Electric boiler for whole house hydronic heat
KimCO
| Posted in General Questions on
Hi Folks. We just purchased and will be starting a remodel of a well-built, 1978, 2 story (with full basement) home. It currently has a new propane boiler with baseboard hydronic heat that works well. We would like to put in solar and switch to all things electric including an electric boiler for the heating. Our architect and plumber are concerned about the current electric boiler technology in the US. Can anyone give me some insight? We are in Colorado, natural gas is not available at our site. thanks!
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Seem to me the smart move in dollars and cents is to install an air-to-air heat pump that will cover most if your heating load and keep the propane for coldest of days cutting the propane bill by 80%.
Electric boilers are dead simple old technology with a low COP of 1. An air-to-air heat pump will have a COP of 4 meaning it will provide 4Xthe heat from the same energy.
There are a few air to water heat pumps they tend to be expensive, problem prone and poorly supported. I would not advise anyone to be the early adopter.
If the plan is to buy enough solar panels cover the heating load the simple electric boiler and lots of panels has the appeal of few moving parts to fail.
Walta
In Colorado, I think solar would work well too, lots of sunny days and snow to reflect the sun. Keeping snow off the panels would be the tough part. If you have the space, you can get a lot of panels for the price of an A2W heat pump and they should last 30+ years.
Just a quick word about terminology: there are two ways of getting heat from electricity, either running it through a resistive wire or using it to run a heat pump. Running it through a resistive wire gets one unit of heat for every unit of electrical energy used, while running it through a heat pump typically gets two to four units of heat for every unit of electricity used (the exact number depends upon the difference between indoor and outdoor temperature and the design of the heat pump).
When people talk about "electric boilers" they usually are limiting the conversation to resistive heat devices. The equivalent device that uses a heat pump is typically referred to as an "air to water heat pump." The "air" in the name comes from the fact that the heat that the heat pump produces is pulled out of the outside air, by cooling it. Since air-to-water heat pumps are more efficient than electric boilers, what you typically see is that electric boilers are the most expensive to operate, heat pumps are the cheapest and fossil fuels are in-between. Although that can vary depending upon what your local fuel costs are like.
As Walta notes, air-to-water heat pumps are a niche product in the US. Much more popular in the US are air-to-air heat pumps, which pull heat out of the outside air and use it to warm air that is blown around the house. The reason they are popular is that in the summer the heat pump can be run in reverse to provide air conditioning.
So if you want or need air conditioning, I would recommend finding out how large an air-to-air heat pump you need to provide the amount of cooling you need. Then find out how much heat that system is capable of producing in the winter. If it can't produce enough, there are several options to make up the difference:
1. Install a larger air-to-air heat pump with enough capacity to meet your heating needs.
2. Keep the propane boiler, and only use it to supplement the heat pump on the coldest days.
3. Get an electric boiler, and use it as in #2.
4. Get an air-to-water heat pump.
All this starts with an assessment of you heating and cooling needs. There are two ways to do that. One is to plug information about your climate and house construction into a computer model, that's known in the trade as a "Manual J." The other is to look at your actual usage history of fuel, that method is detailed in this article:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
You want to stay away from any installer who doesn't size the system using one of these methods. There are no rules of thumb, every house is different.
Thank you so much for your time and thoughtful replies (especially the definitions!). If we change to an air to air heat pump we will need to run ducts through the entire house, is that correct? Also, the forced air is pretty rough in Colorado where it is already arid and exacerbate allergies. I was hoping to stick with a hydronic system for those reasons.
If we leave the propane system as is for a period of time, is it a huge undertaking to change to electric air to water heat pump in the future (renovation long done) if the technology improves? We are not planning air conditioning, at 6500 ft the nights are always cool. The only possible air conditioner would be a mini split in the bedroom on the south side of the house.
It wouldn’t be much of an undertaking. Air-to-water in 2022 is capable of lower temperatures than baseboard might need in the coldest winter days, but the other 90% of the winter might be just fine. If not, you can increase the baseboard output using special high output baseboard or another type of emitter, and/or use backup for that portion of the year. One action you can take now is ensuring the baseboard is not run in series - you want the piping in parallel, like a “home run” or “two pipe” system. The contractor should know what that means. The goal is to deliver the same temperature (hopefully low, say <120) to each emitter.
You can also do resistance heat for the coldest days. My Chiltrix air-to-water heat pump has a spa heater downstream of the heat exchanger.
It kind of depends on the house whether ducting is required or whether a few minisplits can handle it. Every house is different.
Here's the thing about converting your propane system to an air-to-water system: the propane system probably runs hotter. Propane systems will typically be designed for a water temperature of about 170F. Since heat pump efficiency depends upon the temperature difference, heat pumps typically are designed to run with cooler water, probably 115F to 125F.
The reason this is important is that the amount of heat a radiator puts out is directly proportional to the water temperature. So it may be that your existing radiators are too small to work with a heat pump. Then again, it may not be. If you read the article I linked to in post #2 you'll know that the modern era of sizing heating systems using computer models is only a decade or two old, and that before then heating systems were routinely sized two to three times bigger than they needed to be. So it may well be that your radiators are oversized enough that they'll work fine.
Here's how you can do a quick check: size your system using the technique in that article. If you have baseboard radiators, go around the house and measure them with a tape measure, and add up the total length. Baseboards are rated for 550 BTU/hr at 170F. So multiply that measurement by the total length to get the capacity of your radiators. Divide your system size you calculated by the radiator capacity. At 170F your radiators are 100F above room temperature, so multiply that fraction by 100 to get how many degrees above room temperature your radiators need to be. Add that to 70F to get your water temperature. If it's less than about 125F you're a good candidate for an air-to-water heat pump.
This can all be done from the comfort of your home. The folks here can help you with the calculations.
If it turns out your radiators are too small, another approach is to look into increasing the insulation of your home to reduce the heating load. That's probably a whole different thread though.
Here's an example of a house I looked at recently:
The fuel usage history gave a heating load of 42,000 BTU/hr. The house had 144 feet of baseboard radiators; at 550 BTU/hr/ft that gives 79,200 BTU/hr at 170F. To meet the heating load we need to use 42,000/79,200= 53% of the radiator capacity at 170F, which means the water has to be 53F above room temperature, or 123F.
This house is borderline for an air-to-water heat pump conversion, we have to look deeper.
The forced air heat should not dry your air any more than any other type of heat so long as you avoid installing leaky duct work outside of the conditioned space IE attics and crawl spaces.
What dries out a house is when the warm moist air leaks out of the building and is replaced by cold dry air that becomes very dry warm air once heated.
If you can tighten up the house enough excess moisture becomes a problem.
How you chouse to heat the air does not change its moisture content.
If installing ductwork is a non-starter, would you consider ductless heads on your walls? They do make floor mounted heads.
Walta
… the right question to ask is what is the water temp needed for hydronic heat at your design day? If it is 145 or lower you can do an air to water heat pump but as mentioned those are not very mature. Using a regular electric heater to heat water for hydronic will be very expensive as cop as mentioned is 1. A well sized ashp configured as close as possible to 1:1 indoor/ outdoor units is your logical choice.
“Using a regular electric heater to heat water for hydronic will be very expensive as cop as mentioned is 1.”
Expensive yes but the math is ever changing if your electric rate is .25 a kWh or .06 a kWh changes the numbers a lot but it seems the plan is to be net zero by adding enough panel for whatever heater is selected. The real question is what is the cost of install solar in that location.
It may well cost less to install 4X the panels and a cheap simple resistance boiler than fewer panels and a expensive air to water system when you add a few service calls for a complex finnicky system.
Walta
If you want to go that route, resistance heat doesn't care if it's AC or DC or what the voltage is. You can get a modest efficiency gain by taking DC directly off of the solar panels and skipping the inverters. I know there are systems that do that.
You'll likely only gain a few percent going straight DC with today's very efficient inverters, and you complicate your system design based on varying output from the solar panels and much more restrictions on switching and overcurrent protective devices. I'm not so sure the pretty modest efficiency gains are really worth it here, especially if a DC system were to preclude using grid power at night when needed.
Bill
The taco M system is an air to water heat pump that is designed to replace boilers. You can keep the propane boiler as a backup for when the grid goes down. I replaced my old style baseboard in the basement with a low temperature version so I can use 130f water.
This will allow you to gradually move to electric while still providing some resiliency until the grid is upgraded.
Wow everyone, this input is very much appreciated. A lot of it is over my head (my doctrate is NOT in this field!) but I will use it all in the discussion with our plumbing and solar contractors. Thank you....