentation on it to see temperatures. The issue is that outside of very large draws like a tub-fill, the RWT is to high to get much condensation. Running to makeup for standby losses is even worse. Thermal purge only works during the heating season and even up here in the great white north, that is about half the year.

If a bit of cold air is creating mold issues in the house, the house has major building envelope issues. Not saying that those are not out there, but those houses will have mold issues even without a HPWH which should be fixed.

@Kaos , the controls have to be designed to allow stratification of the tank so that continuous cold water make up feeds the plate heat exchanger. The entire system has to be designed properly or you end up with lower performance and the boiler finishes hot which wastes energy (as it does with a conventional indirect tank and combo/instantaneous systems).

Regarding heat pump water heaters, the article indicates this is a common problem that has to be solved before widespread use of heat pump water heaters. This makes sense because the rooms (and outside walls) are chilled as part of normal operation. This introduces a new mold and condensation problem in the outside walls that did not exist before the heat pump water heater was installed. The authors identified the following issues:

1. Inadequate heating capacity compared to advertised specifications.
2. Real-world efficiency significantly lower than rated efficiency.
3. Moisture-related damage and mold growth.

And believe the issues generally track back to two primary causes:

1. Insufficient access to the thermal resources inside the building.
2. Improper handling of cold exhaust air.

Roger

You're welcome, @FirstTimeHomeOwner .

The Accel CS is a modulating condensing boiler and can modulate down to 18,000 BTU/hr.

We recommend that you zone comfort areas, so your idea of separating living areas and bedrooms is a good one, but you could leave your zones as they are if the heat uniformly. Just do not zone each bedroom separately, which would be micro zones (an exception may be a separate zone for a master bedroom with a master bath).

I am not sure if your quote includes the domestic hot water system, your heating pro would need to clarify that for you. Each Accel CS does come with controls so it is ready to add an indirect tank or plate heat exchanger and storage tank with thermal purge (which is much more efficient as the boiler finishes cold each hot water cycle vs finishing hot with an indirect tank).

Best,

Roger