I have a grundfos mq 35 pump to boost pressure in my hot and cold system. due to loss of water recently pump ran dry and shut down. when water returned the system was restarted and pump reset. we now get 3.5 bar pressure in the accumulator but as soon as a tap is turned on the pump in unable to maintain any pressure. any way to fix this

Since 3.5 bar of pressure is obtained using the pump to create static pressure in the accumulator, it appears to me that the pump is delivering at least some water. I assume that by "accumulator" you mean a pressure tank to continue delivering water, even after the pump has shut off, so that it does not need to run each and every time someone turns on a tap to wash their hands. Your pressure tank may be the problem, the symptoms you describe match the condition of a "swamped" pressure tank. These tanks operate by using an air bubble which the incoming water compresses as the pressure builds in the system. In a bladdered pressure tank, the air bubble is on one side of the bladder and the water is on the other side of the bladder to prevent the air from dissolving into the water, thereby reducing the size of the air bubble to nearly or completely nothing. In a bladderless tank the only difference is that the air Will dissolve into the water eventually and it is a given that the air will need to be recharged or replenished in the tank periodically. A bladder tank needs the air pressure and volume checked at least once a year to detect small leaks and prevent the air bubble being "swamped" with water or dissolved away by leaking into the water or the air bubble disappearing due to an external leak, such as a leaking air valve. A bladderless tank the air to be recharged every few days or every few months, depending on the original air mass, the volume of water, and the water temperature. When the air bubble is exceedingly small, then just turning on the tap will drop the water pressure almost instantly. This is not the pumps fault. It is because the pressure was being exerted not against an air bubble that could expand and rebound as the water was expelled, but against the piping and walls of the tank, which are not very elastic and do not rebound much as water is expelled from the system by turning on a faucet. Turn off the water pump, Open a faucet while you Add some air to your pressure tank and Allow the air pressure to push the water out of the faucet until air starts coming out of the faucet. Now, close the air side of your tank, stop pumping air, close the faucet, turn on the water pump, allow the water pump to build water pressure in your system. How try turning on a tap and see if your problem is solved. Since 3.5 bar of pressure is obtained using the pump to create static pressure in the accumulator, it appears to me that the pump is delivering at least some water. I assume that by "accumulator" you mean a pressure tank to continue delivering water, even after the pump has shut off, so that it does not need to run each and every time someone turns on a tap to wash their hands. Your pressure tank may be the problem, the symptoms you describe match the condition of a "swamped" pressure tank. These tanks operate by using an air bubble which the incoming water compresses as the pressure builds in the system. In a bladdered pressure tank, the air bubble is on one side of the bladder and the water is on the other side of the bladder to prevent the air from dissolving into the water, thereby reducing the size of the air bubble to nearly or completely nothing. In a bladderless tank the only difference is that the air Will dissolve into the water eventually and it is a given that the air will need to be recharged or replenished in the tank periodically. A bladder tank needs the air pressure and volume checked at least once a year to detect small leaks and prevent the air bubble being "swamped" with water or dissolved away by leaking into the water or the air bubble disappearing due to an external leak, such as a leaking air valve. A bladderless tank the air to be recharged every few days or every few months, depending on the original air mass, the volume of water, and the water temperature. When the air bubble is exceedingly small, then just turning on the tap will drop the water pressure almost instantly. This is not the pumps fault. It is because the pressure was being exerted not against an air bubble that could expand and rebound as the water was expelled, but against the piping and walls of the tank, which are not very elastic and do not rebound much as water is expelled from the system by turning on a faucet. Turn off the water pump, Open a faucet while you Add some air to your pressure tank and Allow the air pressure to push the water out of the faucet until air starts coming out of the faucet. Now, close the air side of your tank, stop pumping air, close the faucet, turn on the water pump, allow the water pump to build water pressure in your system. How try turning on a tap and see if your problem is solved.