Commercial application: Ground-source heat pump to hydronic fan coil. Ground source loop is alcohol. Building loop (hydronic) is glycol. Designer wants to add a boiler for supplemental heat for cold season and as back-up. Ok, that's fine. So, where do you locate the boiler? On the ground loop just before it enters the heat pump? Or on the building loop? I vote for the second one - put the boiler on the building loop. Designer says 'no'. I'm having a hard time buying into raising the ground water temp before it goes to the heat pump. Who's right? (I'm happy to be wrong, if there's a reasonable explanation.)
Boilers, not mod-cons, work best running flat out. If it is a cast iron boiler it does not want to see return water temps below 130*F. That is too high of a temperature for the heat pump(s) to handle. Last one I did the boiler feeds the load side buffering tank. I have never seen boiler to the heat pump design. Have your designer call me. Mark
Please find a new designer. Piping a 90% eff boiler into the ground loop will net 75% efficiency to the building after the heat pump exchange. The boiler should inject into the building hydronic loop after the buffer tank. The boiler should be mod con and setup to supply geo tolerant temperatures. I also have issues with shooting flames at a heat exchanger with alcohol in it.
I just received the Caleffi essay on radiant cooling. One needs to remember the the lower the lift the better a BTUH squeezing heat pump works, and the more energy it saves. Mark
Either will work. If you think about it heating the load side permits the geo to amplify the heat energy offered by the boiler vs it's .95 COP on the load side. Boiler/cooling tower systems (EWT is elivated or lowered depending on need) were more common 20-25 years ago.
Thanks for the replies. The ground loop pumps would, presumably, circulate continuously (deep winter) and with that I'm having trouble imagining that the heat pump could strip off all of the 'supplemental' heat that the boiler generates fast enough... the result being that the boiler would be heating the ground.
I need to add one bit of clarity... I'm not suggesting that we could not get the system to "work" (with the boiler on the ground loop), albeit by a sophisticated control strategy, it's a matter of 'efficiency'. So, the goal is 'most efficient way the works'. I'm told that the COP of the HP improves with a higher input water temp (and that's why we should put the boiler first) - is this true?. It seems the decision is being made based on this sole 'fact'. Even if it is, I'm not convinced that the initial energy expended by the boiler is worth the purported gain in COP.
My limited understanding of pairing fired boilers with geo systems is that it is NOT about COP (any additional COP is offset by the cost of operating the boiler) but rather about giving a boost to the system when heat load is above the system's capacity. The boiler acts as a large scale equivalent to auxiliary heat strips.
The COP of the HP does improve with higher source temps. However, in this case, one has to consider total system efficiency to come to a reasonable conclusion. COP=System Output/System Input If you are paying to inject boiler heat into the ground loop you have to include that power as System Input along with the power required by the HP. Your System Output will increase but never as much as the boiler injected. Please don't make me do the math. Stop this now or the next thing you know the PV guys will be running their cars at night to shine their headlights on their PV panels to get better efficiency.
Dumping heat in to the loop ONLY makes sense when you do not have a primary use for that heat. You will only recover a % of those btu's.
http://welserver.com/WEL0267/ Boiler can supplement your load side, but it make no or very little sense to connect it to the ground loop. Why do you want to heat up the earth? Don't you want the heat to go in your house? You should not walk away from your designer....you have to RUN away!
Another good reason not to pipe a boiler to your ground loop, besides great loss of efficiency, is the loss of backup heat if the geo side went down.
Yes obviously if you were to use a boiler on the loop side, you would have to isolate it from the ground loops. As I said it has been done historically, but it is an old school design (again 20+ yrs ago). Most every application I've seen today is with boiler on the load side.
>>> Yep, the math is everything here. Or, look at it this way: if putting a boiler at the input to the HP resulted in a gain in COP, then why not simply use only a boiler? The boiler may make sense as a stage 2 heat source with the hot water coils located in the plenum -- as a substitute for electric heat. I am designing a system that way and will use it when our power goes out (often) and the emergency generator kicks in. You can't come out ahead using propane to power a generator to power a gs heat pump when you can use the propane directly in a boiler to heat an air coil. Actually, my boiler will be a 199k btu on-demand water heater.
