DHW cylinder - configuration

[Bobnjudi]

Our MBC passive new build has no conventional heating system. We are now embarking on plumbing.

 

We want to make the right choices for a vented copper DHW cylinder needed for up to 4 - 6 people. The hot water will be needed for 2 gravity-fed mixer / pumped showers, 3 bathroom basins, a kitchen sink and a utiIity sink. 

 

We think we need 3 immersion- type connections on the cylinder:

1. The energy from the solar PV will preferentially divert electricity into an 'immersun' type element (probably in the bottom of the cylinder?). 
2. The main energy source - when the sun isn't shining - is likely to be a conventional off-peak tariff immersion heater (again near the bottom of the cylinder?)  
3. And we think we ought to have a back up immersion for occasional use only, positioned near to the top of the cylinder.

 

The two power showers will need to be fed via (2 no. ? twin) impeller pumps from the cylinder - probably (?) with 2 separate connecting points off or close to the top of the cylinder with some means of ensuring no risk of air bubbles into the pump in-feeds. 

 

We think we're probably looking at a cylinder in the size range of 200 - 300 litres.  Having looked at a number of topics on the forum, we can’t find suggestions that address our specific queries, hence this post. We'd be very grateful for any advice and guidance on selecting the right type & size of cylinder & configuration of connections.

 

[Nickfromwales]

Why open pipe / gravity ? 

[PeterW]

Hi and welcome ..!

 

first and most fundamental question - why gravity and the pumps ..?? Do you have very low water pressure ..??

 

all of what you need can either be done via a 3-400 litre Unvented Cylinder (search for UVC) or probably a pair of Sunamp units (search Sunamp) which will both give mains pressure hot water. 

 

Gravity water systems and shower pumps are the last resort really. @Nickfromwales will be along shortly too ... 

[Nickfromwales]

Yup, but only after some more details . 

[Jeremy Harris]

23 minutes ago, Bobnjudi said:

We think we need 3 immersion- type connections on the cylinder:

1. The energy from the solar PV will preferentially divert electricity into an 'immersun' type element (probably in the bottom of the cylinder?). 
2. The main energy source - when the sun isn't shining - is likely to be a conventional off-peak tariff immersion heater (again near the bottom of the cylinder?)  
3. And we think we ought to have a back up immersion for occasional use only, positioned near to the top of the cylinder.

 

All three of these can use the same immersion heater if you wish.  All excess PV energy diverters can be bypassed to turn the heater on when there is no excess PV generation, either manually, with a timer, or both.  I fitted a timer with a series switch that I can use to disable the boost in summer as the bypass on our unit, and that originally ran a single immersion heater in the base of our old 210 litre thermal store.  The wiring is the same, but we now have it connected to the heating element supply in a Sunamp PV (we removed and sold the big thermal store as it was making the plant room and adjacent bedroom far too hot in summer).

[ragg987]

For 6 people a 300l store makes more sense to me. I would suggest 2 immersion heaters, one half-way up (or a bit above that) and one at the bottom. Logic as follows:

 

Heat top immersion first to give you some usable hot water. This can be either electrical or diverted from PV (Immersun). I would suggest you set the thermostat relatively low - e.g. 50 to 55C to minimise waste through standing losses in pipes or through the DHW lagging.

 

Once above has been satisfied, divert the input to the lower immersion. You can set this thermostat a bit higher to take advantage of excess solar during summer months.

 

Note the Immersun has this logic built in as standard. You can also use the Immersun as the timer for direct electrical heating of the DHW.

[Dreadnaught]

At the risk of going off-topic, as @PeterW mentioned, SunAmp(s) would take up less space and the lower heat loss would in the summer lessen the risk of overheating for your Passive House. An alternative?

[Visti]

39 minutes ago, Dreadnaught said:

At the risk of going off-topic, as @PeterW mentioned, SunAmp(s) would take up less space and the lower heat loss would in the summer lessen the risk of overheating for your Passive House. An alternative?

 

I just raised the opposite in your welcome post @Dreadnaught, what a coincidence!

 

 

At ~12p/kWh heating via electricity is somewhat more expensive than gas at 4p/kWh (average rates, not accounting for E7 or E10). Unless you charge the SunAmp via PV and well timed E7 or E10, you're looking at a higher £/kWh.

 

Noted there are lower losses from storing the heat with the SunAmp unit (about a 1/4 of a conventional cylinder IIRC), which would reduce the total kWh and therefore lower your total £.

 

If you did the calculations (which we need to do shortly) I'm sure the SunAmp unit can get very close to a £/kWh cost as a conventional unvented boiler+cylinder. However, the initial capital outlay when factoring in PV may be significantly more. Hence we're undecided!

 

 

Edited February 14, 2018 by Visti
typos

[Dreadnaught]

@Visti, at the risk of going off at tangent, given that @Bobnjudi was asking a specific question about a cylinder, I am learning that one of the greater risk with a passive house is not the winter space heating but the complication of keeping the home cool in the summer. I believe @JSHarris found this and somewhat to his surprise. 

 

I therefore weigh the overheating benefits of SunAmp highly given how costly cooling can be in terms of either house design (shading installation, etc.) and possible cooling technology (such as air-to-air or air-to-water heat pumps, etc.).

 

Also the ability to have a (marginally) smaller plant room (not applicable to @Bobnjudi I assume given their advance stage) is another cost to be considered.

 

Just a thought. Hope its not too off topic. Sorry @Bobnjudi!

[Bobnjudi]

1 hour ago, Nickfromwales said:

Why open pipe / gravity ? 

 

We have relatively low water pressure. It was measured recently by a local plumber at a nearby outside tap as being 2 bar static pressure delivering a flow of 14 litres per minute. We live next door, and know from experience that the water pressure is variable. Hence planning on a vented gravity system......

[Jeremy Harris]

13 minutes ago, Bobnjudi said:

 

We have relatively low water pressure. It was measured recently by a local plumber at a nearby outside tap as being 2 bar static pressure delivering a flow of 14 litres per minute. We live next door, and know from experience that the water pressure is variable. Hence planning on a vented gravity system......

 

 

2 bar static is fine, so one option might be to just fit an accumulator, to boost the flow rate and maintain 2 bar even when water is being drawn off.  A gravity system is going to deliver a pressure of less than 1 bar (1 bar ~ 10m head) for most normal height homes, and needs large bore pipework to get an acceptable flow rate.

 

 

 

 

[Nickfromwales]

8 minutes ago, Bobnjudi said:

 

We have relatively low water pressure. It was measured recently by a local plumber at a nearby outside tap as being 2 bar static pressure delivering a flow of 14 litres per minute. We live next door, and know from experience that the water pressure is variable. Hence planning on a vented gravity system......

Put a non return valve on the pressure tester and leave it on overnight. That'll capture the maximum static pressure which usually peaks out around 01:00 to 04:00. Once you have that information you can then properly decide on a solution.

Cold mains accumulators sound like they'd sort your problem TBH. 

Sealed potable and simple. 

[Nickfromwales]

Crossed with Jeremy.