Combi and unvented cylinder

[nikbower]

Hi there,

 

I am moving into a terraced Victorian house on 5 floors, and I'll need to redo the plumbing and heating entirely.  There's a 3-storey outrigger in the rear with floors set below the LG, between LG & UG, and between UG & 1st.  The water comes in at LG level.

 

Layout will be kitchen/diner on the LG, guest WC in the LG/UG outrigger, one bathroom in the UG/1st floor outrigger, another on the first floor and a third on the top floor.  There are six of us in the house (we have four boys, aged 5-14).  So the hot water demand is pretty high.  In our current house we have a 300l unvented cylinder and we regularly run out of hot water (though mainly I think because it's a horizontal cylinder that's been installed wrongly with the hot outlet and cold inlet on either side instead of top and bottom - ie it was installed 90 degrees out of whack).

 

Anyway, this is what I'm thinking and I would be very grateful for any advice.

 

- Upgrade water supply to 32mm MDPE, install a 32mm stopcock reducing to 28mm copper pipe (like this?).

 

- Run 28mm cold supply directly from the stopcock to an unvented cylinder in the 1^st-floor outrigger bathroom.  I am thinking 400l.

 

- Immediately after stopcock, tee off 28mm cold supply to a combi boiler on LG, from which:

 

• the hot water side supplies the kitchen tap.  That way the kitchen tap would not drain the unvented cylinder – and given its proximity to the boiler, the water should run hot more quickly than if it’s drawn off the unvented cylinder upstairs.  What diameter of pipe should run hot water to the LG kitchen tap?

 

• The heating side is set up with three zones to supply (i) underfloor heating in the LG floor (two port valve); (ii) rads upstairs; (iii) heating coil for the unvented cylinder

 

- Then the unvented cylinder would supply the UG floor cloakroom (also in the outrigger) and the three bathrooms.

 

What size of combi boiler required to do this efficiently?

 

Thank you!

[JohnMo]

If you are doing UVC, cold ideally wants to come from the UVC pressure reducing valve, so everything hits the tap mixers at the same pressure.

 

26 minutes ago, nikbower said:

tee off 28mm cold supply to a combi boiler

Combi boilers only take a 15mm supply in, so no point running 28mm.

 

26 minutes ago, nikbower said:

What diameter of pipe should run hot water to the LG kitchen tap?

15mm, would do in Hep2O so internal dia is slightly smaller.

 

Issues I see.

Doing everything from the combi heating output misses out on different flow temp. A decent boiler would allow different temps for CH and DHW heating.

 

If it was me

Decent system boiler, that does weather compensation and is plumbed in X plan (not S or Y plan). Run heating at lowest temperature you can, to get around 100% efficiency or better from boiler. Install a heat pump cylinder with 3m2 coil. Again can run boiler at low temperature to get good efficiency or at a higher temp to super quick recovery.

 

If insistent on a combi. Dump the idea of a huge UVC as well.

Get a combi that takes preheated water, Intergas, Atag etc. install a preheat cylinder circa 150L. The preheat cylinder takes the cold water normally fed direct to the combi and heats it. This should be a heat pump UVC, heat as a zone from the central heating to around 30 to 40 degrees. Output from the combi will be huge - I had this set up for a while, don't notice any difference in flow with an UVC.

Edited January 19 by JohnMo

[nikbower]

Thanks John, I will read this carefully.  One question, are you suggesting pairing a gas system boiler with a UVC that's suitable for use with a heat pump, so that a low temp output from the boiler can heat the UVC?  Just checking because I won't be able to install an actual heat pump (conservation area and no outside space).

[JohnMo]

56 minutes ago, nikbower said:

One question, are you suggesting pairing a gas system boiler with a UVC that's suitable for use with a heat pump, so that a low temp output from the boiler can heat the UVC?

Yes. Big coil is the only difference in the actual cylinder.  Two options are run at low temps when your house is empty of kids etc in a few years. Run at normal gas boiler temps to get the max storage of hot water. But with the huge coil your recovery time will be really quick.

[marshian]

X-Plan can even work with a small capacity cylinder even with a high water demand and non of the downsides of a combi

 

 

[nikbower]

Do people generally agree that best approach is to install a condensing system boiler with weather compensation plumbed in X-Plan for PDHW and couple it with an unvented cylinder for all DHW?  Or is there any dissenting thought on this?  And is the idea of using a large-coil cylinder designed for ASHP one that everybody agrees on?

 

Then I guess it would make sense to install both the boiler and the cylinder on the 1st floor bathroom and run all DHW and CH from there?  Pipe runs to the LG for kitchen hot water and UFH will be longer, I guess..

 

Is there any consensus on which brand of UVC is best?  From web searching it seems a lot of people have trouble with Joule and Gledhill (the cheap options), most people don't like Telford (which is actually what I have had for the past 10 years with no issues).  Seems like OSO always highly rated, also seen love for ACV (which I've not heard of before).  Worcester Bosch?

 

Thanks!

 

[JohnMo]

28 minutes ago, nikbower said:

Gledhill

I have one sold with an Ideal badge on it. No issues, charges quickly, has a 3m2 coil, but it is split into several smaller ones in parallel to limit pressure drop. The immersion is silent in operation, unlike a thermal store I had.

 

Run at a low temperature cylinder heat loss is almost zero. Most cylinder losses are down piss poor pipe insulation normally anyway.

 

32 minutes ago, nikbower said:

guess it would make sense to install both the boiler and the cylinder on the 1st floor bathroom and run all DHW and CH from there? 

Do secondary circulation if you have long runs, with thermostat and timer and small bore from a plumbing manifold for hot and cold.water distribution.

[nikbower]

Hi there. 

 

Do you know whether it's possible to connect a system boiler to OSO Delta Geocoil in X Plan?  Their brochure only mentioned S & Y Plan wiring... 

 

The coil in the OSO Delta Geocoil is rated 32kW.  If it's paired with a 30kW system boiler, do you know how I work out the reheat time for the cylinder?

 

This cylinder only comes with a 22mm DHW outflow connection.  From the point of view of maximising flow rates, how much of an issue is that likely the be vs a cylinder that has a 29mm outflow?

 

Thank you!

[JohnMo]

3 hours ago, nikbower said:

do you know how I work out the reheat time for the cylinder

Look at the cylinder datasheet it should have that information.

 

3 hours ago, nikbower said:

you know whether it's possible to connect a system boiler to OSO Delta Geocoil in X Plan

You need to look at the boiler manual not the cylinder manual. X plan uses a 3 port diverter, S and Y use either mid point valve and a two port or two 2 ports valves.

 

Cheap boilers maybe don't do X plan, this is an Intergas wiring for X plan.

X-Plan-diagram-Rapid-HRE-Eco-RF-low-voltage-option-V1.pdf

 

And an Atag boiler all the connections are within the boiler

 

3 hours ago, nikbower said:

This cylinder only comes with a 22mm DHW outflow connection.  From the point of view of maximising flow rates, how much of an issue is that likely the be vs a cylinder that has a 28mm outflow?

 

Last thing you want to do is run hot water in 28mm, as it will never get to the taps. Mine comes out the top of the cylinder in 22mm, goes into a tee and is reduced to 15mm (Hep2O), one side goes to the kitchen and the other side to a manifold and then to shower room, 2x ensuites and a utility room.

 

[marshian]

On 21/01/2024 at 22:28, JohnMo said:

Run at a low temperature cylinder heat loss is almost zero. Most cylinder losses are down piss poor pipe insulation normally anyway.

 

Can you expand on that please - sorry for the slight threadjack

 

I get a heat loss from my jacketed tank of around 0.5 to 0.7 deg per hour when not using any water from the tank which I think is quite poor but if I heat the tank to 60 deg my heat loss is 1.0 to 1.5 deg per hour

 

I have a 112 L std foam jacket vented cylinder with a 3kW heater coil (Yeah and a boiler that has a min of 10kW - smart huh)

 

All the pipework in the area of the tank is well insulated

 

I heat the water to mid 45 deg (+/- 3 Deg to minimise heat loss (because water heating is a fixed time and depending on heat loss of the cyl and what temperature it is when the heating cycle starts I can't control the tank temp at the end of the heating period to a better level of accuracy)

 

Note - once a week I do a legionaires cycle to 60 deg C although we pretty much use almost all the water daily and that's how I know my heat loss massively increases when the tank is heated to a hotter temp.

[JohnMo]

9 hours ago, marshian said:

once a week I do a legionaires cycle

Do yourself a favour and go onto heat geek site and read up on legionnaires, if you consume all your hot water daily there is zero point doing the cycle.

 

If it's vented cylinder you are going to have bigger losses as you have a vertical pipe out the top of the cylinder. Insulate with 25mm wall thickness foam.

 

As a side note - Insulation with gaps isn't any good. So make sure the mitred ends match up with the next piece and tape all the joints nice and tight together, along the length and at the corners - use electric insulating tape. Take the insulation a good couple of meters away from the cylinder. Go to a plumbers merchants they do 2m lengths. Use 19 or 25mm wall thickness at valves or compression joints use 35x9 insulation.

 

 

[marshian]

1 hour ago, JohnMo said:

Do yourself a favour and go onto heat geek site and read up on legionnaires, if you consume all your hot water daily there is zero point doing the cycle.

 

If it's vented cylinder you are going to have bigger losses as you have a vertical pipe out the top of the cylinder. Insulate with 25mm wall thickness foam.

 

As a side note - Insulation with gaps isn't any good. So make sure the mitred ends match up with the next piece and tape all the joints nice and tight together, along the length and at the corners - use electric insulating tape. Take the insulation a good couple of meters away from the cylinder. Go to a plumbers merchants they do 2m lengths. Use 19 or 25mm wall thickness at valves or compression joints use 35x9 insulation.

 

 

Heat Geeks - yep well read on the Legionaires issue but the better half likes a deep long hot bath once a week so I take the tank temp up to 60 on that day to ensure I have a little water left for my shower so it doubles up as a win

 

I think my insulation is pretty good but thanks for the tips on using 35 x 9 - I shall get some to deal with valves and compression joints as they aren't currently insulated

 

Rest of the pipework insulation is all 25 mm wall on 15 mm Copper or 19 mm wall on 22 mm Copper - this way I can set up the mitre box once and it works for both 15 mm and 22 mm

 

Old and new insulation pictured below - thin stuff is std DIY shop and bigger stuff is Turbolit that I bought 50 m of each as all my CH and HW pipes run below a very well ventilated suspended ground floor

[nikbower]

16 hours ago, JohnMo said:

Look at the cylinder datasheet it should have that information.

I'm looking at cylinders designed to work with a heat pump.  They give a re-heat time, but isn't that the time you'd expect if running them off a heat-pump?  I thought the idea of coupling that design of cylinder with a system boiler was to get a much quicker reheat... but they don't seem to have that information

[JohnMo]

Gledhill data as I had on the computer for heat pump cylinder, reheat time given for two flow temps 

 

 

So for a 210L cylinder at 80 degree flow, reheat time is 17 mins or with 55 flow 17 mins. A standard boiler cylinder from Gledhill is 32 mins reheat time at 80 degree flow.

[SimonD]

17 hours ago, JohnMo said:

You need to look at the boiler manual not the cylinder manual. X plan uses a 3 port diverter, S and Y use either mid point valve and a two port or two 2 ports valves.

 

For PDHW you can also use a normally open and normally closed 2-port valve if you're modifying an existing s-plan, but the boiler should ideally support two temps of course.

[nikbower]

4 hours ago, JohnMo said:

So for a 210L cylinder at 80 degree flow, reheat time is 17 mins or with 55 flow 17 mins. A standard boiler cylinder from Gledhill is 32 mins reheat time at 80 degree flow.

Right!  I see that information for Gledhill - 17 minutes at 80 degree flow and 22 mins at 55 degrees.  But that is for heat-up to 50 degrees, whereas the 32 mins given as time to heat the standard cylinder is for heat-up to 60 degrees and it doesn't say at what flow rate...  I wonder how different they are on a like-for-like basis.  OSO doesn't seem to provide the same information on their cylinder