Hey all, So let me describe my problem first and then I'll describe the specifics of my installation. The Problem: Over the past 2 weeks we have began experiencing the cold water sandwich for the first time since installation. It doesn't"t happen consistently, but when it does we often go from scalding hot to ice cold, sometimes more than once during a shower for example. It's also happened at the kitchen sink once or twice, but like I said, not consistently enough for me to identify any kind of pattern. Additionally we've been hearing the pressure relief valve open and release water pressure of the unit sometimes several times within an hour. Installation: The home is a two story old brick carriage housebuilt about 100 years ago, 1700' including a 500' apartment on the first floor. It sits on top of a stone foundation crawlspace which isn't insulated and some voids in the foundation where doors etc used to be are just timber framed with plywood and stucco on the outside, while the floor of the crawlspace is concrete with an exception of about 32' that was dug below grade to accomadate the hieght of the mechanicals which is a dirt floor. 75% of the drafty old wood windows have been replaced and the second floor ceiling is modestly insulated but the attic has no additional insulation on the underside of the roof. The home currently has hot water radiators and a small wood stove providing heat in the colder months. When I bought the home 5 years ago, the Bosch Aquastar had been installed previosuly to us ,I assume to save space since it was cramped in the crawlspace. Back in January we were experiencing an issue with our Bosch Aquastar HG. Incoming water temp was 37F and the aquastar was barely getting it to 104F. The plumber I was consulting with recommended Navien and since I was planning to change out my old Weil-McLain boiler soon, he suggested the combi-boiler. Due to the DHW needs for the small apartment and the remainder of the home being a combination of two full baths, two kitchens with dishwasher and a washing machine, he recommended we go with the largest of the combi-boilers, the NCB-240e. Since I was leary about disconnecting the working boiler in the middle of the coldest part of winter, he suggested I put in a small loop on the space heating side of the unit and then do the complete retrofit when the weather warmed up. I installed a five foot loop at his recommendation. We haven't experienced any problems with the DHW at all over the first 3 months of installation. Since the issue started I took some notes on the specs the unit was reading and here is what I found. Incoming DHW temp - 55F DHW Target Temp - 120F Incoming Space Heating water temp - 109F Space Heating Temp on display - 140F Water pressure - hovers around 19-20PSI When there is a call for hot water, the temp of outgoing water spiked to 139F and then the pressure in the system also spiked up above the PRV setting of 30PSI to around 36PSI and the valve began dripping a little and relieving pressure. The parameters that have changed since installation are that the outside temps are now consistently 20-30 degrees higher than they were in the winter when it was installed and the incoming water temp for the DHW is about 20 degrees higher, I also installed a check valve on the automatic water feed supply line as a back flow preventer about a month ago. Prior to that I had the supply to the auto water feeder turned off and would check and add water whenever I noticed the pressure getting close to the minimum operating pressure. We have noticed no discernible difference in the water pressure in the home that should affect the operation of the DHW system. I checked the screen on the water inlet for the DHW and is clear. My Guess: I think that what's happening is some combination of the outside air temps and incoming water temps being higher are causing both the space heating and DWH systems to heat up more than necessary and increasing the pressure in the system to the point that the PRV opens. What settings should I change to keep this from happening? and is this the root cause of the cold water sandwich or just the frequent PRV openings we are hearing? Thoughts...?