unvented cylinder v combi store

[Jennie]

Hi all,
My customer is renovating a house. Complete renovation of the plumbing.
It's a three-bed house for a young family of 3. It will have one main bathroom (bath and maybe an over-bath shower), plus two shower ensuites.
I have yet to check the incoming water pressures/flow rates to the property. Assuming this is adequate (at least 20l/min, and 1.5bar dynamic pressure)...
Would you go for something like a Greenstar HiFlow (up to 90l/min)? Or alternatively, a system boiler and an unvented cylinder?
If it hadn't been for the third shower, I might have considered a Vailllant Ecotec combi store (20l/min). They're still designing the house, and may have prefered to save space by having a boiler on the wall.
The customer's children are still babies, so will share a bath for the next couple of years or so. But in their late teens, or if family visit, there may well be a lot of demand on the system.
Advice/ideas much appreciated.
Jennie

 

[ShaunCorbs]

Unvented Cylinder and boiler don’t forget about designing the sys for a dt24/25 output

 

[Scott_D]

Boiler and cylinder if they have the suave and budget.

 

[Jennie]

Unvented Cylinder and boiler don’t forget about designing the sys for a dt24/25 output

Unvented Cylinder and boiler don’t forget about designing the sys for a dt24/25 output

Such low dt. Is that due to the new regulation, of sizing radiators for the lower water temperatures that suit the renewables/heat pumps?

 

[ShaunCorbs]

Correct new heating systems must be sized for heat pump capability

 

[Jennie]

Hi everyone,
My customer wants a 35kw combi boiler (I recommended an unvented cylinder, but the combi won). I'm now planning the job. New pipes, new rads, new boiler, new gas pipe.
It's my first entire heating system (since my apprenticeship) so I haven't sized and designed a entire system to new regs yet.
Have I got the below correct?

• T24/25 gives a conversion factor of 0.41 if I'm speccing T50 radiators. For a room with a heat loss of 1,000w, would I need a 1,410 radiator?
• The maximum flow temperature from the new boiler shouldn't be set above 55c (under the newish regs)
• The footprint of the house is 49m2. So total is just under 100m2. I'll still need two heating zones (upstairs and downstairs), but can use one programmer. So maybe two Hive thermostats, with their phone apps?
• I'll need a two-port zone valve for each zone, to give two flow pipes. But can I use one 22mm return pipe through the property? They have screed floors downstairs. So I'll be running all the pipes under the upstairs floors, and dropping down pipes for various radiators.

Thanks for your advice everyone
Jennie

 

[ShaunCorbs]

0.385 for 24 or 0.406 for 25

So 1000w required so at dt 50 you would look at 2600w for 1000w at dt24

Or 1000w required at dt 50 you would look at 2500w for 1000w at dt25

Yes

Stupid rule I know hence why most stick the master on its own zone

I wouldn’t there’s a good chance of reverse circ

 

[Jennie]

0.385 for 24 or 0.406 for 25

So 1000w required so at dt 50 you would look at 2600w for 1000w at dt24

Or 1000w required at dt 50 you would look at 2500w for 1000w at dt25

Yes

Stupid rule I know hence why most stick the master on its own zone

I wouldn’t there’s a good chance of reverse circ

Hi Shaun
Thanks so much for your advice. Big radiators it is then.
Just one more question - where does dt 24/25 come from?
The regs say a max flow temp of 55 (55-20 is dt35). However, I'd lose another 10 percent efficiency by piping up the radiators BOE. That's as far as I've got with it.
Thanks,
Jennie

 

[ShaunCorbs]

Hi Shaun
Thanks so much for your advice. Big radiators it is then.
Just one more question - where does dt 24/25 come from?
The regs say a max flow temp of 55 (55-20 is dt35). However, I'd lose another 10 percent efficiency by piping up the radiators BOE. That's as far as I've got with it.
Thanks,
Jennie

55+35 eg f+r / 2 - 21 eg room temp

And yes for best it’s tboe

 

[Jennie]

Hi Shaun (and everyone else)
I've nearly got my head around the rad sizings.
So... flow temp from boiler 55c gives a DT25. Rads are sold advertised at DT50. Conversion factor is 0.406.
If a room has a heat loss of 1000w, 1000/0.406 = 2463w radiator.
This is under Part L, which came in June 2022. It's not law, but it's recommended as best practise.
My job is to install a new heating system in a house refurb. So I'll do my best to reach the target rad outputs. Though there may be rooms with limited space for big radiators (eg, a small shower room which needs a towel rad), and the utility room.) In which case, I'll fit what I can.
But otherwise, big radiators.
What do you all reckon on the T33 rads? Stelrad and Ultraheat both make them.
Thanks again for your advice
Jennie

 

[ShaunCorbs]

The big problem is technically it doesn’t comply with building regs I would try my best to size to it where practical

Maybe go down the route of upgrading the insulation levels etc which will reduce the Heatloss or tripple glazing etc

T33 are good aslong as you have a hand lifting them and it’s a solid wall / you hafe support / find the studs etc as most are around 60/100kg

 

[Ric2013]

Hi Shaun (and everyone else)
I've nearly got my head around the rad sizings.
So... flow temp from boiler 55c gives a DT25. Rads are sold advertised at DT50. Conversion factor is 0.406.
If a room has a heat loss of 1000w, 1000/0.406 = 2463w radiator.

In practice, I'm sure fitting radiators with an output equal to the heat loss often works because we calculate heat loss at -1 EAT (down south, anyway) and we rarely have it as cold as that. In theory, you've left nil margin for intermittent heating. Most plumbers seem to add a bit on, though I haven't decided what my personal opinion is.