New boiler considerations/advice

[gtg]

Hi everyone

Apologies for the long post, and thanks for reading.

I'm probably going to be getting a replacement boiler soon. I've always found these forums really helpful, and as it's such an expensive purchase, I was hoping to get some opinions from people on here about what will or won't work.

The current install is a bit sloppy, and I'm expecting there will be a fair bit of work to tidy up and bring up to standard. I've got a couple of questions for now:

1. We currently have a system boiler and unvented cylinder in a 5 bed, 2 bathroom house.
For non technical reasons (mainly space) my preference is to fit a combi boiler. I'm aware this is probably against conventional wisdom, but want to look into whether it could work for us.

Am I right in thinking that mains water flow rate is the most important factor here? To measure this, I have just run the outside tap for 1 minute and observed how much the water meter reading moved. It moved on just over 20 litres in a minute. Is this a valid way to test water flow, and is 20 litres enough to supply a mid/large sized house?

2. I've had someone recommend a plate heat exchanger to separate the boiler from the radiator circuit (which is split into upstairs and donwstairs heating zones). I understand the benefits of separating the boiler from old/dirty rads but are there any negatives or complications with this kind of set up that I should be aware of?

Thanks for reading

 

[muggles]

That's a very valid way of testing your flow rate, although that only gives your outside tap flow rate - turn a few taps on at the same time to get a true max flow rate for the house.

No combi boiler on the market will keep up with that kind of flow rate, so you'd get a significant reduction in performance over your current setup if you went down that route.

Separating the heating with a plate is a terrible idea with a condensing boiler. In order for a condensing boiler to actually condense (and therefore be at its most efficient) it needs to run at low flow & return temperatures with a 20°C difference (delta) between those temperatures. They stop condensing when the return temperature rises above 52°C or when the delta is reduced. A plate heat exchanger, on the other hand, requires a high flow & return with a very narrow delta in order to effectively transfer heat to the system, thus preventing your condensing boiler from actually condensing. It's much better to just clean the system thoroughly and fit decent filtration before the new boiler is fitted.

 

[gtg]

Thanks Muggles, that helps a lot. I've done the same test this morning by opening up the cold tap on the bath to full power. I got the same flow rate of 20L/min when doing that. The same test on the cold kitchen tap gave a result of about 13L/min.

Would it be better to run both showers simultaneously at normal warm/hot operating temperature, then measure the flow when doing that?
I've been thinking about the flow coming into the house, but is it more relevant to look at the flow rate that we will actually use?

Very interesting on the plate heat exchanger. As well as keeping the boiler side clean, it was also specced because the flow around the downstairs heating circuit is poor in some parts of the house. We've currently got a pump midway round the downstairs circuit, which the previous owners fitted to help with this. It's not great though.

It was explained to me that having standalone pumps for the upstairs and downstairs zones to distribute flow from the PHE would solve these issues, and we'd be able to do away with the additional pump and get better results. I'm now doubting whether this is the best way.

I'm not sure how clearly I am explaining this, but I do have some floorplans where I've scribbled on what I think the pipe layout is. I could try to upload those.

 

[gtg]

Ok, I have a bit more info to share. I've just run both showers simultaneously at normal temp and the water meter showed 13L/min. I think some of the bigger combis from Vaillant and Worcester should be able to handle that if I understand correctly?