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Old 03-02-16, 08:28 PM   #1
jeff5may
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Default 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.

Quote:
Originally Posted by SDMCF
Quote:
Originally Posted by AC_Hacker:
A GSHP is pretty much the most economical form of heating you can have. But it is not like running you are going to run GSHP right into your ordinary house heating system and expecting everything to work just like it did before, only cheaper.
and
Quote:
You can't use the methods that worked for heating with wood, coal, gas, oil, etc. because these methods produce much hotter temperatures than a GSHP is capable of... things need to be done in a different way.



That's a reasonable rule of thumb but as always "it depends". I just installed a GSHP and connected it into my existing heating system (radiators). I didn't change anything inside the house apart from connecting the GSHP into the central heating pipework. Everything works just like it did before, only cheaper.

Quote:
One of the most important things to bear in mind is that a GSHP will produce hot water at a temperature of 120F max.

That is indeed the crux of the issue, although I would say 55C/130F. My GSHP manual claims 65C/159F but I don't believe that. My radiators run at a maximum of 55C so I just about cope during really cold spells, but of course usually it is not so cold and the radiators don't need to be so hot.

Quote:
There are three big things that can lower the required water temperature:
Minimize the heat loss in your heated space by reducing the infiltration (air leakage) to the very minimum.
Increase the quality and quantity of insulation significantly above the required code level.
Insure that your heat input method, forced-air or water heated radiating panels, are as efficient as possible.

Maybe a 4th option is to install more radiators (or whatever you use).
Quote:
Originally Posted by AC_Hacker
I think you need to give some details regarding your radiators. The reason I say this is that North American radiators are typically about 50 to 70 years behind the technology that is normal in your area.

Additionally, by going to more efficient radiators, which will lower the feed-temperature, the increase in COP is exponential. It is a mild exponential curve, but exponential nonetheless.

-AC
Quote:
Originally Posted by SDMCF
I nearly spat out my coffee! That did make me laugh.
You may be correct that more modern technology is normal here compared to North America, but if you saw my house you would understand why that idea is comical in my case. I have 2 types of radiators.

I have a few radiators something like this:

My guess is they were old, re-used units when they were installed.

Most of the radiators radiators are slightly more modern, something like this:


I believe all the radiators were installed in the 1960s.
Quote:
Originally Posted by AC_Hacker
SDMCF,

Very sorry to hear about your ruined coffee by spitting in it. Did you drink it anyway?

The top heater can still be found. It was originally meant for steam heat, and as it was designed, steam condensed in it and the radiator gave off considerable heat. Just running hot water through it would yield far less heat.

The bottom heater is now being sold in the US as a high-tech premium heat radiator. Not many people have them.

There is another radiator that is between the two you showed.


These are everywhere here in the US, and many people here don't know of any other kind. These things work well, but they are designed for water temps around 150F to 180F. And yes, if you get enough of them it would be possible to heat a house.

But I came across some radiators that are specifically designed for low feed temps. Here is an image:


In this case, they have very large fin area, and the small computer type fans gently keep the air moving. This pic is from a product made by Jaga. It that a Finland firm?

My point is that yes, if your phase change system is running at 120F it will work and will be cheaper, but if you can use a more efficient radiating system, you can use radiators that give you the heat output you need and use lower temp water, and you will be even better off.

-AC



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:
Originally Posted by SDMCF
That's interesting information. FWIW The refrigerant in my system is R407C. I have no idea whether that is in any way better or worse.

I may have given the wrong idea when I said that my system just about copes in the coldest conditions. What I mean is that when it is very cold out the heat pump has to work at the limit of how much heat it can extract from the ground. I normally keep its additional resistive heating turned off, and let the GSHP run hard. If that is not sufficient I do turn on the resistive heating function of the GSHP. I don't sit in the cold adding another horse hair shirt and letting the house grow cold.

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