Home ยป Tech Talk with John Wilson from Ravenheat plate heat exchangers
Check out my new video https://www.youtube.com/watch?v=p77iH66vcUo
I just wanted to do a video on removal of plate heat exchangers or basically the unnecessary removal of plate heat exchangers. One of the major call outs that you guys get, is fluctuating hot water where basically the water’s going hot and cold and your customer can be jumping into the shower like a rabbit’s nose. In and out, in and out. Hot, cold. Hot, cold. Well, obviously not teaching you to suck eggs in any shape or form but that’s usually down to a blocked plate heat exchanger and that blocked plate heat exchanger is not a boiler fault, it’s debris that’s within the heat exchanger.
I’ve done some drawings here of plate heat exchangers and I’ve got a cut down heat exchanger for you guys just to look at. Now here we’ve got a plate heat exchanger were we’ve 82 degrees coming in and let’s say for instance, well 86 coming out. Really that can’t happen but we’ll address that shortly. Here we’ve got a heat exchanger were we’ve 80 degrees C coming in and 70 degrees C coming out. Here we’ve got 81 degrees C coming in and 45 degrees C coming out. Now these two differential clamps here guys, these are clamped on to a piece of copper pipe. Basically the differential between these two, the instrumentation that we carry is not accurate enough to get a reading for the differential because it’s a pipe, so the temperature is the same across that pipe when we’re doing a differential reading.
When we come to a heat exchanger. A heat exchanger in its own definition, it exchanges heat. That’s what it does. Now the reason for this boiler, when you’ve got a boiler that’s firing up and dropping out. Going hold and cold, always make sure that the burner is actually dropping out guys. This 82 degrees C is very, very important and if we looked at these heat exchangers, this one is one that’s working. There is a level of debris in this one but this one is working pretty decent because we’ve got 81 degrees C coming in and 45 degrees C coming out.
This one is way off because we’ve only got a 10 degree differential, so basically it’s not becoming a heat exchanger, it’s becoming a pipe and this one is very important. We’ve 82 degrees C coming in and a hotter temperature coming out. Now we exchanged the water, the cold water comes into the heat exchanger. The flow from the space heating as the diverter valve moves across, obviously we get the transfer of heat. Now as the level of debris comes more severe in this heat exchanger, the differential temperature gets less and less and less and just to put it in simple terms, basically all we’re doing guys, we’re changing the heat exchanger into a pipe and why are we turn it into a pipe because we get to the stage were we’ve one single track through the heat exchanger, so it’s no longer exchanging heat. The course of action there is to take the heat exchanger out and give it a good cleanse.
Now the reason for the video guys, is there is so many plate heat exchangers that are taken out unnecessary and how many times have you done it where you’ve been with your customer, you’ve had your bucket there ready to show the customer the debris and surprise, surprise you look a clown because there’s no debris in there. Well there’s a couple of checks that you need to do before you start dragging plate heat exchangers out of boilers and it will save you a lot of time if you heed what I’m saying.
Now on a boiler we have these space heated thermistors, a primary thermistor what you call them and this is on the central heating flow. When the diverter valve’s in the heating mode, this pipe supplies the space heating. When we’re in domestic hot water the diverter valve mechanism moves across and we move it to supply the plate. Now the central heating thermistor has three functions. The first function is for temperature and modulation of the heating. The second function is built in frost protection at 10 degrees the pump energizers, at five degrees the burner will fire and raise it to 25 degrees C, so it’s pipe stat and frost stat in one and that’s across most boilers these days.
Now, the third function is very important. It’s high limit and high limit is rated between 82 and 85 degrees and that’s pretty much the standard in the industry guys. The high limit is what kicks in when the burner actually drops out, so that’s why the customer gets hot and cold water because it’s the high limit stat that’s reaching 82 because why is this getting so hot. It can’t dissipate the heat because the plate heat exchanger is bunged up with debris, so that’s the reason for it dropping out. It’s the high limit stat that dictates that.
Now, a couple of things that you must know. I mean I’m not saying must, I recommend that you do. This is an old atmospheric appliance and this is a standard Honeywell gas valve and on this gas valve we have a plastic nut and we have a brass nut. The brass nut is the low burner pressure,

Allen Hart

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