How to keep your HRV clean and operating efficiently

We will try to go through this point by point, but it seems your main issue in there is trying to repair / clean / replace the duct work? We of course can’t speak to how difficult or costly that would be without seeing it unfortunately. Here is out page on HRV maintenance, have a look and maybe you can find the problem - 

HRV maintenance: How to keep your HRV Clean & Efficient

If your HRV or ducts were cleaned but they are still full of dirt afterwards, then they weren’t cleaned.

If you had an HVAC contractor come in and they didn’t offer you solutions, then call another, however, that first person may have been correct in diagnosing the fix to your problem as replacing the HRV, I say that because you say it was cleaned but is still dirty, and clearly the previous owners weren’t big on air quality so it’s highly unlikely you have a good quality HRV that is performing well.

Blocked vents – unblocking the was wise, but it’s possible that the HRV was damaged by that move.

Electrical issues in a home is a serious matter for safety and should be addressed immediately.  

Air will naturally leak in the bottom of a home and leak out the top, so if air from the attic is leaking down into the conditioned space then your home may be depressurized due to problems with your HVAC system. I hate to be the bearer of bad news, but I do think you need to get some proper ventilation work done, that starts with getting a second, and maybe third opinion. 

You could also get a good quality portable air filtration unit that will remove the pollutents and allergens that cause stale odors - read more here.

I am not entirely sure what you mean by a ‘second ground loop’ after the system but I’ll take a shot at an explanation anyway. The intake and exhaust direction never change, so what leaves the HRV or ERV is nothing but stale air you are exhausting to the exterior so there is nothing you need to do with that air. I may be missing something in your question and please feel free to elaborate if the following doesn’t properly address your question, but it’s a good topic so I’m going to go through the basics.

An HRV or ERV is a centralized ventilation core where incoming air is pre-heated or pre-cooled by exhaust air. No heating or cooling coil is present, you are simply running a fan, and a heat exchange takes place in the core. Read more  here about choosing between an HRV and ERV here.

To soup that up a little, some builders will install a ground loop as you mention, and I’ll start with a basic explanation of that as well along with its drawbacks -

A classic ground loop pulls air through a long 6-inch pipe below grade (often accompanying footing drains) to feed pre-heated or pre-cooled air into an HRV. They work well to help reduce heating and cooling costs, however, there are air quality concerns if they are not done properly. 

We would first give you an idea of what problems they can bring – as the tubes are below grade, on a hot day they will be much cooler than the air they are drawing in. So there is a risk of condensation forming as warm, humid air come in contact with those tubes. That, combined with any organic matter that may find its way in (which can be as little as a buildup of pollen in the spring) creates an environment where mould (or 'mold' for US readers) can form and introduce mould spores right into your ventilation system. 

There are some people that swear by ground loops and dismiss the risks, so if any readers plan to construct one I would be sure that at the very least you ensure they are graded in a way that would allow any water to drain out. 

To now address the fluid ground loop portion of your question: 

This involves running a ¾ pex loop through the ground (with a glycol mix to prevent freezing) and have the incoming air run through a heat exchanger coil. This way you can get the benefits of the stable ground temperature but eliminated that risk of contaminating your air quality. 

That same warm humid air can instead condense on the heat exchange coil, so you need to make sure you have a way to collect that moisture. This will offer minor benefits in terms of dehumidifying the air. 
Systems such as this are not terribly hard to Jerry-rig yourself, but you can also buy a plug-and-play system from Zehnder, which is a favourite brand with Passive House builders. 

As for how to design the system, you are into the higher end of building efficiency here so if you have limited knowledge in this field I would highly recommend looking into designers or HVAC contractors experienced with such systems. Passive House certified designers would probably have a lot of good ideas to pitch you, as well as knowledge about the types of systems you are talking about here. If that doesn't answer your question let us know, and feel free to let us know what you finally decide on.