Boiler running 2 Cylinders and pipe size question

[scoobydo123]

Hi,

Going to start a refurb (DIY)which will be a HMO (house of multiple occupation). Plan is 6 rooms all with ensuites (1in loft, 3 on first floor, 2 on gnd floor). Was thinking of the following design for DHW. 170Ltr megaflo serving the top 2 floors and a gravity system feeding the Gnd floor and communal kitchen.

First question is, will all boilers support the controls needed (or can I get an independant box) as I assume I will need a mid postion valve serving the 2 cylinders (located next to each other on the Gnd floor). Obviously both have the potential to call for heat simultaneously and independently of each other which can be achieved with the 3 port valve but it needs the ability to be able to be triggered by more than 1 source with the valve diverting the water to the "correct cylinder".

Next one is on pipe size for DHW. Is there any advantage on running 28mm pipe from the cold water storage tank to the standard cylinder? The cylinder only has 22mm tapping for DHW take off. Also is it worth coming from the 22mm DHW draw off and converting immediately to 28mm to the first bathroom then continue in 22mm for the next (better flow perhaps). In respect to the 28mm from the cold water storage I assume I will be at an advantage as I can take a feed from the cold water storage for balanced cold water for the shower valves or would I be better to run a separate pipe from the cold water storage.

I appreciate that there may be some loss of performance (mulitple showers at once) but hopefully my design is ok and will accomadate as best I can within the budget I have, although as always more than willing to listen to advice from the experts.

Thanks a lot

PS just as I posted thought that a zoned system for CH is more or less what I am asking for but on the DHW side (or am I barking up the wrong tree)

 

[bengasman]

In a nutshell: it won't work.
The capacity is nowhere near what it should be, you will have to redo all the plumbing back to the meter.
HMO's are very strictly regulated, I strongly advise you to get expert advice in writing before you start doing anything.

 

[scoobydo123]

Back to the water meter not a problem, have a friend with a mole.

Have another property that is running 3 showers on a 145 litre megaflo with no issues, however this did not need to be licenced. Most LA have very little documentation on HMO's, and I have never seen a minium requirement only that it has to have hot water available. Had based it on around 7 litres a minute of HW mixed with 3 litres of cold water and 5 minute showers was also thinking of installing eco shower heads which should help to reduce this figure.

 

[DP]

Three port valve not a good idea.

 

[scoobydo123]

Ok so I need to create a circuit with relays and some logic to control the opening of 2 port valves according to which cylinder is calling for heat. Out of interest why is the 3 port not a good idea?

 

[electronicsuk]

Ok so I need to create a circuit with relays and some logic to control the opening of 2 port valves according to which cylinder is calling for heat. Out of interest why is the 3 port not a good idea?

Wow, you do like to overcomplicate things. Each cylinder needs a cylinder stat connected to a 2-port valve controlling flow of heating water through the coil. The microswitches inside the valve heads should be wired in parallel and used to call for heat from the boiler. Whether or not you feed the two cylinder stats from a single or two programmer channels is up to you.

I think balancing valves will be rather critical on a property of this size, unless you want the cylinders to steal all the heat. Also may be best to program them to heat well before you bring on the CH radiators to help negate this.

 

[scoobydo123]

So looks like it may be do-able, but I am undersize on storage and need to be clever with timing and balancing. But perhaps my design is not correct or could be tweaked.

I am trying to create good quality affordable housing, but within cost constraints. I would love to be able to install the ideal system (perhaps accumulators heat pumps & solar) but this would then push the price up to beyond affordable for the target market.

The other property seems to work well (never had any complaints of lack of heat or hot water). This property I am trying to achieve the same hence opting for part Gravity so as not to put too much immediate load on the incoming supply.

 

[bengasman]

...hence opting for part Gravity so as not to put too much immediate load on the incoming supply.

Gravity?

 

[kevplumb]

Going to start a refurb (DIY)which will be a HMO (house of multiple occupation). Plan is 6 rooms all with ensuites (1in loft, 3 on first floor, 2 on gnd floor). Was thinking of the following design for DHW. 170Ltr megaflo serving the top 2 floors and a gravity system feeding the Gnd floor and communal kitchen.

methinks cold tank +cylinder

 

[scoobydo123]

Yes gravity to "split the system" So the Gnd floor is served by CWS in the loft to get the max head height I can get (theroy behind this is if Megaflo is filling then my gravity part of the system will still run fine albeit the CWS will have reduced the speed it fills). Is this why Bengasman said I was undersize of storage as he only read 170ltr for 6 showers? And I am now understanding perhaps why a 3 port valve was not a good idea as then I would not be able to balance the 2 cylinders?

 

[electronicsuk]

And I am now understanding perhaps why a 3 port valve was not a good idea as then I would not be able to balance the 2 cylinders?

You could still balance them, but using a 3-port valve adds unnecessary complication. It's much easier to use 2-port valves, as there are simple to drive and spring return, so it's a case of apply 230v and valve opens, remove supply and it closes. Very easy to interface a couple of these to a pair of cylinder stats and a programmer.

 

[DP]

3 port valve is not allowed for unvented cylinder.

Furthermore, you are going to have two HW zones and (possibly ) one CH zone. So, how do you control flow of water through the third zone with one three port valve?

Simple solution would be a two channel timmer for two cylinders and a programmable roomstat (or a three channel programmer if only one 'box' is specified) controlling three two port valves and also fit a system bypass.

 

[mysteryman]

Do you have to have two cyls? It is the ability to transfer heat into the water that matters. Some unvented cyls have poor heat exchangers. Try to find a cyl that will take the full output of the boiler - they do exist.

 

[scoobydo123]

Hi,

Yes a 3 channel programmer sounds like what I need (is this what is usually used for a zoned CH system). Also perhaps during peak demand I could use the immersions on the cylinders.

@ Mysteryman thought the 2 cylinders which are in effect independant systems of different technology was a good idea as it would work almost independantly to what the megaflo was doing apart from the demand of filling the CWS on the incomming mains.

Any advice on the pipe sizing, or should I be having a total rethink on the whole thing. I actually thought it was a reasonable idea

 

[electronicsuk]

@ Mysteryman thought the 2 cylinders which are in effect independant systems of different technology was a good idea as it would work almost independantly to what the megaflo was doing apart from the demand of filling the CWS on the incomming mains.

I think what he was trying to suggest is that a cylinder with a high recovery rate coil will be able to provide significantly more hot water than the capacity of the cylinder alone when the boiler is running at the same time. With a large (running) boiler and good coil, it would be possible to draw off a small flow of water continuously and never run out of DHW.