15M pump required, one large Vs 2 small in series?

[Last Plumber]

I think it sounds a lot to be honest with you.
I still think you would be wise to get a Heating Engineer to spec this job for you. Boiler size, Pump size and Domestic Hot water storage, any Cold water storage that may be necessary. You would benefit from the design and the advice they will be able to give. It will cost you a few hundred pounds but I think you will save a lot more. Plus you will have the peace of mind that it will be correct, the knowledge that the system is a as efficient as possible and someone to blame if it goes wrong.

 

[doitmyself]

I used the default figure in the sizing resistance in copper tube. It was tiny. 3.5M is much bigger. Yes looking at 40CDi regular, but with Robo kit. I thought 35CDi system was just too tight. In that case I would need 8.3M head.

By "default figure" do you mean 11 deg C as on page 2 (46 in the formula for flow rate on same page)? If so, that is incorrect for your boiler which is designed for a 20 deg C differential. 46 then becomes 84. If you redo your flow rates and resistance using 84, you will find that the head is about 30% of what you originally calculated. That's because resistance varies with flow rate squared.

What was "tiny"? Don't understand your reference to 3.5M and 8.3M.

You also need to take the 20C differential and lower operating temperatures (needed to ensure condensing) when selecting your rads, as rad output will be reduced.

It would have been helpful if you had mentioned HMO in your opening post.

Have you thought about having two boilers, a 17kW for the heating and a 20kW for the cylinder? A smaller heating boiler will modulate much lower and will be condensing more often - if the system is correctly designed. You could also include weather compensation.

The HW boiler needs to run with a flow temp of 70-75C and a 10 deg differential, otherwise the cylinder will not heat up to 60C quickly enough. This means the boiler will not condense.

 

[Michael Groves]

You make some very valid points, I think I need to wind this back to make sure I'm getting correct results.
Calculating head is complicated, I'm now thinking about a single boiler using W-Plan. If I also have a 400L cylinder the heat exchanger is 17kW. This way I can use a single 18kW boiler to run both.
Worst scenario if the boiler can't cope I could buy a second 18kW boiler further down the road and have a dedicated boiler for each.
I recall being told by Worchester a while back you want your boiler running at 100% for best efficiency?

Thoughts?

 

[ShaunCorbs]

Think it will be more than 17kw as normal 300l ones have between 20-25kw coils

Also you just got the one cylinder??

 

[Michael Groves]

Seems to be various heat exchangers, Gledhill 300L has 30kW heat exchanger !!!, I'm looking at Grahams Therma.

I'll just go for a single cylinder.

 

[doitmyself]

W Plan is either heating or hot water; you can't have both at the same time. If it's a HMO, you won't have any control over when anyone wants to wash, bath or shower. So there may be up to eight people showering early in the morning when the heating needs to be on. With W plan hot water takes priority; everyone freezes as the heating is not on. You can time the hot water to recover when heating is not required, i.e. overnight or when most of the inhabitants are at work; but this can't be guaranteed to work all the time.

Have you had the water pressure and flow rate checked to make sure it is sufficient for eight simultaneous showers?

You haven't answered my earlier questions, so here they are again:

1. Where did you get the idea from that the CDi 40 heat exchanger was low resistance (tiny?)?
2. Where does the 3.5m head come from?
3. Where does the 8.3m head come from?

 

[Michael Groves]

Water Pressure is very good in the property, they'll be no problem.
Agreed it's one or the other with W-Plan, it'll take some tweaking to get it right. But remember with HMO, they're not all rushing out the door to get to work, showers are spread throughout the day. For those that wash anyway!
1. I used a default figure from calculating copper pipe resistance for average boiler, this was proven to be inaccurate. My bad.
2. You said from Technical Bulletin 48 it was 3.5M ?
3. I calculated 4.8M using calculations supplied previously. I simply added your 3.5M and got 8.3M

 

[Michael Groves]

Just been reading modern boilers prefer a W or S-Plan where each circuit gets 100% of boiler. Most 300 & 400 cylinders have a reheat time of 38-45 minutes. Therefore giving Hot Water 100% for 45 minutes every few hours should not affect Central heating that much.
I was also reading if you give hot water preference, you can reheat as soon as it drops, so maybe only need 15 minutes whenever someone takes a shower, so central heating should not be affected unless you get several showers in a row on a cold night.
They were also saying you want the biggest heat loss to stop boiler cycling, so again W or S-Plan so boiler always running at max.

 

[doitmyself]

1. I used a default figure from calculating copper pipe resistance for average boiler, this was proven to be inaccurate. My bad.
2. You said from Technical Bulletin 48 it was 3.5M ?
3. I calculated 4.8M using calculations supplied previously. I simply added your 3.5M and got 8.3M

1. Modern heat exchangers have very narrow passageways for the water to pass through, so the resistance is very high. Also, the resistance is not fixed, it varies with the flow rate.
2. The figure of 3.5m only applies when the heat exchanger is running at max output with a flow-return differential of 20C. The same hex running at 20kW, with the same differential, has a resistance of approx 0.8m. This is because the flow rate is halved. See attached chart for the CDi hex.
   
3. The resistance of 4.8m assumed a differential of 11C. Resistance is proportional to flow². The flow rate for a 20C drop will be 55% if the flow for a 11C drop (11/20) so the resistance will be 0.55² = 0.30 x 4.8 = 1.45m.
4. The available head on WB system boilers is 2m, which gives a margin of 0.55 metres.

 

[doitmyself]

Just been reading modern boilers prefer a W or S-Plan where each circuit gets 100% of boiler. Most 300 & 400 cylinders have a reheat time of 38-45 minutes. Therefore giving Hot Water 100% for 45 minutes every few hours should not affect Central heating that much.

Only W Plan can guarantee HW priority. That's because it uses a diverter valve (similar to Y plan but without a mid-position).

Most cylinder thermostats have a differential of about 10C. If the hot water is stored at 60C and incoming cold is at 20C, the cylinder temperature will drop to 50C when 25% of the hot water has been used. That's equivalent to two ten-minute showers for a 400 litre cylinder.

They were also saying you want the biggest heat loss to stop boiler cycling, so again W or S-Plan so boiler always running at max.

That doesn't make sense. Can you post a link to this?