Combined furnace and water heater...is this any good?

Heating space with hot water/steam is a very efficient solution, provided it is done well. Well designed hot water systems have less heat loss system wide than a comparable forced air system.

As to whether or not the system you cite is good or bad, I have no idea. If you are building from scratch, consider getting very good, very competent local advice before you buy/build anything. For example, a hot water system may be a great idea in one climate, but if you need A/C it might be better to do a forced air system for example. Also a combination system that is very efficient, may not be so efficient in the summer when the space heating requirement is nil. Some of the better hot water systems I have seen are demand water heater coupled to a radiant heat system.

Good luck.

Icarus

 

isn't this just a domestic hot water heater? cold water comes in and hot water goes out. you need a boiler to recirculate the water for radiators or an air handler. you cannot combine that with domestic (potable) water unless you use a heat exchanger. i think what you want is a high efficiency boiler and seperate domestic water heater that runs off of it.

 

This seems eerily familiar to what my condo had for it's forced air heat. The hot water heater also provided the heat through an exchanger or some sort and the air was blown over it to heat it. The problem I had (and this was 5 years ago, with a 10 yr old unit) is that when it was heating the house and pulling hot water for successive showers, it became warm water and not hot water. I'm sure they've come a long way since then, and I also have no idea how well insulated the hot water pipes were. It was also a propane unit vs NG. All told, it worked pretty well, I just adjusted the programmable thermostat to accommodate. I also recommend speaking with a professional about this as I have no clue on technical information.

 

it probably just wasn't sized properly for the amount of showers you had and they usually are set to prioritize domestic hot water over heat.

 

Subscribed. I have a 12-year old water heater that's due for replacement and an 80% efficient furnace that's getting long in the tooth. We never used the AC in the house, so it's not a big deal.

 

If your domestic hot water needs are small, you might want to consider a smaller on-demand unit, which eliminates the inefficiency of having to keep the water hot all the time.

Hydronic heating systems are great. The radiators tend to stay warm most of the time, and they give off a fair bit of heat. But the system you're looking at appears to use a central radiator to heat air, which is then circulated through your existing ducts. I'm not sure the 'whole system' efficiency would be as high as stated for heating the water, but it's bound to be better than running two separate systems, and hopefully cheaper to install.

 

I agree with HYO.

I would consider a good demand gas water heater (ideally use solar for preheat unless you use hot water recovery A/C) for DHW, and a dedicated boiler for your radiant. That said, you might look for a plumber hvac guy who is versed in the technology. You are making a big investment in your house, and saving energy going forward is a big deal.

I don't know where you are located, but I assume someone has done some calcs as to the proper size BTU rating hvac system for your house. When you know that (and post it here) it might be easier to give a better idea of what might be a better solution.

Icarus

 

If the specs on this are to be trusted, and if I'm reading them correctly, then using a standard on-demand gas water heater probably wouldn't save energy for the water heating.

Here, the unit is claiming 96% efficiency and 1% standby losses.

That 1% standby loss seems seems plausible to me. On a conventional gas water heater, most of the standby loss is due to the uninsulated flue running through the center of the tank. Here, they eliminate the flue.

So, no pilot light loss, 1% standby loss, 96% efficient -- that's hard to beat.

A standard (non-condensing) tankless heater will run maybe 80-something percent efficient. And the condensing tankless units advertise the same 96% as the Polaris.

E.g.,

Tankless Water Heater

So you'd have to go all the way to condensing tankless to be able to beat this. And then, you'd beat it by a percent. And at that point, the cost of the alternative is going to be in the neighborhood of the cost of this unit.

Of more concern to me, when I Googled Polaris water heater, one of the "auto type" options that popped up was Polaris water heater problems. Hard to say from what you read but it looked like a lot of unhappy customers. And the parts prices that they mentioned were kind of breathtaking. Given that, the idea of running this unit really hard, to make it serve as a furnace, I wouldn't find that appealing.

OTOH, not having to buy a furnace would pay for some pretty substantial repairs.

 

I would skip it. There should be information in your area about cost reliability and efficiencies of Heating systems. Trying to make your hot water system heat your house is not straight forward. Good hot water systems are very straightforward, while heating systems are often a regional choice and depend much on the existing unit in the home. Just because natural gas is likely your choice for both does not mean it will be the best system for your house.

From a pease of mind point of view, I'd want my new heating unit to go in and be pretty bullet proof. I doubt you will get a big enough price break up front to make you feel good about problems in the future.

 

We had a similar system when we lived outside of Boston and it worked very well. Ours was a forced hot water furnace that also handled hot water - the furnace heated it on its own zone and then stored it in a tank next to the furnace. Our town did not have gas lines so heating choices were oil, bottled gas and electric so that's why we used oil. All in all I was very happy with the system and would not hesitate to get something similar in the future.

 

That's my situation now in regards to options. Unfortunately, there aren't many options in terms of efficiency and reliability when you're in that position, esp if you don't want another tank. I have an oil furnace, heat pump, and pellet stove for heating (pellet gets used most, as it's "renewable" energy) and the heat pump will be used over oil depending on the costs (I have a PV array on the roof so that comes into play some) and temps outside.

I'm looking at an electric HW heater, that combines a heat pump and electric elements. From what I've read, it's supposed to be about 60% more efficient than a standard electric water heater. I'm debating whether I want to try to sneak it in using the tax credit, or wait a year and see if they start to drop in price.

 

I looked at them, too, but also decided they were not suitable, mostly because we would have needed about three of them. One would not have been up to the simultaneous tasks of showers, dishwashing, laundry, and heating a large older home.

We went with an over 90% efficient boiler for both domestic and hydronic. The biggest efficiency difference is probably the pumps - the system we replaced was originally coal-fired (later converted to natural gas) and relied on the heat to circulate itself. The kitchen had a wood stove with a water jacket and a small tank in the closet. The original home owners had to chop wood before doing the dishes or having a bath. At least it had indoor plumbing.

 

Unless you can find a local installer who has experience with these systems, I would tend to shy away from it for that reason and instead look for a condensing water heater and furnace.

But if you don't use much hot water (for example, my house only uses about 7 therms/mo for heating water, or about $7/mo at current gas prices), that's less than $100/year and condensing water heaters cost a lot more than your typical 80% water heater.

Now, replacing my 30 yo furnace with a 95% variable speed unit reduced gas consumption by over 3x! Worst month went from 70 therms to 20 therms and cut the gas bill over $100 as gas rates are tiered here - so the more you use the more you pay per therm.

In the non-heating months the pilot light alone used about the same amount of gas as my standard water heater.

When I replaced my furnace I also opted to add an AC/heat-pump so that I'd have the option of heating with electricity for a couple reasons.

1. I don't think these low gas prices will last forever - I give them another couple years before they start rising significantly.
2. I have solar panels so if I produce extra electricity it makes sense to heat with the heat pump instead.
3. The addition cost to install a heat-pump was nominal compared to adding just A/C.

Now if you're not wanting to add A/C, it doesn't make sense to install a heat pump because of the cost.

Since you do have solar, you may want to look into a water heater heat-pump retrofit like this one - this you can add to any water heater. Since gas is go cheap right now it's probably not going to make financial sense unless you are generating extra electricity from solar unless your water heater is already electric.

 

I looked into these a few years ago. Back when there was just 1 or 2 units in the US that were an add on to an existing electric water heater. But you already have a heat pump. You already stated that it's a back up to the pellet stove, but what about the summer? You regularly use it for AC? If so, look into a super deheater. It hooks up to the heat pump to use the hot side to provide hot water. It might not be workable if the heat pump is off most of the time without a back up water heater though.

I haven't looked into the newer heat pump water heaters yet, but the ability to attach duct work was plus when I researched them before. This allows you to direct the waste cold air from the water heater into a central AC system during the summer. Of course they are more common in other countries. I saw a model line in Australia that had the cold side of the heat pump as a remote panel. Instead of drawing heat from the closet, basement, or whatever, the panel went on the roof to gather solar heat. A basic solar water heater is probably more efficient, but it doesn't provide additional AC or dehumidifying.

To OP, I think the concept is sound, and it appears Trane and other manufacturers also have a line of similar heaters. So you can comparison shop for it. I think it's just a question of how harsh your winters are. With severe ones, you might want to consider supplemental heating options.

 

From what I understand, if you have regular need for A/C then heat recovery water heat from the heat pump is far and away the most efficient system out there. You gain two ways, first your A/C runs more efficiently, and the waste heat heats your water. Win/Win!

The equation gets a bit dicier when you only use A/C sporadically however.

Icarus

 

I put in a Bosch AE125 Tankless water heater and it works great.
Had it for 5 years now.
Not sure what a "cold water sandwich" tankless are famous for. Never heard of that before. The water temp is very uniform.

It has knocked off $30 off my electric bill every month. Only drawback is it consumes 4 breaker spaces in the elctrical panel.
It uses two 220 breakers. So you need 4 spaces or have to put in a aux box.

Pro:
Saves $$$ every day.
Does not consume any space. Wall mounts.
No maintenance so far been 5 years.
Bought it new in box on eBay for $100.

Con:
Electrical skills to power it. Wire & breaker cost.
Plumbing skills to plumb it.