Electric boiler sizing for hydronic TOD slab

radiantslaber · October 9

Hi, I have a 15kw electric boiler for 2400sqft hydronic slab on grade house and have been using time of day to heat sink it. Have water set point at 118f 48 Celsius to allow the boiler to get slab warm enough at off peak overnight to keep house at 70f all day. Has worked well for last 5 years, but have now added a 1600sqft hydronic garage and ran insulated suppy lines over underground for it but it can’t keep up now as expected. My calculations suggest I need around 80k btu vs my 51k current. My question is the difference from a 15kw to a potential 24kw boiler for a garage I may not always keep fully heated and kw usage difference, should I consider just adding a separate propane hydronic water heater for garage that can run all day vs off peak electric, upsize the boiler or potentially an air to water heat pump(most expensive) to supply both? Winter temps are typically-10 Celsius 14f. My major concern is if I decide to not heat the garage at times, will the 24kw boiler kill any TOD savings I’m currently getting, I know they modulate, but current set up without new garage runs 100% for at least first few hours overnight to heat sink the slab, so it i’d think a larger boiler would still do same, or would water set temp on larger keep up better and even out? Thanks in advance for any insight

hot_rod · October 9

There is no free lunch here. It takes X amopunt of BTU to heat those spaces regardless of when or how you get them.

It the TOD works well without over-heating the space it may be you better option. A slab is a huge flywheel, hard to change output quickly.u

You would need to next compare electric rates on and off peak against LP. Although LP can be a moving target price- wise.

Here is a handy calculator for comparing fuels.

https://coalpail.com/fuel-comparison-calculator-home-heating

mattmia2 · October 9

the larger boiler might heat the slab faster during the off peak time but it will still take more or less the same energy to do it (if it is hot sooner in the off peak period there will be a little more loss from the structure because it is hot for a longer period of time but I think that is negligible)

If you aren't heating the garage some of the time you might want to run glycol in it and use a separate electric boiler so you only have to maintain glycol in that system, otherwise one boiler probably makes sense.

How often do you want to heat the garage. If it is only occasionally then the on peak electric cost probably isn't a lot and the boiler will be a lot cheaper than the propane boiler and tank and piping and venting. if you will be doing a lot of heating of the garage then you need to compare cost of the different fuels and cost of the equipment.

radiantslaber · October 9

https://forum.heatinghelp.com/discussion/comment/1867222#Comment_1867222

yes, no free lunch, I had been considering an air to water heat pump for couple years hoping prices would come down, with TOd and one that could fully replace boiler for btu, I was told could save 50% on kWh. Newer large ones can do 70-80k btu now so might even be able to do garage… but are over 10k

took a year to dial in the flywheel. 48 degree has house at 22 in morning and real cold days-20 it drops at 19 late evening but its bed time anyhow and like that cool

I had solar guys here trying to talk me into keeping it all electric and let the boiler run 24hr to maintain… if it was allowed to run all day it probably could… sounds good but the total cost take 10 years to recoup. I have propane for cooktop dryer and bbq box and a regular propane hot water heater at 40k btu could easily do the garage to keep it 10-15 Celsius. As it’s a garage, could get away with that, but no allowed in house in my area

radiantslaber · October 9

https://forum.heatinghelp.com/discussion/comment/1867224#Comment_1867224

That was what I thought, or was hoping, larger would be able to maintain set temp of water which in turn gets it to temp quicker so in theory would be close kw usage. I did put low concentration of glycol is whole system once garage was connected to protect to -10c if it gets down around there I will be heating it either way. Even those -20 days garage without heat barely gets to zero, lots of windows and well insulated.

Seen some cheap propane water heaters tanks from upgrades lately and be an easy install… which packed my interest as an option… I may even consider a snow melt before I concrete my driveway… but that’s a pipe dream! I’d definitely run it independent if can find recycled foam and good deal on Pex a… dam rebar is pricy now too

ratio · October 9

Electric has basically infinite turndown, limited only by your staging, so the only mechanical downside to a larger electric boiler is more wear on the contactors used on the stages. SSRs would alleviate that.

Cheapest might be a second electric, inline with the current one. Tie it in before the current one, have it come on by return temp when the current boiler can't quite keep up. That may also give you slightly faster recovery when the garage is not being heated as well, since the loop temp will come up faster—but I don't know how much impact that will have.

I imagine this would involve quite a bit of tweaking to find the right setpoint. Using a separate control to manage both boilers would be less fiddling, but more cost and complexity.

hot_rod · 2:19AM

the amount of heat you can put into the slab will be limited by the distribution piping, loop sizes, pump size, etc

A bigger boiler may just short cycle if it can’t move the heat out

A boiler on odr with a infinite turndown could run efficiently 24/7. But it doesn’t take into account TOD pricing

Jamie Hall · 1:37PM

Seems to me the bottom line here is that you'll use the same number of BTU whether it's a hydronic radiant system or an electric one — and an electric one is a lot simpler and cheaper for that size area.

Electric boiler sizing for hydronic TOD slab

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