Basic question about the "backup" heat source

jb9

Hello,

I have a question about hydronic designers consider the “backup” heat source. For my own house, I am planning on using WB-S and considering using an air-water heat pump since I will probably be using electricity. The house design isn’t large by most standards (~1500 sq ft), and for a time, I was actually considering installing a masonry stove (as the primary heat source) but the overall large footprint of the stove has me reconsidering it. When considering the backup source, am I to envision a scenario that the in-floor radiant fails? Or that the boiler fails? I don’t particularly want to have a wood stove in my home (it will be a SIPs enclosed space) and I don’t necessarily want to shoehorn a bulky masonry stove into 2-story home (768 sq ft per floor). Sorry for the basic question, but I am interested to hear how designers approach the “backup” source of heat. Does one backup the boiler or simply have a 2nd more basic source?

Thanks in advance. Again, I am learning more and more from all the contributors here.

MikeSpeed6030

Most residences do not have a full-fledged back-up heat source. Some may have a fireplace. If the boiler craps out, and you still have electrical power, you can plug in portable heaters.

Brewbeer

Some wood stoves have the ability to use outside air that is piped directly to the stove. If you have gas, you could use a gas fired stove as backup. I have a gas fireplace insert that can be used as a backup, but would prefer to have an electric power source to run the boiler and circulators (generator or inverter & batteries).

Fred

@jb9 , just for my clarity, are you talking about a suplimental heat source for those times that the primary may not be able to keep up or are you talking a true back-up, in the event the primary actually fails or some combination of the two?

jb9

Good question @Fred. The house currently has a 22,000 BTU's/hr design load and I am hoping to just use electricity to drive my boiler and the WB. Although I am looking forward to the radiant, the idea that a prolonged failure could lead to a bursting section of pex-al-pex is somewhat scary. So I suppose I ought to consider a true backup. @MikeSpeed6030 pointed out that most homes don't have a full fledged backup source. I actually like @Brewbeer's suggestion that a way to store power (battery) or a generator could also be a viable solution. I guess I am just trying to think about the backup problem and how to approach it. So to answer your question, I would say I am perhaps considering an event that is some combination of a primary failure and a true back-up. Good question though... you guys are definitely helping me think about my options. Hope my design load numbers help folks envision the envelope. Happy to keep answering questions too.

Jamie Hall

I would think, since you are planning to use electricity apparently to heat the water -- and you need a fair amount of it -- that a backup, possibly automatic changeover (though that's pricey) generator would be the way to go if you want a real backup without taking a lot of space in the building (and I can see that you might want that!). The advantage of the automatic changeover would be that it would function even if you or some other responsible person weren't there.

Do you have natural gas? If so, that would be the fuel of choice for the generator. Otherwise, LP would be a good choice.

Size it to handle the building load.

Bart Vaio

You are right, you will need backup heat in some climates for an air to water heat pump. The daikin altherma has a built in resistance heat backup or can be set up for bi-valent system in concert with an alternate heat source such as a gas boiler which is more cost effective to operate than resistance heat.

Gordy

This thread is getting confused.
It seems the OP is heating using electric. The concern is losing power of course. As @Jamie Hall suggested the real solution would be a back up generator properly installed, and sized.

Things to consider are frequency of outages, and duration. SIP construction is low load as seen by the heatloss. So I would imagine outages of shorter duration the structure could carry for quite a while before actual freezing temps occur inside.

Evidently no source of NG, or LP an option?

JUGHNE

22,000 BTUh is only about 7 KW of electric. Is that right? So a full sized generator is not that big. Then this brings some form of fossil fuel into the picture.

But what will back-up the back-up?

I always thought that with burning wood I had a back up for that, it was called the furniture and next the books etc. That was the standard procedure for the one room school house on the prairie when students were snowed in for several days. But I digress...

In your case you might consider glycol antifreeze for your infloor heating. If your thermal envelope is as good as it sounds and you have enough mass in the floor, it might be 2 days before you even notice your heating stopped working. Then a few days before it became really uncomfortable in the house. You would need to protect the domestic plumbing though. One would assume it all within the warm side of the envelope and preferably towards the center of the structure.
I have a pretty tight house with full basement and passive solar gain, I believe it would not drop to freezing inside the structure for a long time.

If you have to have gas for the generator then you could add a gas heater/fireplace with an exterior bump out and not lose floor space.

How are you going to heat domestic (hot) water?

jb9

Good question @Gordy. I have a small lot and am hoping to avoid having a large propane tank. My approach is that I will use electricity and in the next 5-10 years, install some solar (photovoltaic) panels and have some kind of battery storage as the household technology progresses. To answer the question about DHW, I am thinking I will just have a separate tank for DHW (2 1/2 bathrooms). I like the glycol suggestion too but I won't derail my own thread here. On a high level, I am assuming electric "boilers" are of two types, electric resistance and heat pumps. I sometimes get confused when I see folks talk about products by brand name and I don't see what type of a boiler it is. I think the overall suggestion of a backup power source/generator is a good one coupled with a well-designed and well-maintained system. Does anyone think that having the DHW heat source be capable of aiding the radiant system (in a failover mode) sound like a good idea as another possibility?

Jamie Hall

Couple of thoughts here.

First, regarding domestic hot water -- if you are really planning to try to go off grid with your electricity, you will want to get your electric demand as low as possible. There are hybrid electric water heaters (heat pump designs) which, although pricey, use less electricity overall than straight resistance designs. How "green" they are, overall, compared with oil or LP is very much debatable, but that's not the point when one is trying to go to on-site solar photovoltaics.

My own opinion is that I would avoid the use of a device designed for domestic hot water as a source for radiant heating. The demands are quite different, and purpose built devices usually work better.

I wish you good fortune on trying to go off grid with the PV; in my view it will be many years before the cost of adequate storage (batteries) comes down to the point where it is economically beneficial to do so.

jb9

Thanks for the suggestion. Just to clarify, I do not intend to be off-the-grid, merely capture some solar at some point as the technology improves. I will be on grid for sure. The site is in a small growing progressive town.

Brewbeer

You might consider a small propane tank. My first house, which had a heating load similar to yours, used a 120 gallon up-right tank to provide heat, hot water, and fuel for a gas stove. A 120 gallon tank is a little less than 5 feet tall and 2.5 feet in diameter, and is easily hidden behind a bush. Since your primary heat sources is the heat pump, you wouldn't use much propane for space heating. You could use the propane to fire a boiler that would provide hot water, and the boiler could also serve to supplement the radiant space heating when it gets too cold for the heat pump to run efficiently.

jb9

Thanks @Brewbeer. I may actually have a small 20 gallon propane tank for the stove, but I haven't decided on that yet. I will definitely heed the warning above and not combine DHW with the needs of the radiant system. Are there any other systems besides resistance and hybrid (heat pumps) that are driven by electricity?