Anticipated diverter valve problem / question

[WhydidIstart]

Hi,

I'm having a new system boiler fitted in the loft and it will connect to the existing Megaflo indirect tank for hot water. The boiler and tank are very close.

I also want to connect the central heating which is all done in 15mm pipe (old heating system added to an even older back boiler, the boiler will be removed). 15mm pipe seems a little small for the 9 rad system, especially where multiple rads are fed by a single 15mm pipe, but it worked.

My concern is that with the long and small central heating pipes, when a diverter valve is in the mid position, most of the water will want to run through the very close Megaflo tank (22mm pipe) rather than through the house central heating. The restriction valve on the hot water circuit will need to be almost closed to get a good flow through the rads. This means when I select hot water only, the pump will be straining to get a good flow through the tank. Am I right to foresee slowly heating water and frequent pump replacement + noise from the restriction valve? What would you recommend doing in this situation please?

The boiler is 30kW and I think the tank can accept this, but possibly not if there is such a big restriction in place.

 

[Agile]

It all depends on the make/model of boiler. Some do require a good flow rate.

It will all manage itself by the boiler modulating back to a lower output.

But are you totally sure it needs 30 kW? Is it really a six bed+ detached house with poor insulation?

Have you done a whole house heat loss calculation?

The adition for hot water to the heating loss is only 2 kW! Thats already included in most heat loss programs.

Tony

 

[ChrisR]

The adition for hot water to the heating loss is only 2 kW!

Seems pretty dumb if you've got a cylinder like a Megaflo with a 25kW coil.
One of my custs gets through 6-800 litrres of hot water while the family showers, every morning

 

[WhydidIstart]

I'm impressed by the quick responses - thanks!

Boiler is a Vaillant ecoTEC plus 630. You’re right, the house isn’t that big, but I had the boiler already and the fitter said it would be ok as it would just modulate down like you said. There are quite a few people having baths and long showers in the morning. They hopefully don’t use quite the 6-800L figure, but someone did manage to empty the 250L Megaflo tank one morning!!

Exactly ChrisR – seeing as the Megaflo can accept almost 25kW and the boiler can easily supply this, it seems a pity not to use it.

Any ideas on what to do?

 

[Beerlover]

Seeing as you are having a lot done, why not get things done right. Older conventional boilers would muddle by on undersized pipework, where new condensers need the correct max ΔT of 20 ° to work at their optimum. The Vaillant is particularily picky - see the S.53 posts

Pipe sizing (KW)

• Beerlover
• 17 Nov 2010
• 1

BTW the Megaflow has a very quick recovery time and should only rob the heating for short intervals. Would recommend that you set the DHW to come on 1/2 Hr before the heating and that you leave it on allday.

Hope this helps.

edited due to poor spelling

 

[Agile]

Exactly ChrisR – seeing as the Megaflo can accept almost 25kW and the boiler can easily supply this, it seems a pity not to use it.

Any ideas on what to do?

Its normal to time the hot water 1/2 hour before the heating. Furthermore the heating power is only required when the temperature is -1° outside and in an case contains an allowance for heating up from cold.
Consequently the energ saving advice is to add just 2 kW to the heating power.

There is also a serious disadvantage in fitting a too large boiler. That is that it will only modulate down to a certain figure and if the heating demand is less then it can then only modulate on/off which is less efficient.

Tony

 

[WhydidIstart]

Thanks, that pipe work sizing table looks very interesting – with 15mm pipes it looks like I will only get about 1/3 or less of the boiler’s power into the CH. I’m assuming that table is for average flow rates? Does anyone know if the Valient ecoTec+ 630 pump has different speed settings?

From the table, it seems like a good idea to replace the 15mm CH pipes with 22mm up to where they split off to feed a few different sections of the house.

As I already have the boiler and it will cost about £1k for another similar one, do you think the loss in efficiency due to it being oversized will be significant? Re the tank, it looks like what’s being suggested is not to restrict the flow into the tank and just accept that occasionally if both the HW and CH are turned on together from cold, the CH will just take a bit longer to warm up – if the house is that cold, the system can be run in CH only mode for a bit anyway. Normally the water will be up to temperature and won’t affect it much.

Oh for the energy prices of the Middle East where petrol is 7p per Litre and gas/electricity are practically free!!!