Replacing a nat gas combi boiler with electric alternative

fosterph

I have a customer that has an existing nat gas combi boiler. They also have solar thermal with a large horizontal indirect on a shelf that is used as the solar storage tank. This preheat DHW is fed into the existing combi boiler to top up if required for DHW. Also via an external flat plate heat exchanger the same solar storage is used for infloor space heating with supplement from the combi boiler if there is not enough heat.

Quite a neat design not mine, unfortunately the customer has had many issues with the combi boiler and it's end of life. They want to migrate to all electric. As you can see this is a very small mechanical room shared with laundry on a main floor. Ideally I would like to go with wall mounted solutions for the electric replacements so the space is still useable.

Replacing with an electric boiler for the space heat portion is straightforward. The DHW is my concern I don't really want to go with a tank because of floor space. Has anyone tried using an electric boiler with a flow switch and flat plate to create DHW? Am I better to put in a separate on demand DHW electric unit that has all this built in? I understand the electrical service will need to be upgraded. I just like the idea of one unit doing both functions.

Any suggestions or experiences would be greatly appreciated.

Phil

kcopp

That set up is fairly decent. Having the Natural gas is great… many would love to have that option.

I would try to have them just replace the Combi w/ another combi that is tried AND that you know well.

If they are drinking the electrification koolaid and cant be moved to gain the floor space maybe get a stackable washer and dryer?

Hot_water_fan

Air to water heat pump would be the way. But a tank is almost a must have. Stack the W/D and fit a small tank!

fosterph

Looking at ComboMax Ultra from Thermo 2000 as an option, but would need modification due to solar thermal input expects cold water to pre plumbed mixing valve. Another option would be wall mount boiler and separate tank for DHW.

Anyone used an electric on demand water heater for space heating as well?

ratio

Before you go too far down the electric rabbit hole, check & see how much power an equivalent electric heater will require. You might be a few hundred amps low for a direct replacement.

Kaos

A big electric tankless (24kW) is equivalent to a small gas unit (80kW). That already needs a 100A supply with 3x40A connection and won't run much more than about a shower so unless you have a 400A service, electric tankless will be hard. I would find space for a resistance tank.

As for space heat, resistance boiler will definitely work, you would have to see how much natural gas was used and what that would cost in terms of electricity. Sometimes the cost delta is not much but in most places resistance boiler is expensive unless they have a big PV array.

fosterph

Yes customer is set on electric and service upgrade up from 100A is planned already.

Home is very well insulated and sealed and current gas bill is small for heat, dhw and gas range around $30 a month, I believe the solar thermal preheat helps a lot with this.

I need to investigate what the DHW draw requirements are i'm thinking fairly small as well.

Jamie Hall

Where is this? There is a possibility, if it is far enough south, that you might be able to use a air to water heat pump. That will save some electricity.

Otherwise, you will need an electric boiler. If the one presently serving appears to be reasonably sized, the size you need is easy to determine: divide the BTUh rating of the boiler by 3,400 to get the required kW rating of the electric boiler.

The solar component is irrelevant in the sizing, unless the customer has a HUGE thermal storage or a HUGE battery bank.

hot_rod

Do they have solar PV or just thermal?

PeteA

Is there any math here that makes the electric boiler / electric DHW economically feasible as compared to just keeping with the combi on gas that they already have here?

I am honestly just curious because my winter electric in NYC area runs about $100 and my gas runs about $250 (not a combi just a 84 efficiency regular boiler) and this reverses in the summer (actually gas is less than $100 at times in summer) but I would imagine electric boiler and electric hot water would easily beat the yearly costs of this system with the gas combi. Is this not correct with the turndown ratios and outdoor resets?

Like I said just curious.

Kaos

https://forum.heatinghelp.com/discussion/comment/1841629#Comment_1841629

Depends on your local utility costs. Around me gas is about 2.5x cheaper than resistance heat, so hard to argue with switching away unless you have excess PV.

Switching to a heat pump tends to be a net win since you get much better efficiency on yearly costs but that is only really worth it if you are replacing equipment that has failed.

PeteA

Thanks @Kaos

I am expecting many people that have access to gas would feel the same way. Staying with gas is hard to beat if you have it.

Also if there is electrical upgrades required like 100 to 200amp just to fit all the circuits needed you're really adding onto the expenses of switching to all electric then.

Kaos

Switching to heat pump doesn't always mean a service upgrade, in most cases an existing 100A service is fine as long as you don't have an EV. Even with an EV it can work as long as you use one of the smart load management chargers.

Switching to resistance heat is a different story. Those are hard to do unless you have very low loads.

Usually if you are in a place with AC, when your AC is near end of life, changing it out to a properly sized heat pump coil is a not much additional cost and doesn't require any services upgrades. In most cases the heat pump can carry the full house plus you always have the furnace as a backup. This gets you mostly off gas plus a heat pump will also be cheaper to run in the summer for cooling.

Hot_water_fan

Right on @Kaos, but a heat pump isn’t magic - it’s the same efficiency as an equivalent AC in the summer