Running two thermostatic showers

[Daljit]

Hi

I'm developing an HMO rental property which will have 4 tenanted rooms, two bathrooms and a shared dining kitchen over three floors. I'm keen to provide good quality showers in each bathroom and in all likelihood both would operate at the same time (eg mornings). I have limited storage space for a tank. I don't find electric showers can deliver as good a flow rate as thermostatic showers.

The mains pressure hasn;t been measured but i would generally describe it as strong - the mains CW tap gushes out.

There is gas CH in the property, although boiler needs replaced.

I wondered if using two 30 KW combi boilers would be possible on a single domestic gas meter. Gas meter is on ground floor, 6m pipe run to first floor where current boiler is located, and 10m to second floor (possibly location of second boiler).

Would appreciate thoughts.

 

[johnny2007]

With combi boilers, if running 2 showers, flow goes down drastically, especially on top level.
Said that, a 30kw boiler delivers a good flow and when 2 showers operate simultaneously they're still ok.
I have this setup in my house and find it ok.
Although, I prefer to wait 5 minutes if someone is taking a shower so to have a nice strong jet of water falling on me.

 

[Madrab]

An electric shower will never reach the output of a mains fed shower but It is all dependent on the mains and what that can deliver in terms of properly tested dynamic pressure and flow rates.

A smaller combi, say 24kw, will deliver plenty of hot water for 1 shower so 2 of them may be ok and could probably be served by a U6 meter but again it is all dependent on the mains. The ideal would be a large, high rate recovery, unvented HW cylinder and a good boiler on PDHW.

 

[Daljit]

The ideal would be a large, high rate recovery, unvented HW cylinder and a good boiler on PDHW.

I suspect this is the way to go.

I don't know much about these products. Looking at the Megaflow (only brand i've heard of), their slimline version could be accommodated in one of the bathrooms.

 

[foxhole]

Two boiler are possible , have 24kw in annex and 30kw combi in house off same supply .

 

[Justin Passing]

You cannot get two of what most people would call "good" showers at once off one combi.
In round numbers an electric shower is 10kW, 4 litres/minute; for a combi - you should go as big as you can, say 35kW.
So "a bit better" than electric, at half that.
Then put proper in-line FLOW RESTRICTORS (not half-open valves) in the HW to both the showers, say 7l/min.
With two off one , you risk one taking the lion's share of the water if the pipe lengths are a unequal. The FR's will help with that too.

If going for a mains unvented cylinder you can't just assume it'll be fine - you need to measure everything. FRs still help.

Two boilers, makes sense. If one fails you get fewer moans.

 

[Daljit]

You cannot get two of what most people would call "good" showers at once off one combi.
In round numbers an electric shower is 10kW, 4 litres/minute; for a combi - you should go as big as you can, say 35kW.
So "a bit better" than electric, at half that.
Then put proper in-line FLOW RESTRICTORS (not half-open valves) in the HW to both the showers, say 7l/min.
With two off one , you risk one taking the lion's share of the water if the pipe lengths are a unequal. The FR's will help with that too.

If going for a mains unvented cylinder you can't just assume it'll be fine - you need to measure everything. FRs still help.

Two boilers, makes sense. If one fails you get fewer moans.

Interesting. There's not much difference in cost between two Worcester Bosch 1000 30kw combis and a system boiler plus mid-range unvented 250l cylinder. Two things that i need to properly check/seek advice on: mains pressure and pipe sizing from gas meter to boilers (and gas hob).

If i were to use two combis, each would be connected to roughly half the rads in the house, each with own thermostat (and smarts TRVs). So if one has an issue, half house would be cold but at least one shower would still work. Only one would supply HW to the kitchen.

With unvented cylinder, i'd still need to advise tenants to keep showers to below 5mins (which is plenty really) in the mornings. As room rent is inclusive of all bills, there could be a temptation to take longer showers...!

 

[Madrab]

With unvented cylinder, i'd still need to advise tenants to keep showers to below 5mins (which is plenty really) in the mornings. As room rent is inclusive of all bills, there could be a temptation to take longer showers...!

Yeah, same issue with 2 combi's though, and the issue with them is they won't run out of hot water.

 

[Justin Passing]

Get your installer to Check the gas consumption for 2 x 30kw boilers wrt using a U6 (6m³/hr), Check diversity rules.

You CAN use both boilers to share the radiator heat - takes a little working out (Rev return) and control if one fails. Check you don't leave a dangerous electrical situation if one is isolated, using one roomstat. (relay in a box is ok)

If you use two boilers, it would make sense to use two cheapies, like Main (if they still do them) 24kW.

If you don't have any baths you could put a timer on the HW use. Standard modules would do it but again it would mean learning how.

Again you have to actually measure your mains pressure, and flow once it's got through the plumbing, to wherever the boiler /UV cyl output, would be, at the right time of day.
If you only use one boiler, consider putting an electric shower in one of the bathrooms (as well). It would save you all manner of grief if the boiler fails .
If you do instal a UV cyl with an electric heater fitted, put a padlock on it! (psst, removable fuse). It won't keep up anyway.
Remember , having a slightly better HW system probably will not affect the rent much at all, but it may well affect YOUR cost of supply & maintenance hassle.

Oh, and, the slightest unconventionality, even a relay in a box, might dumbfound a plumber. If you're going to be at the mercy of some unknown bloke, go one 30+kW combi boiler, two flow restrictors, and that additional electric shower.

 

[endecotp]

i'd still need to advise tenants to keep showers to below 5mins (which is plenty really)

Oh no it isn’t!

room rent is inclusive of all bills

Don’t do that!

 

[dilalio]

Litres per minute from the main is the most important point here.

 

[Daljit]

Ok ran water test at location of two possible boilers. On first floor, filled 20l/min, while at same time filled 10.5l/min on second floor.

I’m thinking this is enough mains pressure to run two combis to produce 10-12l/min at c38-40 degrees?

I’ve now exposed gas pipe and it would be 5m from meter to first boiler, and further 3m to second boiler.

 

[Madrab]

That is peak flow but if this is to be a HMO then you want to know dynamic pressure and flow to ensure the mains can handle multiple outlets running @ full flow @ the same time.

Run at least 2 CW outlets whilst filling the container from a 3rd. If you have an outside tap then connect a pressure gauge to that and monitor what the mains pressure does when each outlet is opened, with a final reading with 3 running at the same time. That will give you an understanding of what the mains can deliver, dynamically, when say 2 showers are running at the same time and someone then runs another mains tap in the kitchen or the washing machine kicks in.