Re-designing my heating system

[John Carroll]

3 hours ago, JohnMo said:

Me neither and I am over 60, soon as saw BTU I moved to the next thread. Leave BTU to USA.

I'm well (well) over 70 but I just divide BTU by 3412 to get KW.

[John Carroll]

4 hours ago, jayc89 said:

 

Boiler is 33kw, massively oversized for what we need, but that's another story. I believe it's max heating output is 30.3kw, so using your calcs;

 

30.3*860/60/20 = 21.72 LPM

 

3.5*(23/21.72)^2 = 3.924678395

 

Presumably a LLH, or CCT, on the heating loop would be required, with an additional pump for the rads?  

 

Anything running at max, instinctively, feels like it'd be inefficient though? 

If you can't find the HEX dP/flowrate in the boiler MIs, I would ring Baxi and request it, you may well find that its only 3.0M or so at your required flowrate, also ask what is the pump head, most now are 7.0M which should still give ~ 6.0M at 23LPM, a HEX dP of 3.0M then still leaves 3.0M to circulate through the systems which may be sufficiend.

[SimonD]

4 hours ago, jayc89 said:

 

Having pulled them off the internet, I assumed they'd be for dT50 and therefore would need some tweaking for a lower dT. 

 

It makes sense to spec the pipework for a future ASHP too, which I assume would optimally run at dT5, using Heat Geek's Cheat Sheet for pipework at keep velocity below 0.9, it suggests the following pipe sizes would be required;

 

 

If I use the same formula for the UFH supply (minus any correction factor for rad sizes), it suggests that also needs a 35mm run !? They seem pretty beefy. 

 

I will of course calculate the run lengths, elbows, tees etc to confirm these. 

 

I think I'm now getting tempted to say that as soon as I see a mention of 'Heat Geek said this' I'm going to scoll on by!

 

I don't understand how you're arriving at 22mm for bedrooms 1 & 2 for example.

 

1.1kW at dT 5 requires a flow rate of 0.19m3/h which multiplied by 0.28 becomes 0.05kg/s. This gives us a pipe diameter of 15mm with a velocity of under 0.5m/s and a pressure loss of 0.015 meters head/m copper. My calc suggests you could get away with a 10mm pipe for shorter runs at just under 1m/s for these rads.

 

Personally, I'd delete all the bookmarks to blog posts about pipe sizing rules of thumb and just go buy a copy of the CIBSE Domestic Heating Design Guide, which has all the info you need to correctly design your system. The problem with the blog posts is that they're invariably incomplete, come with lots of caveats, and what they're really trying to do is get people to come tothem for their business! My biggest bugbear is that if a post makes claims, I want to have references so I can check out those claims - they're not given that often.

[jayc89]

46 minutes ago, SimonD said:

 

I think I'm now getting tempted to say that as soon as I see a mention of 'Heat Geek said this' I'm going to scoll on by!

 

I don't understand how you're arriving at 22mm for bedrooms 1 & 2 for example.

 

1.1kW at dT 5 requires a flow rate of 0.19m3/h which multiplied by 0.28 becomes 0.05kg/s. This gives us a pipe diameter of 15mm with a velocity of under 0.5m/s and a pressure loss of 0.015 meters head/m copper. My calc suggests you could get away with a 10mm pipe for shorter runs at just under 1m/s for these rads.

 

Personally, I'd delete all the bookmarks to blog posts about pipe sizing rules of thumb and just go buy a copy of the CIBSE Domestic Heating Design Guide, which has all the info you need to correctly design your system. The problem with the blog posts is that they're invariably incomplete, come with lots of caveats, and what they're really trying to do is get people to come tothem for their business! My biggest bugbear is that if a post makes claims, I want to have references so I can check out those claims - they're not given that often.

 

Not a fan of Heat Geek? The sizes in that screenshot were all based on their suggested kW/pipe size table and were very much as a guide until I calculated them more accurately, which I'm taking a stab at now.

 

35mm on a domestic set up does seem, large...

 

I was, perhaps incorrectly, using the "corrected" kW requirement of each radiator. I.e. the size I need to purchase because they're all spec'd at dT50. So I was basing it on 3.6kW for Bedroom 1, not the actual heat loss of the room, which is 1.1kW. 

 

Does the length not factor in when calculating pipe diameter? Is that only relevant when calculating the pump requirements?

 

[SimonD]

15 minutes ago, jayc89 said:

Does the length not factor in when calculating pipe diameter? Is that only relevant when calculating the pump requirements?

 

Yes, like I said before, knowing the resistance of the pipework is essential as in some cases it may mean resistance of the whole system is too much. So, like @John Carroll says, you get the press drop across the boiler HEX and then add your pressure loss across the index circuit to make sure it's all okay. Like above where your mass flow rate allows for a 10mm pipe, it's not going to take a long section to make it unsuitable for an efficient system.

 

The index circuit is the circuit from boiler to radiator that has the highest resistance, including boiler, pipe, fittings, valves and radiators. This figure is used to size the circulator, but obviously you have your pump in the boiler so you need to know the boiler pump and HEX figures.

[jayc89]

13 hours ago, SimonD said:

 

Yes, like I said before, knowing the resistance of the pipework is essential as in some cases it may mean resistance of the whole system is too much. So, like @John Carroll says, you get the press drop across the boiler HEX and then add your pressure loss across the index circuit to make sure it's all okay. Like above where your mass flow rate allows for a 10mm pipe, it's not going to take a long section to make it unsuitable for an efficient system.

 

The index circuit is the circuit from boiler to radiator that has the highest resistance, including boiler, pipe, fittings, valves and radiators. This figure is used to size the circulator, but obviously you have your pump in the boiler so you need to know the boiler pump and HEX figures.

 

Makes sense, thanks. I've dropped Baxi an email, will wait for their Tech Support to get back to me.

 

With regards to the individual radiator runs, is there a recommended number to keep the pressure drop below? I've seen 350Pa/m recommended by CIBSE, but it's the first I've heard of them...

[SimonD]

33 minutes ago, jayc89 said:

 

Makes sense, thanks. I've dropped Baxi an email, will wait for their Tech Support to get back to me.

 

With regards to the individual radiator runs, is there a recommended number to keep the pressure drop below? I've seen 350Pa/m recommended by CIBSE, but it's the first I've heard of them...

 

Not that I'm aware for domestic installations. The 300Pa/m is just a guideline that along with the flow velocity being below 1m/s is a good ballpark for a system that works well.