engines size?

HX systems...
Does the size of the engine correlate with the amount of heat/temp it can spit out HOT water??
Or small engine will only be able to produce a certain temp of water?

And about how much HP needed to run a pump?

Just brain storming here.

An engine is nothing more than a heat exchange and the greater the horsepower, the greater the heat given off by that engine.

Does that translate to hotter hx temps? It depends on how that heat is being transferred. Is it exhaust heat or circulating coolant being used to heat the water? At what rpm is it spinning....higher the rpm, the greater the hp and thus greater the heat.

It just depends on what you had in mind?

As for the pump, I don't think it takes that much to rotate a pump, the blower is the harder thing to turn. If you spin the blower at speed, the pump will be fine with that horsepower.

At least that would be my thoughts.

See manufacture specs for HP needed.

I think..

A small blower needs 15 or so
and a pump about 2-3

Lets see if this works this is off of a blower site the bold numbers are cfm of a # 3 and # 4  blowers and the HP it takes to get it.
3MP 2"-S 0.060 2,265 106 2.0 93 3.3 87 3.9
3MVP 2,770 136 2.4 124 4.0 117 4.7 110 5.4
3,600 186 3.1 174 5.0 167 6.0 160 7.0 156 CFM 7.5 Horse power to get it
1,760 161 3.0 142 4.9 132 5.8
4MP 21
⁄2"-S 0.117 2,190 211 3.7 193 6.0 183 7.2
4MVP 2,620 262 4.4 243 7.1 233 8.6 222 10.0
3,600 376 5.9 358 9.8 348 11.8 337 13.7 331 14.7 325 CFM 15.7 Horse power to get it

Depends on what pump but I would think 3 -5 hp needed for most would be a safe guess.
Yes the size of the motor makes more or less heat because you have a greater burn of energy for larger motors.
How to maybe help is adding more coils to a HX system. So first your incoming water just house pressure through first hx this dumps into water box no safety needed as it is a open system.
Second Hx would need safety due to it being sealed and pumped with pressure pump this makes it sealed, and would need to be able to release if heat gets to high, higher pressure rated plumbing to hold pressure and as it starts to expand due to higher temps.
But the thing is, the smaller motor just doesn't have a lot of btu's so the hot exhaust gas is going to get cooled off much faster.
There is a HUGE amount of difference in the BTU's of my vw motor vs my kohler 20 HP motor.