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Water source heat pumps- please advise (6 Posts)
Water source heat pumps- please adviseI looked at a project today with water source heat pumps. They are supplied with hot water by an 800,000 btu steam boiler connected into (2) 11/2 steam to water heat exchanger plates. The heat exchangers are 11/2 and both connect into a common 11/2 header before tying into 4 inch supply and return mains feeding the various heat pumps throughout the building. Is this a bottleneck and not allowing the full flow of the heat exchangers and boiler?
I am not familiar with heat pumps. The contractor that showed me the job wants to replace the steam boiler with a conventional cast iron hot water boiler and pipe direct to the heat pumps. He further explained that the boiler water needed to supply the heat pumps is in the range of 80-110 degrees. If this is correct, this return water temp is too cool for a conventional boiler and I advised against this.
I considered multiple mod cons, but the venting is nearly impossible. So, I have also considered a 4 way mixing valve, variable speed injection pump, bypass piping or a water to water heat exchanger to separate the boiler from the system and provide boiler return water protection.
Outside boilersYou would really benefit from using mod-cons. Can the boilers be installed outside or build a room for them that would give you the ability to vent properly? I work at a conmunity college that has a steam to water convertor and our gas usage would go down greatly if we switched out the steam boiler and installed mod-cons, quit ripping out the steam radiators and converted to hot water or at least when the radiators get gone use hot water coils in the split systems instead of electric heaters.
Try to find a way to use the mod-cons. What are you using for the ac mode? Cooling tower, ground loops?
Don in MO
Cooling tower for ACA maintenance guy onsite explained that on Oct 15th, they close the valves to the tower loop and open up the valves to the boiler loop. Is this the norm? I have read elsewhere that these two systems sometimes work simultaneously during shoulder seasons as some areas of the building require cooling while others heating. I need to do more research onsite.
I am trying to figure out whether I need to get the "heated water" from the boilers 55 ft up to the cooling tower or if the tower and boilers are isolated, only 40 ft up to the highest heat pump. Need to stay within my 30 psi relief valve setting or may have to add a heat exchanger.
injectionWhat you are describing is a standard injection scheme used with closed loop heat pump systems. A small amount of hot water is blended with the circulating water to add btu's to the system. This is done with standard non-condensing boilers to save money. Valving the heat rejection off in winter is commonly done so the cooling tower doesn't have to be maintained in winter. The premise of these systems is that they can redistribute heat from areas that have a winter load to those areas that need heat. Sadly, most buildings don't fit this profile because there is no or little process load. If a change of heat source is being looked at, by all means a mod-con would be better. One thing to be careful of is the size of the heating plant-usually the individual heat pumps are sized for cooling load, making the heating side greatly oversized. The heat source must be sized for the combined operation of all heat pumps in heating mode, unless there is some way to stage units on in the event of morning or weekend warm-up.
I "see" what you mean by injection, TomThe existing 800,000 btu steam boiler is connected to the 4 inch water loop via a steam to water heat exchanger- 11/2 inch in and out. Therefore, I can see how hot water is being "injected" into the much larger water loop from the boiler.
The total heat pump cooling capacity is 60 tons in the building. To get the heating load, is it simply 60 x 12,000 btu= 720,000 btu? I want to further understand the relationship between heating and cooling load sizing.This post was edited by an admin on September 21, 2010 5:54 PM.
This buildingMust not have a winter cooling load since they are shutting down the tower. What few units are cooling are having no trouble rejecting their heat into the loop. Many of our office buildings locally have this type of system and you can see towers pouring out the heat even in zero degree weather.
Something like an old folks home may have enough rooms wanting heat that they can absorb the heat from common areas and kitchen and still need heat added to the loop.
The loop stays under 100, don't want HPs going off on high head.