Plumbing UVC with Condensing Boiler, How to....

[Gone West]

Calling plumbing gurus, @Nickfromwales, @PeterW, @pocster? How do I plumb a UVC heated by a condensing oil boiler to ensure it condenses when heating DHW. I understand that with a flow temperature of say 60C and a temperature drop of 20C through the radiators it should be in condensing mode for space heating. When heating DHW the UVC coil presumably won't be able to extract enough heat to provide a 20C temperature drop so do I have to restrict the flow through the coil? If so what is the best way to do it and set it up for condensing.

[JohnMo]

Big coil same as a heat pump, run at lower temperature in the cylinder same as a heat pump?

[PeterW]

As @JohnMo says it’s about coil size (surface area) - also as you get toward tank temp then the delta reduces and you’ll struggle to get it to condense anyway. 

[Gone West]

@JohnMo & @PeterW Ok thanks. So I have to have fitted the largest coil that can be squeezed in along with the solar coil and two immersions and accept limited condensing. I guess I'll have to fit a larger UVC than I originally intended then. So there's no advantage to restricting the flow through the coil then?

[PeterW]

Solar coil ..? Going with solar thermal too..?? If so you want a decent size cylinder - counter intuitive but heating toward 300 litres as you want the ability to store a lot of heat as you can’t do anything else with it. Domino tanks and pre-heated solar tanks are all options but they need clever planning. 

[Gone West]

1 minute ago, PeterW said:

Solar coil ..? Going with solar thermal too..??

We already had PV and solar thermal fitted when we moved in, but our existing vented tank is only 140l and we were going to fit a larger UVC.

[Gone West]

42 minutes ago, PeterW said:

Domino tanks and pre-heated solar tanks are all options but they need clever planning. 

I can't find any information on Domino tanks, is that a manufacturer?

[JohnMo]

You could do similar to Mixergy cylinder and not have an internal coil, but use a PHE and pump.

[PeterW]

1 hour ago, Gone West said:

I can't find any information on Domino tanks, is that a manufacturer?

Sorry..!! Domino means to put a pair of smaller tanks in line - basically you have one that is “hot” and then the second that is either cold or pre-heat into the first - basically you’re separating heat sources. Some needs some clever switch over valve work but you can make it work fairly simply.

[Nickfromwales]

Thermal store instead of UVC? Omits the coil and it’s inherent complications in utilising the oil boiler ‘properly’. I very rarely couple an oil boiler with an UVC for this exact reason. 

[Gone West]

12 hours ago, Nickfromwales said:

Thermal store instead of UVC? Omits the coil and it’s inherent complications in utilising the oil boiler ‘properly’. I very rarely couple an oil boiler with an UVC for this exact reason. 

Thanks Nick. I had considered a thermal store first but thought I would use more oil heating it in the spring and autumn. Do you know what sort of temperature I would have to keep the TS at in order to get hot water out at 45C?

[PeterW]

TS needs to be sat at 65°C plus, preferably 70°C to get decent amounts of hot water - you can soon run a TS cold if it’s under sized. 

[Nickfromwales]

1 hour ago, PeterW said:

TS needs to be sat at 65°C plus, preferably 70°C to get decent amounts of hot water - you can soon run a TS cold if it’s under sized. 

With a dual stat setup and an oil boiler, you’d struggle to not get DHW almost on demand tbh. A TS is a beast for producing DHW, and the heat goes almost directly in / out. 
With an oil boiler you can size the TS smaller and it will then recover faster, but if ST and / or PV are present on an install you’ll typically want to size a TS significantly bigger to harvest that ‘free’ energy. 
A TS will give a diminishing output vs an UVC, so needs careful thought and consideration, but if @Gone West has oil as a parachute, they’ll never ever be stuck for DHW with a TS. A 15-20 minute burn from the oil, from cold, and you’ll be able to jump in the shower, assuming a 200L TS vs 21-26kW boiler. 
To maximise the performance, I always specify a 28mm high recovery DHW coil with my TS installs. Trevor at Cylinders2go can get pretty much whatever you want specc’d / made to order.

[dpmiller]

As much as everyone says it won't work, this^ is how I've set up our system on an ASHP. 270l TS, with PV diverter *and* tappings for a boiler stove which isn't installed yet.

 

At "normal" ASHP DHW temps there's enough in the tank to provide a 10min shower without the heatpump cutting in, but if SWMBO has an extended shower the ASHP will happily start, hold and start to restore the temperature during the aft end of said shower, and be fully recovered after a <30min burn.

 

In electric costs it's working out less than half the cost of the previous 9kW shower in the other house...

[Gone West]

55 minutes ago, Nickfromwales said:

To maximise the performance, I always specify a 28mm high recovery DHW coil with my TS installs.

We don't use a lot of DHW so if we had a 250l TS with solar coil, two immersions and a high recovery DHW coil you don't think we would use too much oil, heating the DHW outside of the winter. Would it be best to have a direct or indirect TS for the CH?

[Nickfromwales]

7 hours ago, Gone West said:

We don't use a lot of DHW so if we had a 250l TS with solar coil, two immersions and a high recovery DHW coil you don't think we would use too much oil, heating the DHW outside of the winter. Would it be best to have a direct or indirect TS for the CH?

Tell me which days you don’t need DHW………   Has to be heated ‘howsoever’ for the days you do!

 

You can run the rads via the TS, or directly, even possible to diverter valve it for seasonal swings between the 2. 
But……

On 05/11/2022 at 11:12, Gone West said:

We already had PV and solar thermal fitted when we moved in,

Should this not negate using oil at all in the summer?

[Gone West]

12 hours ago, Nickfromwales said:

Tell me which days you don’t need DHW………

22 hours ago, Nickfromwales said:

With an oil boiler you can size the TS smaller and it will then recover faster, but if ST and / or PV are present on an install you’ll typically want to size a TS significantly bigger to harvest that ‘free’ energy.

 

Every day, but not a large volume, hence would 250l TS be enough with ST and PV?

 

 

12 hours ago, Nickfromwales said:

Should this not negate using oil at all in the summer?

 

Yes, at the moment, with a small vented cylinder, we're ok from around April to September. Don't know what the effect of going from 140l to 250l would be on the amount of oil used in the Spring and Autumn.

[Nickfromwales]

I’d go 300 so you can harvest a lot more lower grade heat as useful energy, eg utilising that to reduce the amount of energy taken to ‘recover’ back to the stat set-point.

A bigger cylinder will promote a longer, more definable ‘burn’ for the oil boiler too so will be better for you all round ( imho ). 

[Gone West]

On 06/11/2022 at 10:15, Nickfromwales said:

To maximise the performance, I always specify a 28mm high recovery DHW coil with my TS installs.

How do you specify a high recovery coil? What would the coil size? be for a 250l thermal store with a feed temperature of 65C, heating the tank from 20C to 60C.

[Nickfromwales]

11 hours ago, Gone West said:

How do you specify a high recovery coil? What would the coil size? be for a 250l thermal store with a feed temperature of 65C, heating the tank from 20C to 60C.

Sorry, higher surface area coil  ( same as a high recovery coil in an UVC ). I specify a 28mm DHW coil to get far better flow rates and connect to a 28mm TMV, then reduce to 22mm pipe work to go to distribution manifolds etc. 
For the TS, you do not have a coil between the TS and the oil boiler, which allows you to dump the huge amount of heat it produces per burn, directly to that internal volume of brine ( primary ) water, by going at that directly. 

[Nickfromwales]

Gives pretty much constant, instant hot water production. Oil + TS is a very good solution for DHW. Same with gas, but gas starts to throttle down as it nears the set temp, oil doesn’t. 

[Gone West]

11 hours ago, Nickfromwales said:

For the TS, you do not have a coil between the TS and the oil boiler, which allows you to dump the huge amount of heat it produces per burn, directly to that internal volume of brine ( primary ) water, by going at that directly. 

 

11 hours ago, Nickfromwales said:

Gives pretty much constant, instant hot water production. Oil + TS is a very good solution for DHW. Same with gas, but gas starts to throttle down as it nears the set temp, oil doesn’t. 

So I presume that method means having an external heat exchanger is not a good idea. I have been talking to Newark about their Aquinox MF thermal store which would have meant having a system oil boiler and coil in the TS. I'm now unsure what is the best way to have an oil boiler, TS, radiators and DHW plumbed and have the boiler condensing. 🤔

 

https://newarkcylinders.co.uk/aquinox-mf.html

[Nickfromwales]

Dead simple. 
Telford TS, 28mm DHW coil, no other coils. 
Oil > TS primary water. 22 or 28mm circs, depending on size of boiler, 21-26 will be fine on 22mm circs. 
Set the stat and boiler flow temps to suit and away we go 👍🏴󠁧󠁢󠁷󠁬󠁳󠁿
You’ll want high and low stat pockets in the cylinder to be able to better reference / respond, vs the one cyl stat. Will give you guest mode then, more stored ( heated ) primary water for sporadic higher usage. A bit like the old “basin / bath” setups on the old E7 / E10 cylinders of yesteryear. 

[Gone West]

1 hour ago, Nickfromwales said:

Dead simple. 
Telford TS, 28mm DHW coil, no other coils. 
Oil > TS primary water. 22 or 28mm circs, depending on size of boiler, 21-26 will be fine on 22mm circs. 
Set the stat and boiler flow temps to suit and away we go 👍🏴󠁧󠁢󠁷󠁬󠁳󠁿
You’ll want high and low stat pockets in the cylinder to be able to better reference / respond, vs the one cyl stat. Will give you guest mode then, more stored ( heated ) primary water for sporadic higher usage. A bit like the old “basin / bath” setups on the old E7 / E10 cylinders of yesteryear. 

So the boiler, radiators and thermal store are all on the same direct circuit and the DHW is heated via a 28mm coil. The solar coil can presumably go in the bottom of the tank along with low and mid position immersions.

[Nickfromwales]

10 hours ago, Gone West said:

So the boiler, radiators and thermal store are all on the same direct circuit and the DHW is heated via a 28mm coil. The solar coil can presumably go in the bottom of the tank along with low and mid position immersions.

Yup, yup…..and thrice, yup.