Large combi advice

[gsportgeorge]

Planning on putting an entirely new system into an old house that has never had one before.

My professional advice so far has been to go for a combi but the house is pretty big and its going to need a fair few rads. like 14-16. over 3 high ceilinged floors.

I have done several different heat loss calcs and get an answer of between 24kw at the lowest and 34kw at the highest. My best guess is that we need 30plus.

So my questions are:-
Is a combi going to cope OK with this? (Probably looking at a condensing one like a Baxi 133HE plus or a Vaillant Ecomax 835e)
I am particularly worried about the pump having to work over 10m of head and 30kws worth of radiators but surely a 35kw boiler will have a pump designed to cope with 35kw of rads?!?!?

Also should I steer clear of a condensing while I still can? As I say it's a big house and we are expecting a biggish bill so the extra effciency would more than pay for a short (10year) life but will we really get the higher efficiency and will it last 10 years?

Thanks

 

[croydoncorgi]

At that scale, you'd be better off with a system boiler and an unvented HW system.
Although most combis are sized for hot water performance and usually eat the CH load for breakfast (!) with that numbere of rads you'll also be running out of pump capacity. Depending on the layout, you'd need AT LEAST 28mm pipes near the boiler, if not 35mm. This is not 'bog standard' Combi territory. Someone will now pop up and tell us about a MASSIVE combi that can drive 25 rads through enormous pipes - but I'd still suggest thinking about a CONDENSING boiler such as a Keston C40 (maybe - do the sizing accurately) or equivalent, and (say) 200 litre UV HW system. That way, you can drive several bathrooms AND have the option of electric water heating when / if the boiler goes legs up. Also, you get useful options such as a reduced-temperature mode for driving underfloor heating. In a big old house with insulation issues, you want every bit of boiler and heating efficiency and thermal insulation you can get!

Specifically, a 35kW Combi does NOT usually have a pump capable of driving 30kWs worth of rads. That's the HW performance, not CH. In most cases, what will happen is that the boiler will NEVER be able to burn at full rate in CH mode - the pump won't get the heat away fast enough.

 

[gsportgeorge]

Thanks a lot. That's the kind of thing I was worried about.

My original thought was to go with an unvented cylinder and system boiler but the appeal of the combi was hard to resist.
What would you say the reasonable limits for a combi are in terms of heating? 28kw and 12 rads?
OR, Is it possible to add a booster pump further up in the system?

The other suggestion I was given was to run two seperate systems with medium sized combis...

 

[gsportgeorge]

I have re-done my calculations to a better level of accuracy and am getting a heat requirement of 22kW so I think I can just about get away with a 28kw combi, but this is where I get confussed.

The combi boilers I am looking at like the Baxi 100 and 133 are quoting CH output powers of 30kw but with only 22mm inlet and outlet connectors?!?!?! and with a max capacity of 125litres.

Now I am finding it hard to see how that can balance with even 28kw of heat output which I reckon needs around 140litres (5litres per kW)???

Also I cant find any specs for the pump in the combi...

 

[htgeng]

At that scale, you'd be better off with a system boiler and an unvented HW system.
Although most combis are sized for hot water performance and usually eat the CH load for breakfast (!) with that numbere of rads you'll also be running out of pump capacity. Depending on the layout, you'd need AT LEAST 28mm pipes near the boiler, if not 35mm. This is not 'bog standard' Combi territory. Someone will now pop up and tell us about a MASSIVE combi that can drive 25 rads through enormous pipes - but I'd still suggest thinking about a CONDENSING boiler such as a Keston C40 (maybe - do the sizing accurately) or equivalent, and (say) 200 litre UV HW system. That way, you can drive several bathrooms AND have the option of electric water heating when / if the boiler goes legs up. Also, you get useful options such as a reduced-temperature mode for driving underfloor heating. In a big old house with insulation issues, you want every bit of boiler and heating efficiency and thermal insulation you can get!

Specifically, a 35kW Combi does NOT usually have a pump capable of driving 30kWs worth of rads. That's the HW performance, not CH. In most cases, what will happen is that the boiler will NEVER be able to burn at full rate in CH mode - the pump won't get the heat away fast enough.

Think you just wasted 15mins of your life typing that reply mate - hes got his heart set on a combi.

 

[kevplumb]

get a 35 kw greenstar doddle

 

[PEDANTICVINDICTIVEMAN]

Why not fit a 35Kw combi and do some of the hot water with that and also fit a hot water cylinder to it with an IH as back up to do some of the other DHW needs, and an electric shower in one of the en-suites, cant lose then can you.
A 35Kw combi will piiss 14 to 16 rads.

 

[kevplumb]

last one i installed had 22 rads on 4 floors

place was like a sauna

 

[gsportgeorge]

Thanks for the replies.

Think you just wasted 15mins of your life typing that reply mate - hes got his heart set on a combi.

Not at all, just seems like the non-combis I have looked at dont have any higher rating than the combis (or am I missing something?) It does mean that I can fit a seperate pump though which means I should be able to get a big enough one.

I was dead set AGAINST combis initially, just trying to keep an open mind...

Why not fit a 35Kw combi and do some of the hot water with that and also fit a hot water cylinder to it with an IH as back up to do some of the other DHW needs, and an electric shower in one of the en-suites, cant lose then can you.
A 35Kw combi will p**s 14 to 16 rads.

Not got that much hot water load to justify that. Kitchen, utility, 1 bath, 1 wc. Though I did consider that option as a later addition if we get round to adding an en-suite.

Re-done the calcs and reckon the actual HEATING load is more like 25-28kW. And the minimum number of rads we can use is 12-13. Be nice if we could fit another couple eventually so capacity up to 15 would be ideal.

Any advice appreciated

 

[gsportgeorge]

last one i installed had 22 rads on 4 floors

place was like a sauna

So what was that set-up? Did you do anything fancy to make it work? What size pipes at the boiler?

 

[ChrisR]

Some like Vaillant will tellyou where to connect an extra pump, but I doubt you'll need it. Did a place with 28kW = 25 or so rads, two bathrooms to be used heavily, with 2 combis. One combi per bathroom for HW but they share the CH. The first few metres to the first T is 28, then 22&15. One combi alone gets all the rads to 75º or so in 20 minutes or so.
2 combis worked out cheaper in terms of installation then 1 boiler + an unvented, and the mains is often well under 2 bar. (35mm copper main).

JUst about to put in a Vaillant Aquaplus - might suit your house. Output to HW, 40kW

 

[kevplumb]

35 kw greenstar in ground floor kitchen

28 flow and return vertical thro floors teed off in 22 at each floor and zoned to the upstairs

majority or rads on trv

ex the 6 cast iron rads the customer wanted

worked a treat

 

[gsportgeorge]

^^^
Thanks. Was that a combi or system boiler? Did it have 28mm connectors or did you just step up from 22 imediately outside the boiler? What pump?

My mums heating guy said he seemed to get called out to a lot of Worcester boilers and to go for a Vaillant or Baxi... Would you agree or think it's just because a lot are getting fitted?

 

[kevplumb]

combi with a cylinder for the bathrooms

22 boiler tappings straight to 28

prob a lot always where fitted

think ive repaired about 15

in the last 8 yrs

 

[gsportgeorge]

Excellent, thanks a lot.
Was that a 35 HE plus? If so didnt you need another expansion vessel for that number of rads?