Baseboard Hot Water Heating System Question

Well yet another heating season is upon us. If I was smart I would have had my heating system serviced earlier in the year, but I didn't.
My heating system is a oil fired hot water boiler that provides my house with both domestic hot water and hot water for the baseboard heaters, three zones. Its a reasonably new furnace so the system works well with a couple of exceptions. Here's the three issues I have;
#1. Occassionally the furnace doesn't turn on when it should. It takes a press of the blower's "reset" button to get it started. This is likely a job for a furnace technician, the cleaning, repair and adjustment of the furnace blower. I'm not touching this job.
#2. There's air in the heating system, I can hear it gurgling through the baseboards everytime the circulators kick in. I've looked for a "bleed" valve which I should find on one of the baseboards but to avail. The system has an expansion tank, would this be an alternate way to deal with the air in the system? Can I simply recharge the expansion tank and if so, to what pressure?
#3. The heating system is sealed, the same water has been circulating about the house for at least the three years I've lived here. How often should it be replaced?, flushed?
I'm going to call the service company today, its getting too cold outside to risk it failing again. Perhaps he'll answer all these questions for me.
One last thing, has anybody with a simular heating system put in a mixing valve for the domestic hot water? At the moment the domestic hot water is at the same temperature as the baseboard heaters, it makes for very very hot showers.
Any advice you can provide me with would be appreciated.
Handi
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Randy wrote:

Depending on the controls, there are all sorts of delays and lockouts to prevent short cycling or running the boiler dry. A technician probably needs to look at it. Boilers are not like hot air furnaces -- don't expect instant responses when you push a button. The system may not start immediately, and it can continue to run after you think it should've stopped.

The system should have no air bubbles or dissolved air in the water. Bubbles will eventually ruin the pump impeller, and dissolved oxygen rusts cast iron parts. Big air pockets are purged by filling and venting, usually at the highest point in the system. Dissolved air is removed with de-airing gadgets - ask the tech to show you where they are. The expansion tank is a safety device, it has nothing to do with removing air.

In a hydronic system running normally, the water is never replaced because it has all of the air removed, and that's what you want. There is usually a manual or automatic fill valve that adds small amounts of water if it's needed due to leaks or venting

A job for the plumber. Using 185F water in your potable DHW system is NOT a good idea! Do you have an indirect DHW tank with a heat exchanger coil? A mixing valve at the output will solve the scalding problems.
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Furnace technician will be of little help. Get a boiler tech since you have a boiler, not a furnace.
There are a few reasons the reset will trip. ONe is oil starvation. If the filter is clogged, or the tank very low, or the nozzle clogged, it will not ignite and trip out. Do NOT keep hitting the reset. It can cause some buildup of oil and when it does light off, BOOM, it really goes.

It may not be the tank. Open the drain valve very slightly while the water is circulating and it will often ltet out any trapped air.

No, you don't want to introduce fresh water and the oxygen in it.

Yes, it can be done. Talk to the service guy.
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