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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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Old 03-02-16, 08:31 PM   #2
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R407c is basically R410a with R134a added to make the pt curve fall (lower) into the range of R22. For HVAC applications, power and capacity are very close to that of R22. The only drawback is that R407c exhibits glide, so it doesn't quite condense and evaporate at constant temperature. It has a bubble point and a dew point, between which the latent heat transfer is not exactly constant. This translates to subtle differences inside the working system, like increased pressure drop and mean temperature difference between condenser and evaporator. In retrofits, minor adjustments in the metering device can balance the system to provide capacity or COP requirements. With a system designed around R407c, everything can be designed and spec'd so the system performs very much the same as an R22 -based system.

Ultimately, with any heat pump system, you are faced with a temperature split vs efficiency trade-off. The higher your indoor supply water temperature needs to reach to meet heating demand, the more of an efficiency penalty you will pay to reach that temperature. I believe Dupont has now released R407f, which is pretty close to the same thing as R407c, with the blend optimized for a little better performance at high delta-t operation.

Oh, and it can't be just dropped into an R22 system; it requires synthetic oil. Along with the other general quirks that the other 400-series refrigerants share. Super clean on the inside, zero tolerance for moisture, etc.

This topic goes right along the lines of this thread. Doing accurate heat load assessments and designing for appropriate heating requirements is not fun or exciting to many, but it is very rewarding in the end.

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Old 03-17-16, 07:04 PM   #3
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I've done a lot of work with both cast iron rads and the steel panels rads (literally 1000s of them) and I will say that the jury is still out as to which one is more efficient, but I know which one is heavier, LOL. I just did a A2W HP with oversized stelrads and have been able to keep this relatively under-insulated house to a max water temp of 40C so perfect for HPs. The oversizing of the rads wasn't that bad either.

The HPs are all 410A because that is mostly what is commercially available but if I could use propane and sell the unit, I would.
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Old 04-14-16, 04:10 PM   #4
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I don't want to unhinge this thread. But I have a friend with a natural gas boiler who would like to go more electricity oriented as she also has solar panels. The house is well insulated and while not PassiveHaus, pretty well sealed.

As a first step, upgrading the radiator panels:
Could increasing the size of her radiator panels (she has the "US as a high-tech premium heat radiator" shown in AC_Hackers post) help her use her hot water more efficiently? What about using the Jaga style?

As a second step: replacing the gas boiler
She also has 2 large heated floor arrays (one per home level). Would ASHP be a suitable upgrade from natural gas?

I know, I know. Many variables unanswered. But if one was to speculate ...
GSHP is not an option. The house is built on granite.
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Old 04-14-16, 04:35 PM   #5
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Creeky,
I would, without hesitation, say that this can be done. Unless old cast iron rads are available cheap or free, then the next best thing is the Stelrad type of radiator which is the standard all throughout Europe. It is not too hard to size them up (in a back of the envelope sort of way) and if done right, she won't need the boiler at all. The HP can do the whole job. That said, I would keep the existing boiler in place as a second stage, just in case.

I would avoid, at all costs, any type of fin tube "convector" similar tot he middle one in ACs post as they are the least efficient thing on the market. Also, the one at the bottom will be prone to failure (it is electric after all) and possibly a bit noisy over time.

I've got 2 just like this up and running this year and I'm doing my own house this way as well.
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Old 04-15-16, 07:16 PM   #6
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OK, so it CAN and HAS been done with success many times before. Which brings on the barrage of questions:

1. With the flat plate radiators, are there any general guidelines in upsizing to maintain comparable BTU output? Assume a drop of 50-60 degF source water temperature.

2. What about the mysterious desuperheaters? What temperature water can be expected out of them? Can they be rigged to a radiator in a room where the existing unit cannot be upsized (small bathroom, etc.)?

3. Once the system is modified, should the existing boiler thermostat be lowered, or if no thermostat, should a mixing valve be installed to lower its source water temperature? What kind of shutoff or disconnect should be installed to keep it from running when the new heat source is operating?
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Old 04-15-16, 08:02 PM   #7
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Here are a couple files for your enjoyment. One gives the sq ft of radiation for various cast iron rad types and the other gives an overview of the stelrad line.

The cast iron one allows you to use a formula to determine the output at various water temps. The Stelrad has them at 110 to 180F but there are multipliers for reducing the temps even farther.

Forget using the desuperheater to try to heat rads. They are small HXs designed for a small amount of flow and perhaps 2F temp drop. You need to work within the limits of the HP which means try to keep below 105F (40C) and this is not hard to do.

The HPs I am installing have an outdoor reset so they only heat the buffer tank to the temp needed and shut off entirely when it gets to 18C outside. There is no need for a mixing valve. An outdoor reset control can be added to any HP anyway and measures the tank temp.

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