Swapping from UVC to Combi

[steveoelliott]

To cut a very long story short, I’ve been offered by Baxi the choice of a replacement 210L UVC which is currently heated by our system boiler or alternatively a Baxi 36KW combi. Initially this was going to be a 40KW combi but the largest they manufacture now is 36KW.

 

We have 20 radiators, 5 bedrooms, 1 bathroom, 1 ensuite and a downstairs toilet. 
 

Our water pressure is ~2 bar so that’s the biggest gating factor even with a cylinder. If the bath hot tap is on then hit to any other outlet is very slow.

 

i had thought of going to a Combi as it’s one less thing to go wrong and be serviced annually. In addition, as our UVC is in the loft, it’s always worried me it leaking etc.

 

I became concerned that swapping to a Combi may be an issue when I wa visiting my parents today who have a Worcester 30CDI combi. When running their bath tap, I was surprised at how much cooler their hot water is. Its of course very cold today 0C but I had expected it to be uncomfortable to leave my hand in the hot water.

 

I suspect swapping the cylinder may be the best approach after all.

[marshian]

I'd avoid a combi for a lot of reasons

 

They are over spec'd for space heating  but frequently not powerful enough for multiple showers or a show and a bath

 

Because they have a huge demand for gas the gas line frequently needs upgrading to a larger size

 

If your water pressure is low on a combi you get what you get same with UVC

 

Without a tank options for solar HW or even HW via PV and immersion are out

 

2 bar mains is going to be about 15 litres per min - need @SteamyTea

[Sparrowhawk]

And keeping the UVC gives you options when the boiler needs replacing. You can go for a 4-pipe boiler (different temps on HW and heating circuits, don't know what they're really called but we're getting great results with our new one), a heat pump, whatever.

 

With only a bathroom and an en-suite, if your boiler can put enough heat into the tank to reheat quickly you can downsize it a bit if you want to save weight in the loft? Though that they doesn't play into heat pump etc in the future...

[JohnMo]

We installed a combi originally, one of the highest flow rates you can get, with preheat cylinder to make flow better. Now have UVC - water flow rates not massively different time to tap big difference. Given a choice between the two - UVC every day. Just set the pressure in your expansion vessel to match the water pressure.

 

A 4 pipe just installs the diverter valve inside the boiler casing instead of outside. So not really needed, just simplifies pipes a little.

 

Size the boiler to the heat demand, no need to go big boiler.

 

Heat loss calculation is required - plumber, scratching head and saying a number isn't good enough. Then do not accept S or Y plan, you need priory demand hot water (PDHW) then you can have high temperature for DHW heating and low flow temps for the radiators either Opentherm or weather compensation. 

[marshian]

3 hours ago, JohnMo said:

We installed a combi originally, one of the highest flow rates you can get, with preheat cylinder to make flow better. Now have UVC - water flow rates not massively different time to tap big difference. Given a choice between the two - UVC every day. Just set the pressure in your expansion vessel to match the water pressure.

 

A 4 pipe just installs the diverter valve inside the boiler casing instead of outside. So not really needed, just simplifies pipes a little.

 

Size the boiler to the heat demand, no need to go big boiler.

 

Heat loss calculation is required - plumber, scratching head and saying a number isn't good enough. Then do not accept S or Y plan, you need priory demand hot water (PDHW) then you can have high temperature for DHW heating and low flow temps for the radiators either Opentherm or weather compensation. 

 

I agree ^ don't need 4 pipe unless you want to get a Viessmann that goes down to 1.2 kWh (which is nice but I needed a boiler that fitted inside a kitchen cupboard

 

Heat only Viessmann with HWDB (HW Demand Box) and external Weather compensation sensor

 

CH set up with a NO zone valve - clean and simple it just runs

 

HW set up to close NO CH valve and open NC HW zone valve

 

Get a decent tank with a 3m coil and you'll have really fast HW recovery

 

This set up with DHWP and what is really a small tank knocks a combi out of the bloody park!!!

 

 

[steveoelliott]

Thanks all for your feedback. I wonder if a 40KW, 42KW or even a 50KW comes close. At Center Parcs they had a 50KW Worcester for the lodge and that did a decent job with two baths / showers.

[marshian]

3 minutes ago, steveoelliott said:

Thanks all for your feedback. I wonder if a 40KW, 42KW or even a 50KW comes close. At Center Parcs they had a 50KW Worcester for the lodge and that did a decent job with two baths / showers.

 

Just bear in mind the modulation range of the boiler for the CH side of the House if going combi

 

Old boiler Glow-Worm Flexicom HX a 24kW boiler with a min modulation of 10 kW

 

My House (Stupid shape 4 Bed 2 Bath 113m2 floor area over two floors) has a calculated heat loss of 4.7 kWh at minus 2.4 Deg C

 

So even in the depths of winter when it's minus 2 my old boiler was only capable of throwing more than double the energy required by the heat loss of the house. For the rest of the winter and the shoulder seasons it was probably 4 to 6 times the energy required.

 

It cycled - it cycled a lot!!!!! Each time throwing energy out of the flue before it had chance to do anything to the water circulating in it

 

Replacement Boiler 16kW with a min modulation of 3.2 kW

 

Running 24/7 heating at a target 32 deg C flow temp at -2 deg C outside - return temp 27 deg and over a 24 hr period running a total of between 14 to 19 cycles (Outside temp never stays the same for long)

[steveoelliott]

That’s interesting, I need to look at calculations. We currently have a 37 kW system boiler running 21 radiators. I said 20 yesterday but I forgot one. Our house is around 2500 square feet.
 

 

[marshian]

12 minutes ago, steveoelliott said:

That’s interesting, I need to look at calculations. We currently have a 37 kW system boiler running 21 radiators. I said 20 yesterday but I forgot one. Our house is around 2500 square feet.
 

 

 

So twice the size of mine 😉 and with 21 rads a much bigger CH circuit.

 

Any heat loss calcs been done or have you done one yourself?

 

This company link below have an online one that I used and found it fairly straightforward - I made a few mistakes - so check and check again before submitting to get the report - the detailed report at the end of it was pretty comprehensive and the few mistakes I made I could manually correct for.

 

https://heat-engineer.com/en/

 

You really only need a tape measure and a reasonable understanding of the fabric of your house and can match it to the selections available - took me a couple of days to go thro the house room by room

 

I wouldn't ever make a boiler purchase without knowing or having a damn good idea of my potential heat loss so I can size the boiler correctly (I made that mistake with the previous boiler - not enough research and just thinking that can't go wrong with bigger cos bigger is best right!!!!)

 

For the new boiler I could have got away with an 11kW Heat only Viessmann but as the 16kW version had the same min output I chickened out - I was also still heating to a schedule rather than 24/7 and you need a more ooomph to get the house back up to temp quickly)

 

In fact all the Viessmann Heat only boilers from 11kW to 32kW have the same 3.2KW min output (only the 11kW to 24kW fit inside a 300 depth kitchen cupboard - the 32kW is much deeper)

 

Turns out that the 11kW version can't be range rated and the 16 kW version can so I got lucky there by accident rather than good research

 

 

[jayc89]

What problem(s) are you trying to solve? For that size of property I don't think a Combi would be a good idea. 

Stick a pressure gauge with a NRV onto your washing machine cold outlet overnight, you'll probably find your "peak" pressure is greater than 2 bar. If it is, an accumulator might be a solution to running water from multiple outlets at the same time. 

[steveoelliott]

16 hours ago, marshian said:

 

So twice the size of mine 😉 and with 21 rads a much bigger CH circuit.

 

Any heat loss calcs been done or have you done one yourself?

 

This company link below have an online one that I used and found it fairly straightforward - I made a few mistakes - so check and check again before submitting to get the report - the detailed report at the end of it was pretty comprehensive and the few mistakes I made I could manually correct for.

 

https://heat-engineer.com/en/

 

You really only need a tape measure and a reasonable understanding of the fabric of your house and can match it to the selections available - took me a couple of days to go thro the house room by room

 

I wouldn't ever make a boiler purchase without knowing or having a damn good idea of my potential heat loss so I can size the boiler correctly (I made that mistake with the previous boiler - not enough research and just thinking that can't go wrong with bigger cos bigger is best right!!!!)

 

For the new boiler I could have got away with an 11kW Heat only Viessmann but as the 16kW version had the same min output I chickened out - I was also still heating to a schedule rather than 24/7 and you need a more ooomph to get the house back up to temp quickly)

 

In fact all the Viessmann Heat only boilers from 11kW to 32kW have the same 3.2KW min output (only the 11kW to 24kW fit inside a 300 depth kitchen cupboard - the 32kW is much deeper)

 

Turns out that the 11kW version can't be range rated and the 16 kW version can so I got lucky there by accident rather than good research

 

 

So not performed any calculations yet and our current system boiler (Vailant Ecotec 637) is running fine. It doesn't appear to cycle frequently and with a crude estimate (double yours), it isn't too far off the mark. Probably don't need bigger for the CH and you are likely correct that larger would be wasteful.

 

16 hours ago, jayc89 said:

What problem(s) are you trying to solve? For that size of property I don't think a Combi would be a good idea. 

Stick a pressure gauge with a NRV onto your washing machine cold outlet overnight, you'll probably find your "peak" pressure is greater than 2 bar. If it is, an accumulator might be a solution to running water from multiple outlets at the same time. 

 

There isn't a problem per se. Water pressure does fluctuate between 2-3 bar at different times of day. I have a PRV with gauge on the incoming main. If we run the bath tap on full then other water outlets are majorly impacted but the same also applies for cold but not completely unusable.

 

The reason for the consideration of swap is owing to an issue with the UVC which Baxi have agreed to swap with either another UVC or Combi Boiler under warranty. I then thought having a combi would mean one less thing to maintain, service or have the potential of leaking. It is however clear that a combi boiler is going to be a downgrade in our performance. I have no issues currently but fear I could going with a combi.

 

I appreciate all the helpful advice and guidance folks have given.