Can you overkill on heat source for radiant heat?

[Mossmilo]

There is a guy on Craigslist with a like new Bradford white high efficiently HWT that was used for radiant floor heat. He bought the house and the PO have the pex all messed up so he took everything out and just put in a furnace instead. He had a good deal on the tank pump expansion tank ect. The tank is a 80gallon 75k btu. When I had a guy draw up my system he said I would need a 40-50 gallon 50k btu unit. Would this 80 gallon be a waste? I understand that it will have to heat up more water when the system is so to speak on standby when not running but would it out way the good. Being 75k btu I would think tht it would be able to also heat te floor faster so maybe that would save some money. By the way the garage is 32x48 8ft ceilings r13 walls r19 ceiling and I live in north east ohio. Any input would be appreciated.

 

[jvitez]

Being 75k btu I would think tht it would be able to also heat te floor faster so maybe that would save some money.

Nope, doesn't work that way. It takes X amount of energy input to heat your floor to your desired temp. An 80K heater will deliver more heat than a 50K heater, but the 50K one will run longer. Remember it's btu/HOUR input rating. If you need 150K btu to heat up your floor, you 50,000 btu/hr heater will run for 150/50 or 3 hours (at a theoretical 100% efficiency) then shut off. The 80,000 btu/hr heater will run for 150/80 or 1.875 hours then shut off. Amount of NG used: identical.

Now for real world specs, the 80K unit will probably run less efficiently than the 50K because it runs less and spends less time at peak steady state efficiency. For fuel burning appliances the best efficiency occurs when it's sized to run 100% of the time on the coldest day of the year. It will not be using more fuel to do this. On the contrary it will be using less because it's running at peak combustion efficiency.

For electric heaters it makes essentially no difference.

 

[Mossmilo]

Thanks any other inputs out there????

 

[jack stand]

Now for real world specs, the 80K unit will probably run less efficiently than the 50K because it runs less and spends less time at peak steady state efficiency. For fuel burning appliances the best efficiency occurs when it's sized to run 100% of the time on the coldest day of the year. It will not be using more fuel to do this. On the contrary it will be using less because it's running at peak combustion efficiency.

For electric heaters it makes essentially no difference.

But if he can get a great deal with this new/used unit over buying the smaller but properly sized unit, are your (correct) points above, sort of splitting hairs?
I am asking, not arguing your points at all, you sound like you've got experiance here.

 

[jlckmj]

I guess if it was me the question I would ask myself is how much is it going to cost me to try this system? Will it cost you the same as buying the correct system? if so I would walk away.

Will you be able to get it for 20cents on the dollar of what the correct system would cost? I might take a chance at that if all the valves and expansion tanks, and pumps are already there.

Jim

 

[jvitez]

But if he can get a great deal with this new/used unit over buying the smaller but properly sized unit, are your (correct) points above, sort of splitting hairs?
I am asking, not arguing your points at all, you sound like you've got experiance here.

You're absolutely correct.

The OP asked if it would be cheaper to run a bigger heater as it would heat faster and I gave him the physics of why it isn't so.

Now if he can get a less efficient heater for a great deal, the money saved will buy a lot of fuel if it runs somewhat less efficiently. It's the capital cost vs operating cost balance.

But I think it's good for gents to know the physics: oversizing fuel burning heaters reduce operating efficiency and increase costs. This is independent of capital cost issues.

 

[Mossmilo]

Thanks for the info guys.

 

[ptschram]

I'm curious about the theories behind this as well.

Recognizing that it is nearly impossible to estimate heat loss and many other variables, what is the impact on having too small a heat source? Would one be better off with an over-sized versus undersized?

 

[pseudorealityx]

I'm curious about the theories behind this as well.

Recognizing that it is nearly impossible to estimate heat loss and many other variables, what is the impact on having too small a heat source? Would one be better off with an over-sized versus undersized?

Oversized:
-less efficient
-costs more
-short warm up phase

Undersized:
-doesn't maintain temp
-very long warm-up phase
-may cost less
-may be 'enough' on 90% of days, but not that last 10%

 

[1953mercury]

if this a conventional boiler With multiple burners you can de-rate it buy plugging off one or two of the burners. Mike