Mod con conundrum

Unknown

if the new radiant has mass, I don't think having the bigger boiler will hurt that much.

or perhaps the bigger boiler, and a buffer tank..?

Wayco Wayne_2

I'm sizing a job

for an addition to a 200 year old house. (Way Cool!) Balloon framing with Rock Wool Blown in. New additon walls 2 x 6 with R-19. I've come up with a 109,000 btuh heat loss. I usually use the Prestige mod con boiler which is sized for inputs of 110,000 and 175,000. One's too small...One's too large. If it were my house I'd take a chance with the smaller and risk my wife's fury on design days. (ACK!) but I will not take a chance like that with a customer. I am considering using another brand boiler that is sized mo better. What do you guys recommend? WW

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Ross_7

what about 2 Prestige 110's

used with a Tekmar 265 boiler control? 265 can be used up to 3 mod/con boilers. On the days that you need a little more btuh, the second one will kick in. See if you can upsell the home owner. Good luck, Wayne
Ross

tim smith

WW, what about improving the envelope some

I think that would make the most sense. Look close at existing and see if there are any out of the box ways to do so. I had the same situation on a house but ended up w/ the 175, even though oversized it was not micro zoned so will still work ok. Not ideal but I had the same feeling, in my house ok but not in customers. I could not figure a good way to take care of envelope on theirs and it was a fairly large house. Tim

Unknown

Design temp?

What is it and what is the output on the small boiler? will you see design temp during the day or just late at night? How often? I'm trying to remember if you're the guy in Alaska.

I ran into this same situation once and then one where even the 110 was too big.

Tim's idea is good too, sounds like all you need would be a couple extra 1000 btus one way or another.

Wayco Wayne_2

Wahington DC area

This job is in Great Falls VA. Also don't know if the existing insulation has settled in the walls. You can see the outline of the holes cut in the tops of the walls so you know it was done once upon a time. The house is historical so some windows can not be replaced unless with same. They have already installed storms. I looked up the Knight boiler specs. It has a 150,000 size, but my brain is lazy and would not look forward to learning a whole new system. Plus I really like the Prestige's vertical HX.

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JackR

Here's an idea

I went to a Siggy class a few months back and he threw an novel idea at us, use a Mod/con for the first stage and a 85% unit for the second stage, the Mod/con will take care of your load 80 to 90% of the time and during design temp you will generally be looking for 150+ water temps anyway which a standard eff boiler would due. When Siggy explained it in the class it made a hell of a lot of sense. I can't do it justice.

Uni R_2

Does Manual J = True Heat Load?

The Manual J has significant recovery capacity built-in, even at design temperatures. Most of what I have learned about this is that the true heating load is typically 65 - 75% of the Manual J.

What does the customer say? Would they be fine wearing a sweater if they had a freaky record low that lasted a few days. If I were the customer, I would certainly want the 110 which seems perfectly sized for this application.

Brad White_200

Hybrid System

I agree that such a system has merit when applied correctly. There is another engineer in the Boston area who, when doing a retrofit (to a school usually), puts in as good a condensing boiler as the budget will afford, sized for the time of year (and majority of hours it seems) when condensing and modulation are most favored.

When it is colder than what best supports optimum efficiency, he then brings on about the least expensive medium-efficiency boiler he can get (82% for example) and carries the load with that.

Another example, not saying this is exactly what Siggy recommended but it may be close.

Wayne, the dual boiler approach if the budget allows, makes a lot of sense. There are some jobs where, to my regret, I did not specify two smaller boilers, even a TT Solo 60 and 110 in your case, to cover the bases, if not a pair of 110's.

Efficiency aside, you have some redundancy and standby capability should one need service or fail for any reason.

Stuff happens and some airplanes have two engines, not just for looks

Mike T., Swampeast MO

Have you or can you do a blower door test to nail down the actual infiltration? If not, with a 200 year-old home things could be better or worse than typical infiltration assumptions. Are there lots of fireplaces? If so, they're serious heat suckers...

That's quite early for balloon framing. A quick history check shows that balloon-like framing was "used in 17th century Virginia when time was of the essence". Source said however that the ribbons supporting upper floor joists were a much later construction practice.

joel_19

boilers

i have used a condening boiler primary and standard boiler as second on a job about 5 years back works quite well. I do agree with others though that with a little envelope work you should be fine. how many zones? odds are they will never all call at once.

Mad Dog_2

I agonize over this on every job wayne

and not just mod cons...its always "right on the border line." All I can tell you is this: I have yet to have to replace a hot water boiler, but I did once with The Steam -my house. Mark Hunt and Darin Cook encouraged me to stop worrying so much on the hot water side and since then I have played it close and never had a problem. Mad Dog

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scott markle_2

Two 60's would be even better. Still close to the line on capacity 120k x .9 = 108k, but a huge modulation range, redundancy and efficiency.

How many dollars per hr. are your clients willing to burn. When average people consider what 100k/hr is going to cost in the near future it may give them second thoughts about the importance of maintaining a 72 degree room temp when it's -5 outside.

It's a bit like the SUV, many felt they needed a vehicle that could transport half the soccer team pull the boat and clime a steep slippery incline. The comfort of this capacity is fading with the reality of $80 fill-ups.

If it's understood that a a slightly undersized system will have higher efficiency than one sized for worst case conditions, some may be willing to take the chance that they may need to dress a bit warmer on occasion. The energy saved would probably pay for a hotel room on those super cold nights.

scott markle_2

Brad a joke about the twin engine planes I heard from a pilot goes something like this- when one engine fails the other helps gets you to the crash site faster.

Wayco Wayne_2

Mike!

you are the man. I mis-spoke. The house was built in the 1870's. The infiltration is the wild card. I'm betting the old side of the house is leaky. A blower door test would take the guesswork out of it for sure. I installed a similar job downtown near the National Cathedral. I took out a 500K cast iron beast and installed a Prestige 175K. We had a day last Winter that was 2 degrees below design temps and I got a call. The boiler was maxxed out and maintaining 68 degrees while the tstat was set at 70. I went around the house and in the basement you could feel high velocity infiltration coming in every crack and hole where pipes came through the walls. I suggested a blower door to tighten up the envelope. They declined and bought an electric heater for the TV room. Their gas bill had been cut in half, I guess they werent interested in saving more. Humph! WW

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Wayco Wayne_2

I played with

the infiltration on my Wright Soft Heat Load. It looks like we could be as high as 105,000 on the heat loss. Since the output on the Solo 110 is 99,000 things are looking grim. Perhaps I should sell them some fireplace heaters for backup. That would eliminate the infiltration from the fireplaces. Hmmm. WW

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scott markle_2

A 1500 watt space heater would give you the the 5000 btus your potentially short.

If the client understands the advantages sizing close to the load, maybe they would be OK with that.

tim smith

Wayne, another idea on this

I have talked with customers before and arranged to set up system with a separate set of tees (prim. /secondary arrangement and informed them full that I really feel the one boiler will do the job but... if it does have trouble in real cold weather, sub design, we could then easily add the 2nd boiler w/ out repipe. It takes a little time educating the customer on heat loss and the worst case scenario i.e. you may lose a couple degrees in house at extreme temps but it would be pre layed out and simple to add. Make sure gas, power and vent are easily added for 2nd boiler also. Most customers get it! 2nd, document, document. Just another idea. Customers like the idea of saving money initially and in the long run.

tim smith

Related to this but OT

I had mentioned in earlier post on this thread that I had the same situation this summer w/ real close design load on 5000 ft house. I forgot to mention this. The house I was working on was an old clients 5000ft house, older couple living in Germany on assignment for his corp. He had me go through his sone in Calif for arrangements. We discussed the undersize vs oversize at length and he also was bringing up Viessmann. Come to find out he and his company were part pat. holders on Viessmanns heat x design I think on the vitodens. They are a software design company for Engineering. He I think has a phd in engineering or software or both if I remember right. I think he is the chief tech officer. Just a interesting tid bit. Made for interesting discussions.

Dave Stroman

You did not say what the radiation was. If it is cast iron, a larger boiler might give faster response, and would be less expensive than 2 boilers. I have found that the heat emitters have as much to do with comfort as the boier size. Also, heat loss programs do not factor in any other heat sources, like lighting, cooking, solar gain.

On the other hand, I have seen brand new houses that felt like the wind was blowing right through them. No boiler sizing will make up for that.

When it comes right down to it, sometimes it is a shot in the dark. Most people just put the larger boiler in and let the modulation of the boiler match the load of the house.

Dave Stroman

Wayco Wayne_2

Here's an idea....

The original house has cast iron radiators. I'm pushing for radiant floors in the addition. Why not install 2 boilers for each heat emmiter. It breaks down to 70,000 for the original house, (cast iron) and 36000 for the additon (radiant floor).

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