03-02-16, 08:28 PM | #1 | |||||
Supreme EcoRenovator
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Retrofitting existing boiler heating to heat pump
This was a topic that ran away with itself in the Homemade Heat Pump Manifesto. It weighs design considerations with actual results when performing a retrofit to an existing systems. Due to the technical nature of the conversation, and the resultant wandering of the Manifesto, I deleted my posts there and will reiterate the relevant points here. Hopefully it will help others in understanding the importance of considering the system as a whole from the very beginning.
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This is typical of R-22 and R-410a systems. The critical temp of R-22 is 96 degC, and of 410a is only 72 degC, so when pushed to its limits, a system filled with R-22 (or propane) can reach a higher condensing temperature ceiling. The main issue with running a high (medium) temperature condenser is the heavenly operating pressure (around 600-650 psi). Everything containing refrigerant must be made of stronger, thicker, tougher materials to handle the pressure. This includes the compressor and all its moving parts. In systems designed to reach this range, R-134a is by far the most widely used modern refrigerant of choice. Its lower vapor pressure at (medium) target temp is only 300 psi @ 160 degF and it has a critical temperature above the boiling point of water. Even at 190 degF, the condensing pressure is (only) 430 psi. Surprisingly, there are lots of systems designed to operate in this range. They are almost all in automotive and marine refrigerating systems. They are not highly efficient, but they will withstand a hot day in Death Valley or the Sahara. Just because it is possible in vehicles does not mean it is a good choice to retrofit an old boiler system. With respect to the purpose, a retrofit is intended to improve the operation of the system as a whole. As SDMCF has stated, the system he has installed has its limitations, due to the radiators he is running. When it gets frigid cold outside, he has been coping with the cold creeping in, rather than feeding his heating system with a backup source of more expensive heat. However, many people are not so tolerant, and will configure their systems to burn backup energy on those frigid spells every winter. Just as when climbing a steep hill when driving, some people put the pedal to the floor to maintain their speed, paying the price the next time they get gas. It's not THAT expensive, right?? It is VERY important to consider these factors when planning any system, new or retrofit. Yes, upgrading hydronic slabs or floors, convective or forced air radiators, baseboard heaters, etc. will add (maybe thousands of $$$) upfront cost to the initial install. But like extra insulation, this is a one-time expense that will have a permanent effect on the heating system. Along with the outdoor loop sizing, the indoor heat exchange system is the most important aspect to consider (and scrutinize) to maximize the performance of the whole system. What is barely adequate to cover 90% of heating needs running heat exchangers at 160 degF supply water temperature will be much more adequate with heat exchangers designed to provide the same heating capacity at 130 degF supply water. This over-sizing of radiators does not have the typical diminishing rate of return, as with brazed plate heat exchangers. Due to the combination of increased radiating area, lower temperature difference between indoor and outdoor temperatures, and decreased compressor work necessary to pump the same amount of BTU's, the relationship is pretty much linear until the radiators become very large (by home heating standards). The increase in performance will not only add to the comfort of the home, it will directly reduce the power bill A LOT. Compared to the ground loop, the indoor heat exchangers are less expensive (MUCH less expensive if you farm out the ground work), so it does not make economic sense to "cheap out" on them. In the end, the choice is easy to make. In most cases, it all hinges on the budget vs potential future payback. Luckily, the indoor heaters are relatively less difficult to replace or upgrade after the initial installation than most of the rest of the system. Quote:
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