A sobering thought on direct heating of water.
I was working on a rheem instant hot water unit, and noticed the nameplate stated that it uses over 55kw for the 25lpm model for 25C temp increase.... it is not easy stuff.
Water heating takes immense amounts of power as you point out. This is why I have gone with using a waste oil burner.
I have the heat exchanger from gas heater that was used in an Olympic pool. It's rated at 200 Kw and my burners can over power it easily. I also have a smaller spa heater and a water heater I have converted.
The water heater is only a small one at 125L but once I get it up to heat, the thing will run as continuous basically. The kids have tried to run it out but I can run it hard enough that it will keep up with demand. Just have to watch when they are finished and get out that it does not over heat before the next one gets in. I'm looking at controls to fix that problem.
The thing works so well I have plumbed it into the house ht water circuit. I have valves so I can Feed from the electric or the oil burner Heater.
I'm on the lookout for a 400L unit to convert. With this I would only need to do a fire up every 2-3 days on what we use on average. I would set this up as a feed for the electric heater. Even if the Oil powered heater was a bit cool, it could be feeding water at say 40o to the electric heater instead of say 15o therefore making the electric heat up faster and cost less.
I have a 70Kl Swimming pool and I can heat that from winter temps to 35+ in 24 Hours. Takes a 44 gallon drum full of oil and a bit more But it's awesome to have a pool party in winter that's for sure.
I was going to use the pool as my thermal storage for home heating but I have been looking at moving for some time so haven't bothered setting that up.
I did use another 125 L heater this year this winter for home heating. Had the burner going controlled by a fuel pump on a timer that just pumped for 2 sec then switched off for 4 and circulated the water with a small electric pump. The high tech heat exchanger was a car radiator with an electric fan.
The system worked really well and we were very cosy but it needed refining.
My house is basically a front and back area with a door between, the back could be too warm and the front too cold. It really needed another radiator in the front section or what I was thinking would be even better, Run the existing radiator into the house and after that have another Under the quite well sealed floor area to allow the whole place to heat up.
Again looking to move so not worth the effort here but the wife has been encouraging me to do it anyway for Summer.
We had a very warm day ( or what seemed like it) a few weeks ago. I just pumped water directly from the pool through the radiator and it made for very effective poor mans AC. I figure the pool will always be cooler than the ambient summer temps and It might be effective to spray the return water to cool it or blow a fan over the top at night.
Evaporation isn't an issue, I put some IBC's Under the downpipes years ago and with the summer rains always have more than enough water to offset the evaporation that occurs. Often one good rain shower overfills the pool level on it's own and theres really no problem with running it a bit low. I have a lot of margin between high and low to play with.
The Mrs wants me to put the Radiator in the front section of the house so we can have the bedrooms Cool to sleep at night. The front AC broke so no point replacing that now, plus we never liked it on while we were sleeping as it always gave us Chills no matter where we set it.
Not that I'm worried, but it also is a lot less power to run a 300W pump than an AC unit.
The other unit I have is a spa size heater rated at 100Kw which will be plenty of over kill for the home heating and has the various sensors from the gas burner control that I can tap into to control the oil burner. At this stage I see it as basicaly a constant burn system with safety cutouts but even a high and low burn rate would make the thing entirely practical. I could set the low burn under what I normally need and the high above and just have a thermostat switch between the 2 as needed.
That's the great thing with my burners. Commercial units seem to have about a 4:1 turn down ratio. I have read of some Gas units with a 10: turndown ratio. I can get a 50:1+ on my oil burners without much trouble at all. Anything from a Litre an hour to a litre a Minute on the same burner. If I had the thing idling at 10 KW and burning at 40, That would be all I'd need and just cycle between the 2.
The other option of course depending on where we end up, is to just Co gen my Lister engine and make heat with that and tap off the power as a side benefit which is really the best way to look at those sort of setups.