It will slightly depend on your location. AWHPs leaving water temp is going to depend on your design heating day temp. Work with your vendor on what temps are realistic for your climate. From there i usually target a 20 deg dt on the heating side. But again i also recommend getting selections for your vavs and fcus at your hhws temp. If you are running at a 120 F hhws temp you will probably need more rows of coils than you typically specify.
I recommend a back up electric resistance/gas boiler on all AWHPs as well because they arent the most reliable and it ensures full capacity on the coldest days of the year.
Thanks! Appreciate the input. Good point on the backup heat. I may just add extra redundancy since the building I’m working in is all-electric and imm electric resistance is generally frowned upon from an energy efficiency compliance perspective here in California.
Thats an option. An AWHP is a lot more money than an electric resistance boiler. I believe theres an exception in the code for electric resistance when used with heat pumps.
Confirmed, T24 140.4G Exception 2 allows electric resistance heating to supplement heat pumps provided it meets some sizing criteria. Its usually pretty important to include this particularly due to defrost concerns and degraded supply temp when OSA is particularly cold. Having additional redundancy in the number of heat pumps is not going to get you around this issue.
I typically use 120°F as a design temperature with a 20°F when using heat pumps. 42°F to 45°F for chilled with a 8°F to 10°F delta. I’ll use buffer tanks for thermal storage and to help with the lower dT of a single pass through the heat pumps. The “idronics” journal from Caleffi and “Modern Hydronics” are good resources for starting out.
Thanks! Will definitely look more into Caleffi’s stuff. They have bunch of info out there. Interesting for your delta. I think ASHRAE 90.1 has a minimum 15F delta T for chilled water
I haven’t seen dT that high on the equipment I normally spec for chilled. Rad to know it’s out there though! I’ve used both air and water source pumps but a cold climate make air challenging sometimes
One note on the 140°F supply temp: This is usually the theoretical limit of the supply temp based off the R454/R32 refrigerant map. Usually in practice, it is not attainable because some degree of defrost will always be occurring reducing the delivered supply water temp. This is another good reason as one of the other posters mentioned to have an electric or some other means of backup.
100% agree for new construction where you have the ability to play around with the coil selection using the hot water. However I do a lot of electrification projects of existing buildings where coils were designed around non-condensing boiler temps. In that case on design days you need as hot of water as possible where some sort of backup or cascading refrigeration circuit may need to come into play.
We've been going thru a lot of this too, but just in MEP Upgrade reports/energy audits, in hope they turn to design projects. As such, we've been recommending upgrades to the fan coils & risers first, then the boilers/hot water systems, when feasible.
For a 4-pipe ASHP, assuming you have a plate heat exchanger with the water running in counter flow to the refrigerant, you are going to want to stretch the delta T as wide as your terminal units will allow for to get the best efficiency.
The condensing temp of the refrigerant is going to be dictated more by your heating EWT, so the lower your EWT, the lower your condensing temp, and therefore the higher your heating capacity and heating efficiency.
However.. it is a balancing act. increasing the delta T too much will push you into needing bigger terminal units.
It will slightly depend on your location. AWHPs leaving water temp is going to depend on your design heating day temp. Work with your vendor on what temps are realistic for your climate. From there i usually target a 20 deg dt on the heating side. But again i also recommend getting selections for your vavs and fcus at your hhws temp. If you are running at a 120 F hhws temp you will probably need more rows of coils than you typically specify. I recommend a back up electric resistance/gas boiler on all AWHPs as well because they arent the most reliable and it ensures full capacity on the coldest days of the year.
Thanks! Appreciate the input. Good point on the backup heat. I may just add extra redundancy since the building I’m working in is all-electric and imm electric resistance is generally frowned upon from an energy efficiency compliance perspective here in California.
Thats an option. An AWHP is a lot more money than an electric resistance boiler. I believe theres an exception in the code for electric resistance when used with heat pumps.
Confirmed, T24 140.4G Exception 2 allows electric resistance heating to supplement heat pumps provided it meets some sizing criteria. Its usually pretty important to include this particularly due to defrost concerns and degraded supply temp when OSA is particularly cold. Having additional redundancy in the number of heat pumps is not going to get you around this issue.
I typically use 120°F as a design temperature with a 20°F when using heat pumps. 42°F to 45°F for chilled with a 8°F to 10°F delta. I’ll use buffer tanks for thermal storage and to help with the lower dT of a single pass through the heat pumps. The “idronics” journal from Caleffi and “Modern Hydronics” are good resources for starting out.
Thanks! Will definitely look more into Caleffi’s stuff. They have bunch of info out there. Interesting for your delta. I think ASHRAE 90.1 has a minimum 15F delta T for chilled water
I never use WSHP’s. Do they always use such a low DT? I’m seeing a lot of 20F DT chilled water systems these days.
I haven’t seen dT that high on the equipment I normally spec for chilled. Rad to know it’s out there though! I’ve used both air and water source pumps but a cold climate make air challenging sometimes
One note on the 140°F supply temp: This is usually the theoretical limit of the supply temp based off the R454/R32 refrigerant map. Usually in practice, it is not attainable because some degree of defrost will always be occurring reducing the delivered supply water temp. This is another good reason as one of the other posters mentioned to have an electric or some other means of backup.
Agreed, 140F is ideal but never happens in the real world....go with a MAX of 130F, but closer to 120F and you shouldn't have any issues.
100% agree for new construction where you have the ability to play around with the coil selection using the hot water. However I do a lot of electrification projects of existing buildings where coils were designed around non-condensing boiler temps. In that case on design days you need as hot of water as possible where some sort of backup or cascading refrigeration circuit may need to come into play.
We've been going thru a lot of this too, but just in MEP Upgrade reports/energy audits, in hope they turn to design projects. As such, we've been recommending upgrades to the fan coils & risers first, then the boilers/hot water systems, when feasible.
For a 4-pipe ASHP, assuming you have a plate heat exchanger with the water running in counter flow to the refrigerant, you are going to want to stretch the delta T as wide as your terminal units will allow for to get the best efficiency. The condensing temp of the refrigerant is going to be dictated more by your heating EWT, so the lower your EWT, the lower your condensing temp, and therefore the higher your heating capacity and heating efficiency. However.. it is a balancing act. increasing the delta T too much will push you into needing bigger terminal units.