Installing a 50kw combi boiler

[blueflatsky]

Hi all

I just had a conversation with a heating engineer and he did not want to install my requested boiler (Worcester 50kw combi boiler) saying he would only install a 25kw or 30kw boiler

The reason I would like a 50kw boiler is the flow rate 20 l/min. He recommend a pressure relief value but in my experience these just cause other problems

Any chance someone knows why a 50kw would cause a problem?

Thanks in advance for any comments

Mark

 

[steadyon]

Depending on pipe sizes and total installation gas usage, could be classed as a commercial installation , which requires additional qualifications beyond domestic.

 

[SJB060685]

Whats the heating design load requirements? Would you consider an unvented cylinder instead?

 

[gmartine]

As mentioned you're verging on the size of a commercial sized boiler so I doubt you have the water and gas flow rates to fully utilise such a boiler size.

A Viessman 200W 30 - 35kw has a max flow rate of 17l/m, that should do you. Pricey but Vitodens 222-F 26kW storage Combi is rated at 23 litres per minute. Vaillant 938 storage combi probably has the flow rate you desire at a lesser cost.

A PRV for what?

 

[Last Plumber]

Maybe he measured your water flow rate and it was too low to provide 20 lpm? Do you have 20 lpm in the property?
I don't follow the pressure relief valve remark?

 

[blueflatsky]

Thanks all, your comment are very helpful

1) Okay the design load / commercial aspects are probably the reason, I am in a 2 bed flat with 1 bathroom
2) Is an unvented cylinder a megaflow? Don't think I can have this as I am the bottom two stories of a 4 story house
3) The only Viessman Vitodens I can see on the sites I being quoted for offer a max flow rate of 14 l/min
4) I measured the flow rate myself after buying one of those flow cups, it was upto 22 l/min
5) The pressure relief value was mentioned after I said the shower was not getting hot enough with my existing 28 kw combi

Mark

 

[Gasmk1]

If you have the flow look at the viessmann 111 this will supply 20 lpm

 

[gmartine]

18l/m Gm1 but near enough, as said you need a suitable flow rate whatever sized combi.

 

[ShaunCorbs]

At a guess gas pipe work isn’t enough for your new boiler and it’s a pain to route a new pipe

 

[EvilDrPorkChop]

2 bed flat with 1 bathroom

50kw combi for a 2 bed flat? Heard it all now 😂

Nearly as bad as the 46kw combi Rowland put in my mates 3 bed semi, on a undersized 22mm gas main when meters on other side of the house 🙄 😱

 

[gmartine]

Thanks all, your comment are very helpful

1) Okay the design load / commercial aspects are probably the reason, I am in a 2 bed flat with 1 bathroom
2) Is an unvented cylinder a megaflow? Don't think I can have this as I am the bottom two stories of a 4 story house
3) The only Viessman Vitodens I can see on the sites I being quoted for offer a max flow rate of 14 l/min
4) I measured the flow rate myself after buying one of those flow cups, it was upto 22 l/min
5) The pressure relief value was mentioned after I said the shower was not getting hot enough with my existing 28 kw combi

Mark

My friend you really need to listen to the advice you've been given otherwise you'll end up making some very costly mistakes.

What make and model is your existing boiler and is it in good order and been serviced regularly because 28kw should really be enough unless it's been poorly set up or you have a monsoon shower head the size of a dustbin lid.

Viessmann is one of the better boiler brands, the 200W has the best modulation rate on the market and can produce a flow rate of around 18l/m as can the 111W storage combi which should be ample for one outlet. A storage combi is really for two bathrooms as is an unvented cylinder (like Megaflow).

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Your flow rate is fine then.

 

[SJB060685]

2 bedroom flat and you want a 50 kw combi? Admittedly this is HW output but your heating load will be tiny, probably far less than the minimum heat output of 5kw. That boiler will cycle frequently.

 

[Timmy D]

Put a Vaillant 938 in, plenty for a 1 bed flat.

Also bear in mind, you’re not calculating your hot water demand from your hot water supply.

The water coming out of the boiler will be too hot to use directly, so will be mixed with cold to lower the temp.
This means that you’ll get a higher flow rate of shower temperature water than the stated boilers hot water output.

Depending on incoming supply temp, you’ll have a hot to cold ratio around 60/40 - 70/30.
So if the boiler outputs, let’s say 12l/m, you’ll be getting around 20l/m - 18l/m of shower temp water.

 

[Gasmk1]

18l/m Gm1 but near enough, as said you need a suitable flow rate whatever sized combi.

To be pedantic >18 lpm not being picky though😂

 

[gmartine]

It's a 2 bed flat mate, the 938 has a modulation rate of 6:1 so isn't there a good chance it'll short cycle?

 

[Timmy D]

It's a 2 bed flat mate, the 938 has a modulation rate of 6:1 so isn't there a good chance it'll short cycle?

Forgot about that bit, too focused on the hot water. Do you know how low you can set the 938?

 

[ShaunCorbs]

Forgot about that bit, too focused on the hot water. Do you know how low you can set the 938?

around 6.5kw min output

 

[SJB060685]

around 6.5kw min output

So a 2 bed flat would cycle frequently even with an outside temperature of say -5°c? Can the cycling be timed and even if could how much of an affect would it have

 

[blueflatsky]

Thanks all

This is a really useful discussion, and outside my normal sphere of knowledge, so I am having to learn alot

1) Current boiler is a Worester Greenstar 28i junior, it has not been serviced in a while
2) There was a problem with an overhead water tank, in that the cold water pressure was so high it was forcing the tank to over fill and run out of the over flow pipe whenever anyone had a shower, this has now been removed hence the request for a high flow rate boiler
3) I am okay with spending more on a boiler (and just about understand modulation rates now, so happy to rule out the Vaillant 938)
4) Reading through gmartine post the difference between the 200W and 111W is if I want hot water out of 1 or 2 outlets at once (the shower is not thermostatic, is there a sub-option of the 200W and installing a thermostatic shower?). Are there any other trade offs (such as running costs)? It seems I should be choosing one of these two boilers

Mark

 

[gmartine]

Hi Mark

Modulation rates are an important consideration for smaller residences because as with your place which perhaps has three possibly four rooms it probably only needs between 4 and 8 kw to heat which is why originally wanting to install such a large rated boiler wasn't appropriate. An oversized boiler unable to modulate low enough will short cycle repeatedly by cutting on and off, unable to condense, run poorly and inefficiently shortening it's working life etc.

Bare this in mind, every heating engineer has their particular favorite but I think 200-W takes some beating for a number of reasons even though it is costlier than most. If however you intend to install any other large rated boiler besides the 200-W you should probably try and do some heat loss calculations for your flat so you know what the boilers minimum heating output should be.

The 200-W doesn't have a thermostatic shower mode as such but you'll be able to change the HW output temperature easily enough, your commissioning engineer will set or change your parameters as necessary. I think we were all trying to mention boilers with high HW output rates but the 111W is a storage combi like the 938 so would really suit a larger place with more than one bathroom and it doesn't modulate down as well (6:1) compared to the the 200-W (17:1). If you can afford it I wouldn't hesitate, with properly utilised weather comp it's one of the most if not the most efficient boilers out there and that will also be reflected in your running costs.