air to water conversion
I have a 5000Sf wood frame house – two stories and finished partially above grade basement that was built in 1995. Nantucket Mass – Zone 5A. The heating system is oil – hot water with Peerless cast iron boiler. We use about 1500 gal of oil per year for heat and hot water (80 gal tank). The system is divided into 6 heating zones:
3 bedrooms with separate thermostats, 1 basement zone, 1 zone for 30 x 40 LR/DR combination, 1 zone for 1st floor hall way that is circulating water in floor under stone floor and 1 zone for a small sunroom with circulating water under brick floor. The house is comfortable even in coldest days.
Boiler says: 20,800 BTU/HR rating; burner capacity 2.00 gal/hour
The question is: Is is feasible to convert to an air to water heat pump system to replace boiler?
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Replies
I think you dropped a zero on your boiler capacity, 2 gallons per hour sounds more like 200K BTU/hr.
The answer is maybe. Air-to-water heat pumps exist. Whether they can be dropped into an oil-fired system depends on the system. First, read this article to learn how to estimate the size of a replacement system based on the fuel usage of the current system:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
As Dana notes in the article, the typical boiler is oversized by a factor of three. That may work in your favor.
The biggest issue is that oil-fired boilers typically operate around 180F and heat pumps have trouble going above about 125F. The output of a radiator is determined by the heat of the water going through it, so if your radiators are appropriately sized for 180F water they're not going to be able to produce enough heat to keep you warm at 125F. The heat output is proportional to the difference in water temperature and room temperature, so the output at 180F is about twice what it is at 125F. If your radiators are actually oversized by a factor of two, that works too.
If not, you need to either add more heating capacity, or reduce the heating load, or both. Reducing the heating load by adding insulation is always a good idea. If you're thinking of adding heating capacity, one thing to think about is whether you want to add air conditioning. Your climate needs much less air conditioning than heating, but adding a heat pump that can do both may be enough extra heating to close the gap.
Excellent summary. Another option is keeping the oil boiler and installing the heat pump as a supplement. Gets you AC and might end up eliminating the majority of oil, which is a win economically and environmentally.
Caleffi and John Siegenthaler have done a number of good YouTube presentations on Air to water heat pumps and how retrofit older hot water installs. Good source for making an informed decision
Thank you all for your help.
I will read Califfi and Siegenthaler.
I did the math DC contrarian suggested and came up with 148,915 BTU/hour for HDD 60 and 133,931 for HDD 65. But not sure what that tells me about heat pump conversion. I think that tells me my existing boiler is about 2x But I am a lawyer not an engineer!!
I got different numbers: closer to 68kbtu without subtracting out domestic hot water. What this means is that you have a better shot of finding a heat pump that fits, as they usually top out around 60kbtu. You can install several in parallel, but one + keeping the existing boiler would be much more cost effective. Do you know how much radiation you have? The in-floor heating will work great with a heat pump, but the other radiation will need a closer look. It's explained well in the Idronics journals.
By the way, your attachments didn't load for me, unclear if others can see them.
This sounds more realistic, and inline with the observation that boilers tend to be oversized by a factor of three.
One of the differences between heat pumps and oil burners is oil burners scale much better. An oil burner that produces 200k BTU doesn't cost appreciably more than one that produces 20K. For heat pumps it's pretty linear, 200k is ten times as much as 20k. This is why heat pumps are really popular in houses with modest loads, like in the 20-40K range. If your house is really in the 130-140K range that's a big lift for a heat pump. I'd be thinking maybe heat pumps to meet half of your needs and keep the oil burner for the other half.
The next step would be to inventory your radiators and try to figure out their capacity. If they are baseboard radiators it's pretty simple, they have a constant output per foot, you just measure the length. If they are other types it's more complicated, you have to ID the manufacturer and find the manufacturer's rating. There are guidebooks that help with that.
Thank you. I will redo math and measure cast iron baseboard.
I took actual usage for two weeks in MidFebruary for 2021 and 2022 and used the larger number.
Baseboard cast iron radiator
That's a classic. Burnham Baseray.
It's rated at 550 BTU per foot at 180F. Measure how many feet you have.
Typically an installation like that also uses the boiler for domestic hot water. So your oil usage will also include hot water production, so your actual heating load is somewhat smaller than what the oil usage indicates. But it also means you have to figure out a way to produce hot water if the boiler is replaced.
John Siegenthaler is scheduled to be our guest on The BS+Beer Show on Nov. 3, for a topic of air-to-water heating strategies. You might want to join that discussion and ask questions in the chat box.