The question I would be asking is what caused it. Drainage problem could be a part as mike-home pointed out. Klem1 danced around the nutrient source a little. Filtering is one issue and leaks in the return letting in unfiltered air, from who knows where, is another. Furthermore, what are the humidity levels in the house at this point? If the RH is as low as it should be, mold issues, everywhere, will be lower. I don't see any health problem with leaving the old unit in the attic. That space is outside of the house envelope. There may be structural or other reasons to remove it. Note that I love my ductless AC, but heat pumps have become an expensive way to heat a home with today's nat gas prices. I live where it is hot, so it is not such a big deal for me. Wanna see some scary pictures of mold in homes? Check out "My House is Killing me" by Jeffrey C. May. Some molds are more dangerous than others. As far as I can tell, a lot of them are bad for me. My respiratory system is a very sensitive detector. I would not likely be able to stay in your home for longer than a few minutes. I'd not want my family exposed to constant levels of mold fragments blowing around my house.

Hi Lyn, So glad you have joined in with posting. Gads feeling a little DOH here. What is a coil? Your house sounds darling. Chris

I have the Grohe K7 in chrome and love it. Once a week or so I wipe around the coil with a damp towel. There is no build up or grime getting stuck anywhere.

This is typical behavior for a scaled-up coil. There could be, however, another cause. Do you have a good service company? Oil-fired boilers should be checked and cleaned annually, no question about it. I've only personally dealt with boilers that are, or would be over 50 years old at this point so I have to ask you some questions about how this is set up. Your hot water is supplied by an indirect tank heated by an third zone from the boiler, or is there an another coil in addition to the space heating coil? If the former, you should not have scale build-up on the coil. I an not saying it did not happen. It cold be caused by a leak leading to excessive water turnover in the boiler. That would be hard to miss but most of the damage may have been done before you moved in and then something fixed. If you have one coil and your DHW performance is poor, your space heat may be suffering as well. I don't think that I have to point out that if that is the case, you really need to get this fixed to avoid excessive oil outlays. (If you have one coil supplying hot water and heat with no heat exchanger somewhere, you have some big problems.) If you have an indirect tank, is there a temp gauge on it? What does it run when you are not stressing it as opposed to use? I suppose there could be something wrong with the tank so that you are filling at the top or drawing from the bottom. I don't know how they are built or how they may break. The coils/boilers that I am familiar with are not tricky at all to remove and clean with hydrochloric (muriatic) acid. You kinda hafta know a little about what you are doing so you don't overdo it and dissolve the metal. My parents went through a period between village water supply shift to harder well water from spring source and installation of a water softener to fix that. During that period, the coil was removed by (very good) service people and brought back with the annual service. My dad got tired of being without hot water for a half day and bought a stand-by coil. that they swapped out. He figured that it might wear out and need replacement eventually anyway so he might as well buy one "now" and have the convenience. Ah, I see now that you probably don't have an indirect since you wrote about that option in the future. I missed that.. Are you thinking about an electric "tankless" I don't think that is going to be your best option. If you have hard water, don't do it. it will be a more difficult task than keeping the boiler coil clear. Even if you don't have hard water, I don't think it is your best option. Are fuel oil , propane and electricity your only options? Just crunch the numbers to see which gives you the best outcome. If it is electricity, get an electric tank or figure out if a heat pump water heater makes sense. (If electricity is less expensive, it will be to heat the house as well so you might want to get an electric boiler if you like the HW baseboard.) If fuel oil is a less expensive source, and you have hard water, get the coil cleaned and install an indirect tank if you want a permanent fix. (Either that or install a water softener. My parents never regretted their decision. you might need to clean the coil every few years, but it will not be nearly as often.) You know, if you are in a temperate climate, and this is a hot-start boiler that makes your basement too warm in the summer, you might be better off turning it off and running an electric (HPWH?) tank in the summer. That is especially truer if the basement is humid. Go back to making hot water with the boiler in the winter if the investment in an indirect tank is not too much to recoup. The boiler is idle most of the time under those conditions and it costs a lot to keep it warm. You could consider having your plumber install a flat plate heat exchanger to heat water in your electric tank so you don't have to invest in the indirect and have that take up space in addition to that water heater..