Unvented cylinder vs thermal store with UFH?

[Rich-]

Hi all, 

 

We have just bought our first house, which is a 200 year old stone built cottage in Northumberland. It is currently heated with electric storage heaters with an electric cylinder for DHW. I want to get the new system in before the winter but im struggling with too many options. We have/are fitting Wunda overfloor heating system throughout, with the exception of possibly 2 towel rails, but these could be electric and used on our off peak meter if needs must. No mains gas in the village, so we have already created the slab for a 2500l oil tank and have the bunded tank on order. The bathroom will have a large waterfall shower fitted, which has kind of steered me away from a Combi. There will be 2 showers and a bath in the house. 

 

Before doing any research, i was going to go for a largeish combi to get the higher water flow, and certainly one other plumber has told me to do this, but now im doing more reading im thinking this isnt the way to go. We have limited space inside for a boiler, so we are set on external boiler. 

 

One plumber that came around suggested an unvented cylinder with a system boiler. He was however adamant that the cylinder would have to go in the garage, on the other side of the wall to the boiler, which i wasnt keen on and now understand is not the case at all, especially as its at the opposite side of the house to the main bathroom. Im going off the idea of an unvented cylinder due to the fact that A) i cant install or maintain it myself and B) it needs annual servicing. 

 

I have now been reading up on thermal stores instead. These seem like a better option for me, because they dont have the regulations of unvented cylinders, but also because it seems to be a more effective buffer for the UFH than an unvented cylinder? Im aware that because its an above slab heating system, its far more likely to cycle than something with an 80mm thick slab of concrete attached to it, so i think this is something important to consider at this stage to prevent short cycling, and another nail in the combi’s coffin for me? 

 

It seems like most of the negatives of a thermal store seem to be from renewable heating sources that cant get the cylinder hot enough? And the fact that because the water is stored at a higher temp than an unvented cylinder, heat loss can be worse. Surely this can be fixed/reduced with some more insulation? For my application it seems like i can still put it wherever i want in the house, dont need certification to fit, doesn’t need to be regularly serviced and will be more efficient for my boiler in that it will be a better buffer for the UFH to prevent short cycling? Plus i can get a cylinder with a PV input incase i decide to add some panels at a later date. 

 

With doing as much work as we are and funds being tight, you can imagine the idea of me being able to fit the system myself, then get it adjusted and passed off by a plumber is a much more attractive proposition than paying a plumber many days labour to do the work i can do myself. 

 

Final one, one plumber was singing the praises of Navien boilers, with their stainless steel construction, meaning low return temps from UFH arent a problem apparently? I cant find much info online about them, does anyone have any experience with them? I was previously looking at the Grantt external boiler range, but these all have a minimum 40 degree return temp, so i would have to fit a back end protection load valve to shunt hot water back into the boiler to prevent condensation? Not the end of the world, but doesnt seem like the most efficient way to run a boiler? 

 

Thanks in advance,

Rich

 

[simon45089]

Hi Rich,

Like you I moved to a stone farmhouse in Northumberland with woeful night storage heaters and no access to gas. This was 15 years ago however. We converted the house and added 4 en suite bathrooms. We got an 1100 litre oil tank installed and an oil boiler in the kitchen. We then got a megaflo unvented cylinder, which went next to the boiler. The plumber had not installed one at that time , but has obviously done many since. It has worked superbly, never letting us down. I didnt realise they needed servicing yearly. We have never had it serviced !

I have since converted two smaller barns and use smaller electric megaflos in them which have also been great. Loads of hot water at mains pressure.I would strongly recommend this system. My plumber is in Haltwhistle if thats anywhere near you. Happy to pass on details.

Good luck,

 

Simon.

 

 

[SteamyTea]

How much insulation are you putting under the UFH pipework?

If there is not much, heat will just leak out to the ground.

Have you done any detailed heat loss calculations?

 

The biggest problem with combining heating and hot water is that they are different things.  You may find that you are better off heating your water with a simple E7 system, then using oil, or a heat pump, to heat the house.

 

Regarding 'woeful' night storage heaters, sounds like they are sized incorrectly.

Taking the simple example of one 21 kWh storage heater, this is charged with a 3 kW supply.

It can deliver, on average, 875 W.

Now to the best of my knowledge, no one makes a gas or oil boiler that small.  So it is not fair to compare a 1 kW heater to a 12 kW heat source.

Sizing is everything.

 

[Rich-]

Thanks Simon, an hour away unfortunately! Im just north of Morpeth. 
 

The Wunda system is self contained, its an over floor panel system that the pipework is laid into. I did some but Wunda also did a lot of the calculation too. 
 

How do i go about getting mains pressure electrically heated hot water? With an electric unvented cylinder? 
 

Thanks for the replies so far

Rich

[PeterW]

A direct UVC is just that - electric heated.  You can get them with a pair of 3kW immersions or a 6kW immersion and a 3kW booster. 
 

Heating UFH with electric is the sweet spot for an ASHP - 35-40c flow all day long gets maximum CoP so even at 18p/kWH a decent ASHP will be costing you 6p so it’s equivalent to oil. 
 

In terms of servicing, there is nothing to change on a UVC - it is a safety check. On an oil boiler budget £200/yr for service and other bits. 

[Rich-]

Thanks. An ASHP is certainly not on the cards at the moment as it just doesnt fit into budget, as i say an oil tanks already on its way to us, so we’re quite committed to the oil heating route, im just quite surprised that having the UFH running from the boiler and the DHW electrically heated can be cheaper! 
 

ASHP seem to cost considerably more than a boiler too so surely the payback will be quite considerable? 
 

Thanks 

Rich

[Temp]

18 hours ago, Rich- said:

The Wunda system is self contained, its an over floor panel system that the pipework is laid into. I did some but Wunda also did a lot of the calculation too. 
 

 

How thick is it? Our UFH system has 80mm of PIR and I wish we had more. We have engineered Wood flooring and have found in some cold winters the flow temperature has had to be cranked up to 55C to ensure enough heat goes into the rooms.

 

 

Edited September 16, 2020 by Temp