Tankless coil with Electric hot water question

[Kenny Diehl]

need some advice. My house has a Burnham Hydronic heating system with a tankless coil for domestic HW. The boiler is located at the opposite side of the house as most ofmthe Domestic demand(kitchen,2 baths). Tankless coils IMHO are ineffeciant to begin with but in my case I can barely get hot water to the other side of the house. Ok, to put a separate HW heater in a more central location limits me to Indirect or Electric because of flue venting restrictions. I decided to go electric since the house has solar electricty. My question is I want to cut the electric HW in after the tankless coil but before any of the fixtures .
My reason would be in case of a power failure I would still have the hot water from the furnace. I can run the furnace off a smaller genset. I want to put an “H” style bypass above the electric HW so in a no power scenario, I can just close two valve and open the bypass to eleminate the elec HW otherwise I would have to push 50 gals of cold water out first. Now of course this create a possible safety concern. If someone bypasses the Elec and forgets to shut it off, it could come on when power is restored and build up excessive pressure. They have a relief on the side but as an added level of safety and protection, I was thinking install and expantion tank between the cold water inlet on the electric and the cold water shut off. I’ll attach a very poor drawing(sorry my art skills are not good). Opinions ?

 

[Dana]

The terms "boiler" and "furnace" aren't really interchangable. In the HVAC world the latter refers exclusively to hot-air heat delivering.

Do you live in some third-world part of Connecticut where lengthy power outages are common?

If you normally have to run a dehumidifier in the summer to keep the "musty basement smell" at bay, the right solution here is to install a heat pump water heater, and turn the boiler off for the summer.

During the summer the water heater will do much of the heavy lifting on the dehumidification, turning latent heat of moisture drawn from the air into sensible heat as water temperature inside the tank rather than as sensible heat in the room air, the way a room dehumidifier does.

During the winter it will "harvest" the distribution and standby losses of the boiler, lowering the basement air temp a degree or two, which lowers the heat loss out of the basement.

Even at CT electricity pricing a heat pump water heater is a cheaper way to heat water than a tankless coil in a cast iron boiler. In heat-pump-only mode it doesn't draw much power at all, though many/most have a "hybrid" mode where it can draw 4000-5000 watts if the resistance element engages. Unless you have a big soaker tub to fill or like to take 30 minute showers most 2 bath homes will do just fine with a 50 gallon heat pump water heater running in heat-pump-only mode.

Keeping a ridiculously oversized boiler at 150F+ just to be able to deliver mediocre to middlin' hot water service is a waste. Retrofitting the boiler with a heat purging boiler control and letting it purge to 140F is safe for oil boilers, even 130F is usually safe for natural gas, and that takes a huge chunk off the idling losses of the boiler, even during the heating season. This is "worth it" if you're keeping the boiler for more than one more heating season, DIY-able for less than $200 for those with electrician skills.

Feeding an electric water heater or heat pump water heater with overheated water from the tankless coil isn't really saving anything- it's still an inefficient way to heat water. Running cold water though a tankless coil when it's off during the summer would result in condensation on the boiler plates, so it's really best to just divorce the space heating from the hot water, until/unless you have a right-sized (for the space heating load) condensing gas boiler.